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2010-05-22
|
Helical edge magnetoplasmon in the quantum Hall effect regime
|
We present the microscopic treatment of edge magnetoplasmons (EMPs) for the
regime of not-too-low temperatures defined by the condition $\hbar
\omega_{c}\gg k_{B}T\gg \hbar v_{g}/2\ell_{0}$, where $v_{g}$ is the group
velocity of the edge states, $\ell_{0}=\sqrt{\hbar /m^{\ast}\omega_{c}}$ is the
magnetic length and $\omega_{c}$ is the cyclotron frequency. We find a weakly
damped symmetric mode, named helical edge magnetoplasmon, which is localized at
the edge states region for filling factors $\nu =1, 2$ and \textit{very strong
dissipation} $\eta_{T}=\xi /k_{x}\ell_{T}\agt\ln (1/k_{x}\ell_{T})\gg 1$, where
the characteristic length $\ell_{T}=k_{B}T\ell_{0}^{2}/\hbar v_{g}\gg
\ell_{0}/2$ with $\xi $ being the ratio of the local transverse conductivity to
the local Hall conductivity at the edge states and $k_{x}$ is the wave vector
along the edge; here other EMP modes are strongly damped. The spatial structure
of the helical edge magnetoplasmon, transverse to the edge, is strongly
modified as the wave propagates along the edge. In the regime of \textit{weak
dissipation}, $\eta_{T}\ll 1$, we obtain exactly the damping of the fundamental
mode as a function of $k_{x}$. For $\nu=4$ and weak dissipation we find that
the fundamental modes of $n=0$ and $n=1$ Landau levels (LLs) are strongly
renormalized due to the Coulomb coupling. Renormalization of all these EMPs
coming from a metal gate and air half-space is studied.
|
1005.4154v1
|
2010-06-02
|
Do Damped and Sub-damped Lyman-alpha Absorbers Arise in Galaxies of Different Masses?
|
We consider the questions of whether the damped Lyman-alpha (DLA) and sub-DLA
absorbers in quasar spectra differ intrinsically in metallicity, and whether
they could arise in galaxies of different masses. Using the recent measurements
of the robust metallicity indicators Zn and S in DLAs and sub-DLAs, we confirm
that sub-DLAs have higher mean metallicities than DLAs, especially at $z
\lesssim 2$. We find that the intercept of the metallicity-redshift relation
derived from Zn and S is higher than that derived from Fe by 0.5-0.6 dex. We
also show that, while there is a correlation between the metallicity and the
rest equivalent width of Mg II $\lambda 2796$ or Fe II $\lambda 2599$ for DLAs,
no correlation is seen for sub-DLAs. Given this, and the similar Mg II or Fe II
selection criteria employed in the discovery of both types of systems at lower
redshifts, the difference between metallicities of DLAs and sub-DLAs appears to
be real and not an artefact of selection. This conclusion is supported by our
simulations of Mg II $\lambda 2796$ and Fe II $\lambda 2599$ lines for a wide
range of physical conditions. On examining the velocity spreads of the
absorbers, we find that sub-DLAs show somewhat higher mean and median velocity
spreads ($\Delta v$), and an excess of systems with $\Delta v > 150$ km
s$^{-1}$, than DLAs. Compared to DLAs, the [Mn/Fe] vs. [Zn/H] trend for
sub-DLAs appears to be steeper and closer to the trend for Galactic bulge and
thick disk stars, possibly suggesting different stellar populations. The
absorber data appear to be consistent with galaxy down-sizing. The data are
also consistent with the relative number densities of low-mass and high-mass
galaxies. It is thus plausible that sub-DLAs arise in more massive galaxies on
average than DLAs.
|
1006.0298v1
|
2010-08-31
|
A SINFONI Integral Field Spectroscopy Survey for Galaxy Counterparts to Damped Lyman-alpha Systems - I. New Detections and Limits for Intervening and Associated Absorbers
|
Detailed studies of Damped and sub-Damped Lyman-alpha systems (DLA), the
galaxies probed by the absorption they produce in the spectra of background
quasars, rely on identifying the galaxy responsible for the absorber with more
traditional methods. Integral field spectroscopy provides an efficient way of
detecting faint galaxies near bright quasars, further providing immediate
redshift confirmation. Here, we report the detection of H-alpha emission from a
DLA and a sub-DLA galaxy among a sample of 6 intervening quasar absorbers
targeted. We derive F(H-alpha)=7.7+/-2.7*10^-17 erg/s/cm^2 (SFR=1.8+/-0.6
M_sun/yr) at impact parameter b=25 kpc towards quasar Q0302-223 for the DLA at
z_abs=1.009 and F(H-alpha)=17.1+/-6.0*10^-17 erg/s/cm^2 (SFR=2.9+/-1.0
M_sun/yr) at b=39 kpc towards Q1009-0026 for the sub-DLA at z_abs=0.887. These
results are in line with low star formation rates previously reported in the
literature for quasar absorbers. We use the NII 6585/H-alpha ratio to derive
the HII emission metallicities and compare them with the neutral gas H I
absorption metallicities derived from high-resolution spectra. In one case, the
absorption metallicity is actually found to be higher than the emission line
metallicity. For the remaining objects, we achieve 3-sigma limiting fluxes of
the order F(H-alpha)~10^-17 erg/s/cm^2 (corresponding to SFR~ 0.1 M_sun/yr at
z~1 and ~1 M_sun/yr at z~2), i.e. among the lowest that have been possible with
ground-based observations. We also present two other galaxies associated with C
IV systems and serendipitously discovered in our data.
|
1009.0025v1
|
2010-12-22
|
Abstract Wave Equations and Associated Dirac-Type Operators
|
We discuss the unitary equivalence of generators $G_{A,R}$ associated with
abstract damped wave equations of the type $\ddot{u} + R \dot{u} + A^*A u = 0$
in some Hilbert space $\mathcal{H}_1$ and certain non-self-adjoint Dirac-type
operators $Q_{A,R}$ (away from the nullspace of the latter) in $\mathcal{H}_1
\oplus \mathcal{H}_2$. The operator $Q_{A,R}$ represents a non-self-adjoint
perturbation of a supersymmetric self-adjoint Dirac-type operator. Special
emphasis is devoted to the case where 0 belongs to the continuous spectrum of
$A^*A$.
In addition to the unitary equivalence results concerning $G_{A,R}$ and
$Q_{A,R}$, we provide a detailed study of the domain of the generator
$G_{A,R}$, consider spectral properties of the underlying quadratic operator
pencil $M(z) = |A|^2 - iz R - z^2 I_{\mathcal{H}_1}$, $z\in\mathbb{C}$, derive
a family of conserved quantities for abstract wave equations in the absence of
damping, and prove equipartition of energy for supersymmetric self-adjoint
Dirac-type operators.
The special example where $R$ represents an appropriate function of $|A|$ is
treated in depth and the semigroup growth bound for this example is explicitly
computed and shown to coincide with the corresponding spectral bound for the
underlying generator and also with that of the corresponding Dirac-type
operator.
The cases of undamped (R=0) and damped ($R \neq 0$) abstract wave equations
as well as the cases $A^* A \geq \epsilon I_{\mathcal{H}_1}$ for some $\epsilon
> 0$ and $0 \in \sigma (A^* A)$ (but 0 not an eigenvalue of $A^*A$) are
separately studied in detail.
|
1012.4927v2
|
2011-02-18
|
The First Observations of Low Redshift Damped Lyman-alpha Systems with the Cosmic Origins Spectrograph
|
We report on the first Cosmic Origins Spectrograph (COS) observations of
damped and sub-damped Lyman-alpha (DLA) systems discovered in a new survey of
the gaseous halos of low-redshift galaxies. From observations of 37 sightlines,
we have discovered three DLAs and four sub-DLAs. We measure the neutral gas
density Omega(HI), and redshift density dN/dz, of DLA and sub-DLA systems at
z<0.35. We find dN/dz=0.25 and Omega(HI)=1.4x10^-3 for DLAs, and dN/dz=0.08
with Omega(HI)=4.2x10^-5 for sub-DLAs over a redshift path delta z=11.9. To
demonstrate the scientific potential of such systems, we present a detailed
analysis of the DLA at z=0.1140 in the spectrum of SDSS J1009+0713. Profile
fits to the absorption lines determine log N(H I)=20.68pm0.10 with a
metallicity determined from the undepleted element Sulfur of [S/H]=-0.62pm0.18.
The abundance pattern of this DLA is similar to that of higher z DLAs, showing
mild depletion of the refractory elements Fe and Ti with [S/Fe]=+0.24pm0.22 and
[S/Ti]=+0.28pm0.15. Nitrogen is underabundant in this system with
[N/H]=-1.40pm0.14, placing this DLA below the plateau of the [N/alpha]
measurements in the local Universe at similar metallicities. This DLA has a
simple kinematic structure with only two components required to fit the
profiles and a kinematic width of 52 km/s. Imaging of the QSO field with WFC3
reveals a spiral galaxy at very small impact parameter to the QSO and several
galaxies within 10". Followup spectra with LRIS reveal that none of the nearby
galaxies are at the redshift of the DLA. The spiral galaxy is identified as the
host galaxy of the QSO based on the near perfect alignment of the nucleus and
disk of the galaxy as well as spectra of an H II region showing emission lines
at the QSO redshift. A small feature appears 0.70" from the nucleus of the QSO
after PSF subtraction, providing another candidate for the host galaxy of the
DLA. (abb)
|
1102.3927v1
|
2011-05-23
|
BCS - BEC crossover and quantum hydrodynamics in p-wave superfluids with a symmetry of the A1 - phase
|
We solve the Leggett equations for the BCS - BEC crossover in the three
dimension resonance p-wave superfluid with the symmetry of the A1 - phase. We
calculate the sound velocity, the normal density, and the specific heat for the
BCS-domain (\mu > 0), BEC-domain (\mu < 0), and close to important point \mu =
0 in 100% polarized case. We find the indications of quantum phase - transition
close to the point \mu(T = 0) = 0. Deep in the BCS and BEC-domains the
crossover ideas of Leggett and Nozieres, Schmitt-Rink work pretty well. We
discuss the spectrum of orbital waves, the paradox of intrinsic angular
momentum and complicated problem of chiral anomaly in the BCS A1 - phase at T =
0. We present two different approaches to a chiral anomaly: one based on
supersymmetric hydrodynamics, another one on the formal analogy with the Dirac
equation in quantum electrodynamics. We evaluate the damping of nodal fermions
due to different decay processes in superclean case at T = 0 and find that we
are in a ballistic regime \omega\tau >> 1. We propose to use aerogel or
nonmagnetic impurities to reach hydrodynamic regime \omega\tau<< 1 at T = 0. We
discuss the concept of the spectral flow and exact cancellations between
time-derivatives of anomalous and quasiparticle currents in the equation for
the total linear momentum conservation. We propose to derive and solve the
kinetic equation for the nodal quasiparticles both in the hydrodynamic and in
the ballistic regimes to demonstrate this cancellation explicitly. We briefly
discuss the role of the other residual interactions different from damping and
invite experimentalists to measure the spectrum and damping of orbital waves in
A-phase of 3He at low temperatures.
|
1105.4438v1
|
2011-07-12
|
Considerations on the accretion of Uranus and Neptune by mutual collisions of planetary embryos in the vicinity of Jupiter and Saturn
|
Modeling the formation of the ice giants Uranus and Neptune is a long-lasting
problem in planetary science. Due to gas-drag, collisional damping, and
resonant shepherding, the planetary embryos repel the planetesimals away from
their reach and thus they stop growing (Levison et al. 2010). This problem
persists independently of whether the accretion took place at the current
locations of the ice giants or closer to the Sun. Instead of trying to push the
runaway/oligarchic growth of planetary embryos up to 10-15 Earth masses, we
envision the possibility that the planetesimal disk could generate a system of
planetary embryos of only 1-3 Earth masses. Then we investigate whether these
embryos could have collided with each other and grown enough to reach the
masses of current Uranus and Neptune. Our results point to two major problems.
First, there is typically a large difference in mass between the first and the
second most massive core formed and retained beyond Saturn. Second, in many
simulations the final planetary system has more than two objects beyond Saturn.
The growth of a major planet from a system of embryos requires strong damping
of eccentricities and inclinations from the disk of gas. But strong damping
also favors embryos and cores to find a stable resonant configuration, so that
systems with more than two surviving objects are found. In addition to these
problems, in order to have substantial mutual accretion among embryos, it is
necessary to assume that the surface density of the gas was several times
higher than that of the minimum-mass solar nebula. However this contrasts with
the common idea that Uranus and Neptune formed in a gas-starving disk, which is
suggested by the relatively small amount of hydrogen and helium contained in
the atmospheres of these planets. Only one of our simulations "by chance"
successfully reproduced the structure of the outer Solar System.
|
1107.2235v2
|
2011-08-19
|
The ALFALFA HI Absorption Pilot Survey: A Wide-Area Blind Damped Lyman Alpha System Survey of the Local Universe
|
We present the results of a pilot survey for neutral hydrogen (HI) 21 cm
absorption in the Arecibo Legacy Fast Arecibo L-Band Feed Array (ALFALFA)
Survey. This project is a wide-area "blind" search for HI absorption in the
local universe, spanning -650 km/s < cz < 17,500 km/s and covering 517.0 square
degrees (7% of the full ALFALFA survey). The survey is sensitive to HI
absorption lines stronger than 7.7 mJy (8983 radio sources) and is 90% complete
for lines stronger than 11.0 mJy (7296 sources). The total redshift interval
sensitive to all damped Lyman alpha (DLA) systems (N_HI >= 2x10^20 cm^-2) is
Delta z = 7.0 (129 objects, assuming T_s = 100 K and covering fraction unity);
for super-DLAs (N_HI >= 2x10^21 cm^-2) it is Delta z= 128.2 (2353 objects). We
re-detect the intrinsic HI absorption line in UGC 6081 but detect no
intervening absorption line systems. We compute a 95% confidence upper limit on
the column density frequency distribution function f(N_HI,X) spanning four
orders of magnitude in column density, 10^19 (T_s/100 K)(1/f) cm^-2 < N_HI <
10^23 (T_s/100 K)(1/f) cm^-2, that is consistent with previous redshifted
optical damped Ly alpha surveys and the aggregate HI 21 cm emission in the
local universe. The detection rate is in agreement with extant observations.
This pilot survey suggests that an absorption line search of the complete
ALFALFA survey --- or any higher redshift, larger bandwidth, or more sensitive
survey, such as those planned for Square Kilometer Array pathfinders or a low
frequency lunar array --- will either make numerous detections or will set a
strong statistical lower limit on the typical spin temperature of neutral
hydrogen gas.
|
1108.4011v1
|
2011-09-22
|
Tidal Dissipation in Planet-Hosting Stars: Damping of Spin-Orbit Misalignment and Survival of Hot Jupiters
|
Observations of hot Jupiters around solar-type stars with very short orbital
periods (~day) suggest that tidal dissipation in such stars is not too
efficient so that these planets can survive against rapid orbital decay. This
is consistent with recent theoretical works, which indicate that the tidal Q of
planet-hosting stars can indeed be much larger than the values inferred from
stellar binaries. On the other hand, recent measurements of Rossiter-McLaughlin
effects in transiting hot Jupiter systems not only reveal that many such
systems have misaligned stellar spin with respect to the orbital axis, but also
show that systems with cooler host stars tend to have aligned spin and orbital
axes. Winn et al. suggested that this obliquity - temperature correlation may
be explained by efficient damping of stellar obliquity due to tidal dissipation
in the star. This explanation, however, is in apparent contradiction with the
survival of these short-period hot Jupiters. We show that in the solar-type
parent stars of close-in exoplanetary systems, the effective tidal Q governing
the damping of stellar obliquity can be much smaller than that governing
orbital decay. This is because for misaligned systems, the tidal potential
contains a Fourier component with frequency equal to the stellar spin frequency
(in the rotating frame of the star). This component can excite inertial waves
in the convective envelope of the star, and the dissipation of inertial waves
then leads to a spin-orbit alignment torque, but not orbital decay. By
contrast, for aligned systems, such inertial wave excitation is forbidden since
the tidal forcing frequency is much larger than the stellar spin frequency. We
derive a general effective tidal evolution theory for misaligned binaries,
taking account of different tidal responses and dissipation rates for different
tidal forcing components.
|
1109.4703v2
|
2011-10-20
|
The First Observations of Low-Redshift Damped Lyman-α Systems with the Cosmic Origins Spectrograph: Chemical Abundances and Affiliated Galaxies
|
We present Cosmic Origins Spectrograph (COS) measurements of metal abundances
in eight 0.083<z<0.321 damped Lyman-\alpha (DLA) and sub-damped Ly-\alpha\
absorption systems serendipitously discovered in the COS-Halos survey. We find
that these systems show a large range in metallicities, with -1.10<[Z/H]<0.31,
similar to the spread found at higher redshifts. These low-redshift systems on
average have subsolar metallicities, but do show a rise in metallicity over
cosmic time when compared to higher-redshift systems. We find the average
sub-DLA metallicity is higher than the average DLA metallicity at all
redshifts. Nitrogen is underabundant with respect to \alpha-group elements in
all but perhaps one of the absorbers. In some cases, [N/\alpha] is
significantly below the lowest nitrogen measurements in nearby galaxies.
Systems for which depletion patterns can be studied show little, if any,
depletion, which is characteristic of Milky Way halo-type gas. We also identify
affiliated galaxies for 3 of the sub-DLAs using spectra obtained from
Keck/LRIS. None of these sub-DLAs arise in the stellar disks of luminous
galaxies; instead, these absorbers may exist in galaxy halos at impact
parameters ranging from 38 to 92 kpc. Multiple galaxies are present near two of
the sub-DLAs, and galaxy interactions may play a role in the dispersal of the
gas. Many of these low-redshift absorbers exhibit simple kinematics, but one
sub-DLA has a complicated mix of at least 13 components spread over 150 km/s.
We find three galaxies near this sub-DLA, which also suggests that galaxy
interactions roil the gas. This study reinforces the view that DLAs have a
variety of origins, and low-redshift studies are crucial for understanding
absorber-galaxy connections.
|
1110.4557v2
|
2011-11-01
|
On the misalignment of the directly imaged planet β Pictoris b with the system's warped inner disk
|
The vertical warp in the debris disk Beta Pictoris -- an inclined inner disk
extending into a flat outer disk -- has long been interpreted as the signpost
of a planet on an inclined orbit. Direct images spanning 2004-2010 have
revealed Beta Pictoris b, a planet with a mass and orbital distance consistent
with this picture. However, it was recently reported that the orbit of planet b
is aligned with the flat outer disk, not the inclined inner disk, and thus
lacks the inclination to warp the disk. We explore three scenarios for
reconciling the apparent misalignment of the directly imaged planet Beta
Pictoris b with the warped inner disk of Beta Pictoris: observational
uncertainty, an additional planet, and damping of planet b's inclination. We
find that, at the extremes of the uncertainties, the orbit of Beta Pictoris b
has the inclination necessary to produce the observed warp. We also find that
if planet b were aligned with the flat outer disk, it would prevent another
planet from creating a warp with the observed properties; therefore planet b
itself must be responsible for the warp. Finally, planet b's inclination could
have been damped by dynamical friction and still produce the observed disk
morphology, but the feasibility of damping depends on disk properties and the
presence of other planets. More precise observations of the orbit of planet b
and the position angle of the outer disk will allow us to distinguish between
the first and third scenario.
|
1111.0297v2
|
2011-11-25
|
Application of a damped Locally Optimized Combination of Images method to the spectral characterization of faint companions using an Integral Field Spectrograph
|
High-contrast imaging instruments are now being equipped with integral field
spectrographs (IFS) to facilitate the detection and characterization of faint
substellar companions. Algorithms currently envisioned to handle IFS data, such
as the Locally Optimized Combination of Images (LOCI) algorithm, rely upon
aggressive point-spread-function (PSF) subtraction, which is ideal for
initially identifying companions but results in significantly biased photometry
and spectroscopy due to unwanted mixing with residual starlight. This
spectro-photometric issue is further complicated by the fact that algorithmic
color response is a function of the companion's spectrum, making it difficult
to calibrate the effects of the reduction without using iterations involving a
series of injected synthetic companions. In this paper, we introduce a new PSF
calibration method, which we call "damped LOCI", that seeks to alleviate these
concerns. By modifying the cost function that determines the weighting
coefficients used to construct PSF reference images, and also forcing those
coefficients to be positive, it is possible to extract companion spectra with a
precision that is set by calibration of the instrument response and
transmission of the atmosphere, and not by post-processing. We demonstrate the
utility of this approach using on-sky data obtained with the Project 1640 IFS
at Palomar. Damped-LOCI does not require any iterations on the underlying
spectral type of the companion, nor does it rely upon priors involving the
chromatic and statistical properties of speckles. It is a general technique
that can readily be applied to other current and planned instruments that
employ IFS's.
|
1111.6102v1
|
2012-02-01
|
CMB at 2x2 order: the dissipation of primordial acoustic waves and the observable part of the associated energy release
|
Silk damping of primordial small-scale perturbations in the photon-baryon
fluid due to diffusion of photons inevitably creates spectral distortions in
the CMB. With the proposed CMB experiment PIXIE it might become possible to
measure these distortions and thereby constrain the primordial power spectrum
at comoving wavenumbers 50 Mpc^{-1} < k < 10^4 Mpc^{-1}. Since primordial
fluctuations in the CMB on these scales are completely erased by Silk damping,
these distortions may provide the only way to shed light on otherwise
unobservable aspects of inflationary physics. A consistent treatment of the
primordial dissipation problem requires going to second order in perturbation
theory, while thermalization of these distortions necessitates consideration of
second order in Compton scattering energy transfer. Here we give a full 2x2
treatment for the creation and evolution of spectral distortions due to the
acoustic dissipation process, consistently including the effect of polarization
and photon mixing in the free streaming regime. We show that 1/3 of the total
energy (9/4 larger than previous estimates) stored in small-scale temperature
perturbations imprints observable spectral distortions, while the remaining 2/3
only raises the average CMB temperature, an effect that is unobservable. At
high redshift dissipation is mainly mediated through the quadrupole
anisotropies, while after recombination peculiar motions are most important.
During recombination the damping of the higher multipoles is also significant.
We compute the average distortion for several examples using CosmoTherm,
analyzing their dependence on parameters of the primordial power spectrum. For
one of the best fit WMAP7 cosmologies, with n_S=1.027 and n_run=-0.034, the
cooling of baryonic matter practically compensates the heating from acoustic
dissipation in the mu-era. (abridged)
|
1202.0057v2
|
2012-02-28
|
The Last Stages of Terrestrial Planet Formation: Dynamical Friction and the Late Veneer
|
The final stage of terrestrial planet formation consists of the cleanup of
residual planetesimals after the giant impact phase. Dynamically, a residual
planetesimal population is needed to damp the high eccentricities of the
terrestrial planets after the giant impact stage. Geochemically, highly
siderophile element (HSE) abundance patterns inferred for the terrestrial
planets and the Moon suggest that a total of about 0.01 M_Earth of chondritic
material was delivered as `late veneer' by planetesimals to the terrestrial
planets after the end of giant impacts. Here we combine these two independent
lines of evidence for a leftover population of planetesimals and show that: 1)
A residual planetesimal population containing 0.01 M_Earth is able to damp the
eccentricities of the terrestrial planets after giant impacts to their observed
values. 2) At the same time, this planetesimal population can account for the
observed relative amounts of late veneer added to the Earth, Moon and Mars
provided that the majority of the late veneer was delivered by small
planetesimals with radii <10m. These small planetesimal sizes are required to
ensure efficient damping of the planetesimal's velocity dispersion by mutual
collisions, which in turn ensures that the planets' accretion cross sections
are significantly enhanced by gravitational focusing above their geometric
values. Specifically we find, in the limit that the relative velocity between
the terrestrial planets and the planetesimals is significantly less than the
terrestrial planets' escape velocities, that gravitational focusing yields an
accretion ratio Earth/Mars~17, which agrees well with the accretion ratio
inferred from HSEs of 12-23. For the Earth-Moon system, we find an accretion
ratio of ~200, which is consistent with estimates of 150-700 derived from HSE
abundances that include the lunar crust as well as mantle component. (Abridged)
|
1202.6372v2
|
2012-03-13
|
Trapping in three-planet resonances during gas-driven migration
|
We study the establishment of three-planet resonances -similar to the Laplace
resonance in the Galilean satellites- and their effects on the mutual
inclinations of the orbital planes of the planets, assuming that the latter
undergo migration in a gaseous disc. In particular, we examine the resonance
relations that occur, by varying the physical and initial orbital parameters of
the planets (mass, initial semi-major axis and eccentricity) as well as the
parameters of the migration forces (migration rate and eccentricity damping
rate), which are modeled here through a simplified analytic prescription. We
find that, in general, for planetary masses below 1.5 M_J, multiple-planet
resonances of the form n3:n2:n1=1:2:4 and 1:3:6 are established, as the inner
planets, m1 and m2, get trapped in a 1:2 resonance and the outer planet m3
subsequently is captured in a 1:2 or 1:3 resonance with m2. For mild
eccentricity damping, the resonance pumps the eccentricities of all planets on
a relatively short time-scale, to the point where they enter an
inclination-type resonance (as in Libert & Tsiganis 2011); then mutual
inclinations can grow to ~35{\deg}, thus forming a "3-D system". On the other
hand, we find that trapping of m2 in a 2:3 resonance with m1 occurs very
rarely, for the range of masses used here, so only two cases of capture in a
respective three-planet resonance were found. Our results suggest that trapping
in a three-planet resonance can be common in exoplanetary systems, provided
that the planets are not very massive. Inclination pumping could then occur
relatively fast, provided that eccentricity damping is not very efficient so
that at least one of the inner planets acquires an orbital eccentricity higher
than e=0.3.
|
1203.2960v1
|
2012-10-04
|
Plasmonic Waves on a Chain of Metallic Nanoparticles: Effects of a Liquid Crystalline Host or an Applied Magnetic Field
|
A chain of metallic particles, of sufficiently small diameter and spacing,
allows linearly polarized plasmonic waves to propagate along the chain. In this
paper, we consider how these waves are altered by an anisotropic host (such as
a nematic liquid crystal) or an applied magnetic field. In a liquid crystalline
host, with principal axis (director) oriented either parallel or perpendicular
to the chain, we find that the dispersion relations of both the longitudinal
($L$) and transverse ($T$) modes are significantly altered relative to those of
an isotropic host. Furthermore, when the director is perpendicular to the
chain, the doubly degenerate $T$ branch is split by the anisotropy of the host
material. With an applied magnetic field ${\bf B}$ parallel to the chain, the
propagating transverse modes are circularly polarized, and the left and right
circularly polarized branches have slightly different dispersion relations. As
a result, if a linearly polarized transverse wave is launched along the chain,
it undergoes Faraday rotation. For parameters approximating that of a typical
metal and for a field of 2T, the Faraday rotation is of order 1$^o$ per ten
interparticle spacings, even taking into account single-particle damping. If
${\bf B}$ is perpendicular to the chain, one of the $T$ branches mixes with the
$L$ branch to form two elliptically polarized branches. Our calculations
include single-particle damping and can, in principle, be generalized to
include radiation damping. The present work suggests that the dispersion
relations of plasmonic waves on chain of nanoparticles can be controlled by
immersing the chain in a nematic liquid crystal and varying the director axis,
or by applying a magnetic field.
|
1210.1509v1
|
2012-11-12
|
The explosion energy of early stellar populations: The Fe-peak element ratios in low metallicity damped Lyman-alpha systems
|
The relative abundances of the Fe-peak elements (Ti-Zn) at the lowest
metallicities are intimately linked to the physics of core-collapse supernova
explosions. With a sample of 25 very metal-poor damped Lyman-alpha systems, we
investigate the trends of the Fe-peak element ratios with metallicity. For nine
of the 25 DLAs, a direct measurement (or useful upper limit) of one or more of
the Ti,Cr,Co,Ni,Zn/Fe abundance ratios could be determined from detected
absorption lines. For the remaining systems (without detections), we devised a
new form of spectral stacking to estimate the typical Fe-peak element ratios of
the DLA population in this metallicity regime. We compare these data to
analogous measurements in metal-poor stars of the Galactic halo and to detailed
calculations of explosive nucleosynthesis in metal-free stars. We conclude that
most of the DLAs in our sample were enriched by stars that released an energy
of < 1.2 x 10^51 erg when they exploded as core-collapse supernovae. Finally,
we discuss the exciting prospect of measuring Fe-peak element ratios in damped
Lyman-alpha systems with Fe/H < 1/1000 of solar when 30-m class telescopes
become available. Only then will we be able to pin down the energy that was
released by the supernovae of the first stars.
|
1211.2805v3
|
2012-12-07
|
Circumstellar disks can erase the effects of stellar fly-bys on planetary systems
|
Most stars form in embedded clusters. Stellar flybys may affect the orbital
architecture of the systems by exciting the eccentricity and causing dynamical
instability. Since, incidentally, the timescale over which a cluster loses its
gaseous component and begins to disperse is comparable to the circumstellar
disk lifetime, we expect that closer, and more perturbing, stellar flybys occur
when the planets are still embedded in their birth disk. We investigate the
effects of the disk on the dynamics of planets after the stellar encounter to
test whether it can damp the eccentricity and return the planetary system to a
non-excited state. We use the hydrodynamical code FARGO to study the
disk+planet(s) system during and after the stellar encounter in the context of
evolved disk models whose superficial density is 10 times lower than that of
the Minimum Mass Solar Nebula. The numerical simulations show that the planet
eccentricity, excited during a close stellar flyby, is damped on a short
timescale (~ 10 Kyr) in spite of the disk low initial density and subsequent
tidal truncation. This damping is effective also for a system of 3 giant
planets and the effects of the dynamical instability induced by the passing
star are quickly absorbed. If the circumstellar disk is still present around
the star during a stellar flyby, a planet (or a planetary system) is returned
to a non-excited state on a short timescale. This does not mean that stellar
encounters do not affect the evolution of planets, but they do it in a subtle
way with a short period of agitated dynamical evolution. At the end of it, the
system resumes a quiet evolution and the planetary orbits are circularized by
the interaction with the disk.
|
1212.1561v1
|
2013-01-21
|
Mass-metallicity relation from z=5 to the present: Evidence for a transition in the mode of galaxy growth at z=2.6 due to the end of sustained primordial gas infall
|
We analyze the redshift evolution of the mass-metallicity relation in a
sample of 110 Damped Ly$\alpha$ absorbers spanning the redshift range
$z=0.11-5.06$ and find that the zero-point of the correlation changes
significantly with redshift. The evolution is such that the zero-point is
constant at the early phases of galaxy growth (i.e. no evolution) but then
features a sharp break at $z=2.6\pm 0.2$ with a rapid incline towards lower
redshifts such that damped absorbers of identical masses are more metal rich at
later times than earlier. The slope of this mass metallicity correlation
evolution is $0.35 \pm 0.07$ dex per unit redshift.
We compare this result to similar studies of the redshift evolution of
emission selected galaxy samples and find a remarkable agreement with the slope
of the evolution of galaxies of stellar mass log$(M_{*}/M_\odot) \approx 8.5$.
This allows us to form an observational tie between damped absorbers and
galaxies seen in emission.
We use results from simulations to infer the virial mass of the dark matter
halo of a typical DLA galaxy and find a ratio $(M_{vir}/M_{*}) \approx 30$.
We compare our results to those of several other studies that have reported
strong transition-like events at redshifts around $z=2.5-2.6$ and argue that
all those observations can be understood as the consequence of a transition
from a situation where galaxies were fed more unprocessed infalling gas than
they could easily consume to one where they suddenly become infall starved and
turn to mainly processing, or re-processing, of previously acquired gas.
|
1301.5013v2
|
2013-01-22
|
Effect of partial ionization on wave propagation in solar magnetic flux tubes
|
Observations show that waves are ubiquitous in the solar atmosphere and may
play an important role for plasma heating. The study of waves in the solar
corona is usually based on linear ideal magnetohydrodynamics (MHD) for a fully
ionized plasma. However, the plasma in the photosphere and the chromosphere is
only partially ionized. Here we investigate theoretically the impact of partial
ionization on MHD wave propagation in cylindrical flux tubes in the two-fluid
model. We derive the general dispersion relation that takes into account the
effects of neutral-ion collisions and the neutral gas pressure. We take the
neutral-ion collision frequency as an arbitrary parameter. Particular results
for transverse kink modes and slow magnetoacoustic modes are shown. We find
that the wave frequencies only depend on the properties of the ionized fluid
when the neutral-ion collision frequency is much lower that the wave frequency.
For high collision frequencies realistic of the solar atmosphere ions and
neutrals behave as a single fluid with an effective density corresponding to
the sum of densities of both fluids and an effective sound velocity computed as
the average of the sound velocities of ions and neutrals. The MHD wave
frequencies are modified accordingly. The neutral gas pressure can be neglected
when studying transverse kink waves but it has to be taken into account for a
consistent description of slow magnetoacoustic waves. The MHD waves are damped
due to neutral-ion collisions. The damping is most efficient when the wave
frequency and the collision frequency are of the same order of magnitude. For
high collision frequencies slow magnetoacoustic waves are more efficiently
damped than transverse kink waves. In addition, we find the presence of
cut-offs for certain combinations of parameters that cause the waves to become
non-propagating.
|
1301.5214v1
|
2013-02-07
|
Secular Orbital Evolution of Compact Planet Systems
|
Recent observations have shown that at least some close-in exoplanets
maintain eccentric orbits despite tidal circularization timescales that are
typically shorter than stellar ages. We explore gravitational interactions with
a distant planetary companion as a possible cause of these non-zero
eccentricities. For simplicity, we focus on the evolution of a planar
two-planet system subject to slow eccentricity damping and provide an intuitive
interpretation of the resulting long-term orbital evolution. We show that
dissipation shifts the two normal eigenmode frequencies and eccentricity ratios
of the standard secular theory slightly, and that each mode decays at its own
rate. Tidal damping of the eccentricities drives orbits to transition between
periods of pericenter circulation and libration, and the planetary system
settles into a locked state where the pericenters are nearly aligned or
anti-aligned. Once in the locked state, the eccentricities of the two orbits
decrease very slowly due to tides rather than at the much more rapid
single-planet rate, and thus eccentric orbits, even for close-in planets, can
often survive much longer than the age of the system. Assuming that an observed
close-in planet on an elliptical orbit is apsidally-locked to a more distant,
and perhaps unseen companion, we provide a constraint on the mass, semi-major
axis, and eccentricity of the companion. We find the observed two-planet system
HAT-P-13 might be in just such an apsidally-locked state, with parameters that
obey our constraint well. We also survey close-in single planets, and found
that none provide compelling evidence for unseen companions. Instead, we
suspect that (1) orbits are circular, (2) tidal damping rates are slower than
our assumption, or (3) a recent event has excited these eccentricities. Our
method should prove useful for interpreting the results of current and future
planet searches.
|
1302.1620v2
|
2013-02-13
|
Low mass planets in protoplanetary disks with net vertical magnetic fields: the Planetary Wake and Gap Opening
|
We study wakes and gap opening by low mass planets in gaseous protoplanetary
disks threaded by net vertical magnetic fields which drive
magnetohydrodynamical (MHD) turbulence through the magnetorotational instabilty
(MRI), using three dimensional simulations in the unstratified local shearing
box approximation. The wakes, which are excited by the planets, are damped by
shocks similar to the wake damping in inviscid hydrodynamic (HD) disks. Angular
momentum deposition by shock damping opens gaps in both MHD turbulent disks and
inviscid HD disks even for low mass planets, in contradiction to the "thermal
criterion" for gap opening. To test the "viscous criterion", we compared gap
properties in MRI-turbulent disks to those in viscous HD disks having the same
stress, and found that the same mass planet opens a significantly deeper and
wider gap in net vertical flux MHD disks than in viscous HD disks. This
difference arises due to the efficient magnetic field transport into the gap
region in MRI disks, leading to a larger effective \alpha within the gap. Thus,
across the gap, the Maxwell stress profile is smoother than the gap density
profile, and a deeper gap is needed for the Maxwell stress gradient to balance
the planetary torque density. We also confirmed the large excess torque close
to the planet in MHD disks, and found that long-lived density features (termed
zonal flows) produced by the MRI can affect planet migration. The comparison
with previous results from net toroidal flux/zero flux MHD simulations
indicates that the magnetic field geometry plays an important role in the gap
opening process. Overall, our results suggest that gaps can be commonly
produced by low mass planets in realistic protoplanetary disks, and caution the
use of a constant \alpha-viscosity to model gaps in protoplanetary disks.
|
1302.3239v1
|
2013-05-10
|
Nonlinear Development of the R Mode Instability and the Maximum Rotation Rate of Neutron Stars
|
We describe how the nonlinear development of the R mode instability of
neutron stars influences spin up to millisecond periods via accretion. Our
arguments are based on nearly-resonant interactions of the R mode with pairs of
"daughter modes". The amplitude of the R mode saturates at the lowest value for
which parametric instability leads to significant excitation of a particular
pair of daughters. The lower bound on this limiting amplitude is proportional
to the damping rate of the daughter modes that are excited parametrically.
Based on this picture, we show that if modes damp because of dissipation in a
very thin boundary layer at the crust-core boundary then spin up to frequencies
larger than about 300 Hz does not occur. Within this conventional scenario the
R mode saturates at an amplitude that is too large for angular momentum gain
from accretion to overcome gravitational loss to gravitational radiation. We
conclude that lower dissipation is required for spin up to frequencies much
higher than 300 Hz. We conjecture that if the transition from the fluid core to
the crystalline crust occurs over a distance much longer than 1 cm then a sharp
viscous boundary layer fails to form. In this case, damping is due to shear
viscosity dissipation integrated over the entire star; the rate is slower than
if a viscous boundary layer forms. We use statistical arguments and scaling
relations to estimate the lowest parametric instability threshold from first
principles. The resulting saturation amplitudes are low enough to permit spin
up to higher frequencies. Further, we show that the requirement that the lowest
parametric instability amplitude be small enough to allow continued spin up
imposes an upper bound to the frequencies that may be attained via accretion
that may plausibly be about 750 Hz. Within this framework, the R mode is
unstable for all millisecond pulsars, whether accreting or not.
|
1305.2335v2
|
2013-07-28
|
Constraint damping of the conformal and covariant formulation of the Z4 system in simulations of binary neutron stars
|
Following previous work in vacuum spacetimes, we investigate the
constraint-damping properties in the presence of matter of the recently
developed traceless, conformal and covariant Z4 (CCZ4) formulation of the
Einstein equations. First, we evolve an isolated neutron star with an ideal gas
equation of state and subject to a constraint-violating perturbation. We
compare the evolution of the constraints using the CCZ4 and
Baumgarte-Shibata-Shapiro-Nakamura-Oohara-Kojima (BSSNOK) systems. Second, we
study the collapse of an unstable spherical star to a black hole. Finally, we
evolve binary neutron star systems over several orbits until the merger, the
formation of a black hole, and up to the ringdown. We show that the CCZ4
formulation is stable in the presence of matter and that the constraint
violations are one or more orders of magnitude smaller than for the BSSNOK
formulation. Furthermore, by comparing the CCZ4 and the BSSNOK formulations
also for neutron star binaries with large initial constraint violations, we
investigate their influence on the errors on physical quantities. We also give
a new, simple and robust prescription for the damping parameter that removes
the instabilities found when using the fully covariant version of CCZ4 in the
evolution of black holes. Overall, we find that at essentially the same
computational costs the CCZ4 formulation provides solutions that are stable and
with a considerably smaller violation of the Hamiltonian constraint than the
BSSNOK formulation. We also find that the performance of the CCZ4 formulation
is very similar to another conformal and traceless, but noncovariant
formulation of the Z4 system, i.e. the Z4c formulation.
|
1307.7391v2
|
2013-08-05
|
Peculiar Velocity Decomposition, Redshift Space Distortion and Velocity Reconstruction in Redshift Surveys. II. Dark Matter Velocity Statistics
|
Massive spectroscopic redshift surveys open a promising window to accurately
measure peculiar velocity at cosmological distances through redshift space
distortion (RSD). In paper I of this series of work we proposed to decompose
peculiar velocity into three eigen-modes (v_\delta, v_S and v_B) in order to
facilitate the RSD modeling and peculiar velocity reconstruction. In the
current paper we measure the dark matter RSD related statistics of the velocity
eigen-modes through a set of N-body simulations, including the velocity power
spectra, correlation functions, one-point probability distribution functions,
cumulants and the damping functions describing the Finger of God effect. (1)
The power spectrum measurement shows that these velocity components have
distinctly different spatial distribution and redshift evolution. In
particular, we measure the window function \tilde{W}(k,z), which describes the
impact of nonlinear evolution on the v_\delta-density relation. We confirm that
it can induce a significant systematic error of O(10%) in RSD cosmology. We
demonstrate that \tilde{W} can be accurately described by a simple fitting
formula with one or two free parameters. (2) The correlation function
measurement shows that the correlation length is O(100), O(10) and O(1) Mpc for
v_\delta, v_S and v_B respectively. These correlation lengths determine where
we can treat the velocity fields as spatially uncorrelated. (3) The velocity
PDFs and cumulants quantify non-Gaussianities of the velocity fields. We
confirm speculation in paper I that v_\delta is largely Gaussian, nevertheless
with non-negligible non-Gaussianity, v_B is significantly non-Gaussian. We also
measure the damping functions. Despite the observed non-Gaussianities, the
damping functions and hence the FOG effect are all well approximated as
Gaussian ones at scales of interest.
|
1308.0886v4
|
2013-10-25
|
A SINFONI Integral Field Spectroscopy Survey for Galaxy Counterparts to Damped Lyman-alpha Systems - V. Neutral and Ionised Phase Metallicities
|
The gas-phase and stellar metallicities have proven to be important
parameters to constrain the star formation history of galaxies. However, HII
regions associated with recent star-formation may not have abundances typical
for the galaxy as a whole and it is believed that the bulk of the metals may be
contained in the neutral gas. It is therefore important to directly probe the
metal abundances in the neutral gas, which can be done by using absorption
lines imprinted on a background quasar. Recently, we have presented studies of
the stellar content of a small sample of such quasar absorbers with HI column
densities measured to be in the sub-Damped Lyman-alpha to Damped Lyman-alpha
range. Here, we present observations covering 300 nm to 2.5 microns of emission
line spectra of three of these absorbing-galaxies using the long-slit
spectrograph X-Shooter on the VLT. This allows us to compare the neutral and
ionised phase metallicities in the same objects and relates these measures to
possible signature of low-metallicity gas accretion or outflows of gas enriched
by star formation. Our results suggest that the abundances derived in
absorption along the line-of-sight to background quasars are reliable measures
of the overall galaxy metallicities. In addition to a comparison of abundances
in different phases of the gas, a potential observational consequence of
differences in fueling mechanisms for disc galaxies is the internal
distribution of their chemical abundances. We present some evidence for small
negative metallicity gradients in the three systems. The flat slopes are in
line with the differences observed between the two phases of the gas. These
results suggest that a comparison of the HI and HII metallicities is a robust
indicator of abundance gradients in high-redshift galaxies and do not favour
the presence of infall of fresh gas in these objects.
|
1310.6865v1
|
2014-04-10
|
Thirty-six New, High-Probability, Damped Ly-alpha Absorbers at Redshift 0.42 < z < 0.70
|
Quasar damped Ly-alpha (DLA) absorption line systems with redshifts z<1.65
are used to trace neutral gas over approximately 70 per cent of the most recent
history of the Universe. However, such systems fall in the UV and are rarely
found in blind UV spectroscopic surveys. Therefore, it has been difficult to
compile a moderate-sized sample of UV DLAs in any narrow cosmic time interval.
However, DLAs are easy to identify in low-resolution spectra because they have
large absorption rest equivalent widths. We have performed an efficient
strong-MgII-selected survey for UV DLAs at redshifts z=[0.42,0.70] using HST's
low-resolution ACS-HRC-PR200L prism. This redshift interval covers ~1.8 Gyr in
cosmic time, i.e., t~[7.2,9.0] Gyrs after the Big Bang. A total of 96 strong
MgII absorption-line systems identified in SDSS spectra were successfully
observed with the prism at the predicted UV wavelengths of Ly-alpha absorption.
We found that 35 of the 96 systems had a significant probability of being DLAs.
One additional observed system could be a very high N(HI) DLA (N(HI)~2x10^22
atoms cm^-2 or possibly higher), but since very high N(HI) systems are
extremely rare, it would be unusual for this system to be a DLA given the size
of our sample. Here we present information on our prism sample, including our
best estimates of N(HI) and errors for the 36 systems fitted with damped
Ly-alpha profiles. This list is valuable for future follow-up studies of
low-redshift DLAs in a small redshift interval, although such work would
clearly benefit from improved UV spectroscopy to more accurately determine
their neutral hydrogen column densities.
|
1404.2914v2
|
2014-05-03
|
3D MHD simulation of linearly polarised Alfven wave dynamics in Arnold-Beltrami-Childress magnetic field
|
Previous studies [Malara et al ApJ, 533, 523 (2000)] considered
small-amplitude Alfven wave (AW) packets in Arnold-Beltrami-Childress (ABC)
magnetic field using WKB approximation. In this work linearly polarised Alfven
wave dynamics in ABC magnetic field via direct 3D MHD numerical simulation is
studied for the first time. Gaussian AW pulse with length-scale much shorter
than ABC domain length and harmonic AW with wavelength equal to ABC domain
length are studied for four different resistivities. While it is found that AWs
dissipate quickly in the ABC field, surprisingly, AW perturbation energy
increases in time. In the case of the harmonic AW perturbation energy growth is
transient in time, attaining peaks in both velocity and magnetic perturbation
energies within timescales much smaller than resistive time. In the case of the
Gaussian AW pulse velocity perturbation energy growth is still transient in
time, attaining a peak within few resistive times, while magnetic perturbation
energy continues to grow. It is also shown that the total magnetic energy
decreases in time and this is governed by the resistive evolution of the
background ABC magnetic field rather than AW damping. On contrary, when
background magnetic field is uniform, the total magnetic energy decrease is
prescribed by AW damping, because there is no resistive evolution of the
background. By considering runs with different amplitudes and by analysing
perturbation spectra, possible dynamo action by AW perturbation-induced
peristaltic flow and inverse cascade of magnetic energy have been excluded.
Therefore, the perturbation energy growth is attributed to a new instability.
The growth rate appears to be dependent on the value of the resistivity and
spatial scale of the AW disturbance. Thus, when going beyond WKB approximation,
AW damping, described by full MHD equations, does not guarantee decrease of
perturbation energy.
|
1405.0587v1
|
2014-11-25
|
Investigation of toroidal acceleration and potential acceleration forces in EAST and J-TEXT plasmas
|
In order to produce intrinsic rotation, bulk plasmas must be collectively
accelerated by the net force exerted on them, which results from both driving
and damping forces. So, to study the possible mechanisms of intrinsic rotation
generation, it is only needed to understand characteristics of driving and
damping terms because the toroidal driving and damping forces induce net
acceleration which generates intrinsic rotation. Experiments were performed on
EAST and J-TEXT for ohmic plasmas with net counter- and co-current toroidal
acceleration generated by density ramping up and ramping down. Additionally on
EAST, net co-current toroidal acceleration was also formed by LHCD or ICRF. For
the current experimental results, toroidal acceleration was between - 50 km/s^2
in counter-current direction and 70 km/s^2 in co-current direction. According
to toroidal momentum equation, toroidal electric field (E\-(\g(f))),
electron-ion toroidal friction, and toroidal viscous force etc. may play roles
in the evolution of toroidal rotation. To evaluate contribution of each term,
we first analyze characteristics of E\-(\g(f)). E\-(\g(f)) is one of the
co-current toroidal forces that acts on the plasma as a whole and persists for
the entire discharge period. It was shown to drive the co-current toroidal
acceleration at a magnitude of 10^3 km/s^2, which was much larger than the
experimental toroidal acceleration observed on EAST and J-TEXT. So E\-(\g(f))
is one of co-current forces producing cocurrent intrinsic toroidal acceleration
and rotation. Meanwhile, it indicates that there must be a strong
counter-current toroidal acceleration resulting from counter-current toroidal
forces. Electron-ion toroidal friction is one of the counter-current toroidal
forces because global electrons move in the counter-current direction in order
to produce a toroidal plasma current.
|
1411.6744v1
|
2015-01-07
|
Understanding resonance graphs using Easy Java Simulations (EJS) and why we use EJS
|
This paper reports a computer model- simulation created using Easy Java
Simulation (EJS) for learners to visualize how the steady-state amplitude of a
driven oscillating system varies with the frequency of the periodic driving
force. The simulation shows (N=100) identical spring-mass systems being
subjected to (1) periodic driving force of equal amplitude but different
driving frequencies and (2) different amount of damping. The simulation aims to
create a visually intuitive way of understanding how the series of amplitude
versus driving frequency graphs are obtained by showing how the displacement of
the system changes over time as it transits from the transient to the steady
state.
A suggested how to use the model is added to help educators and students in
their teaching and learning, where we explained the theoretical steady state
equation, time conditions when the model starts allowing data recording of
maximum amplitudes to closely match the theoretical equation and steps to
collect different runs of degree of damping. We also discuss two design
features in our computer model: A) displaying the instantaneous oscillation
together with the achieved steady state amplitudes and B) explicit world view
overlay with scientific representation with different degrees of damping runs.
Three advantages of using EJS include 1) Open Source Codes and Creative
Commons Attribution Licenses for scaling up of interactively engaging
educational practices 2) models made can run on almost any device including
Android and iOS and 3) allows for redefining physics educational practices
through computer modeling. 2015 resource:
http://iwant2study.org/ospsg/index.php/interactive-resources/physics/02-newtonian-mechanics/09-oscillations/88-shm24
|
1501.01535v4
|
2015-04-20
|
Forward Modeling of Reduced Power Spectra From Three-Dimensional k-Space
|
We present results from a numerical forward model to evaluate one-dimensional
reduced power spectral densities (PSD) from arbitrary energy distributions in
$\mathbf{k}$-space. In this model, we can separately calculate the diagonal
elements of the spectral tensor for incompressible axisymmetric turbulence with
vanishing helicity. Given a critically balanced turbulent cascade with
$k_\|\sim k_\perp^\alpha$ and $\alpha<1$, we explore the implications on the
reduced PSD as a function of frequency. The spectra are obtained under the
assumption of Taylor's hypothesis. We further investigate the functional
dependence of the spectral index $\kappa$ on the field-to-flow angle $\theta$
between plasma flow and background magnetic field from MHD to electron kinetic
scales. We show that critically balanced turbulence asymptotically develops
toward $\theta$-independent spectra with a slope corresponding to the
perpendicular cascade. This occurs at a transition frequency
$f_{2D}(L,\alpha,\theta)$, which is analytically estimated and depends on outer
scale $L$, critical balance exponent $\alpha$ and field-to-flow angle $\theta$.
We discuss anisotropic damping terms acting on the $\mathbf{k}$-space
distribution of energy and their effects on the PSD. Further, we show that the
spectral anisotropies $\kappa(\theta)$ as found by Horbury et al. (2008) and
Chen et al. (2010) in the solar wind are in accordance with a damped critically
balanced cascade of kinetic Alfv\'en waves. We also model power spectra
obtained by von Papen et al. (2014) in Saturn's plasma sheet and find that the
change of spectral indices inside $9\,R_\mathrm{s}$ can be explained by damping
on electron scales.
|
1504.04995v2
|
2015-07-13
|
The role of low-energy phonons with mean-free-paths >0.8 um in heat conduction in silicon
|
Despite recent progress in the first-principles calculations and measurements
of phonon mean-free-paths (MFPs), contribution of low-energy phonons to heat
conduction in silicon is still inconclusive, as exemplified by the
discrepancies between different first-principles calculations. Here we
investigate the contribution of low-energy phonons with MFP>0.8 um by
accurately measuring the cross-plane thermal conductivity of crystalline
silicon films by time-domain thermoreflectance (TDTR), over a wide range of
film thickness 1-10 um and temperature 100-300 K. We employ a dual-frequency
TDTR approach to improve the accuracy of our cross-plane thermal conductivity
measurements. We find from our cross-plane thermal conductivity measurements
that phonons with MFP>0.8 um contribute 53 W/m-K (37%) to heat conduction in Si
at 300 K while phonons with MFP>3 um contribute 523 W/m-K (61%) at 100 K, >20%
lower than the first-principles predictions by Lindsay et al. of 68 W/m-K (47%)
and 695 W/m-K (77%), respectively. Using a relaxation times approximation (RTA)
model, we demonstrate that macroscopic damping (e.g., Akhieser's damping)
eliminates the contribution of phonons with mean-free-paths >30 um at 300 K,
which contributes 15 W/m-K (10%) to heat conduction in Si according to Lindsay
et al. Thus we propose that omission of the macroscopic damping for low-energy
phonons in the first-principles calculations could be one of the possible
explanations for the observed discrepancy between our measurements and
calculations by Lindsay et al. Our work provides an important benchmark for
future measurements and calculations of the distribution of phonon
mean-free-paths in crystalline silicon.
|
1507.03422v4
|
2015-08-03
|
Using coronal seismology to estimate the magnetic field strength in a realistic coronal model
|
Coronal seismology is extensively used to estimate properties of the corona,
e.g. the coronal magnetic field strength are derived from oscillations observed
in coronal loops. We present a three-dimensional coronal simulation including a
realistic energy balance in which we observe oscillations of a loop in
synthesised coronal emission. We use these results to test the inversions based
on coronal seismology.
From the simulation of the corona above an active region we synthesise
extreme ultraviolet (EUV) emission from the model corona. From this we derive
maps of line intensity and Doppler shift providing synthetic data in the same
format as obtained from observations. We fit the (Doppler) oscillation of the
loop in the same fashion as done for observations to derive the oscillation
period and damping time.
The loop oscillation seen in our model is similar to imaging and
spectroscopic observations of the Sun. The velocity disturbance of the kink
oscillation shows an oscillation period of 52.5s and a damping time of 125s,
both being consistent with the ranges of periods and damping times found in
observation. Using standard coronal seismology techniques, we find an average
magnetic field strength of $B_{\rm kink}=79$G for our loop in the simulation,
while in the loop the field strength drops from some 300G at the coronal base
to 50G at the apex. Using the data from our simulation we can infer what the
average magnetic field derived from coronal seismology actually means. It is
close to the magnetic field strength in a constant cross-section flux tube that
would give the same wave travel time through the loop.
Our model produced not only a realistic looking loop-dominated corona, but
also provides realistic information on the oscillation properties that can be
used to calibrate and better understand the result from coronal seismology.
|
1508.00593v1
|
2015-08-27
|
Nonlinear Landau damping and modulation of electrostatic waves in a nonextensive electron-positron-pair plasma
|
The nonlinear theory of amplitude modulation of electrostatic wave envelopes
in a collisionless electron-positron (EP) pair plasma is studied by using a set
of Vlasov-Poisson equations in the context of Tsallis' $q$-nonextensive
statistics. In particular, the previous linear theory of Langmuir oscillations
in EP plasmas [Phys. Rev. E {\bf87}, 053112 (2013)] is rectified and modified.
Applying the multiple scale technique (MST), it is shown that the evolution of
electrostatic wave envelopes is governed by a nonlinear Schr{\"o}dinger (NLS)
equation with a nonlocal nonlinear term $\propto
{\cal{P}}\int|\phi(\xi',\tau)|^2d\xi'\phi/(\xi-\xi') $ [where ${\cal P}$
denotes the Cauchy principal value, $\phi$ is the small-amplitude electrostatic
(complex) potential, and $\xi$ and $\tau$ are the stretched coordinates in MST]
which appears due to the wave-particle resonance. It is found that a subregion
$1/3<q\lesssim3/5$ of superextensivity $(q<1)$ exists where the carrier wave
frequency can turn over with the group velocity going to zero and then to
negative values. The effects of the nonlocal nonlinear term and the
nonextensive parameter $q$ are examined on the modulational instability (MI) of
wave envelopes as well as on the solitary wave solution of the NLS equation. It
is found that the modulated wave packet is always unstable (nonlinear Landau
damping) due to the nonlocal nonlinearity in the NLS equation. Furthermore, the
effect of the nonlinear Landau damping is to slow down the amplitude of the
wave envelope, and the corresponding decay rate can be faster the larger is the
number of superthermal particles in pair plasmas.
|
1508.06903v2
|
2015-10-03
|
Systematic investigations of deep sub-barrier fusion reactions using an adiabatic approach
|
To describe fusion hindrance observed in fusion reactions at extremely low
incident energies, I propose a novel extension of the standard CC model by
introducing a damping factor that describes a smooth transition from sudden to
adiabatic processes. I demonstrate the performance of this model by
systematically investigating various deep sub-barrier fusion reactions. I
extend the standard CC model by introducing a damping factor into the coupling
matrix elements in the standard CC model. I adopt the Yukawa-plus-exponential
(YPE) model as a basic heavy ion-ion potential, which is advantageous for a
unified description of the one- and two-body potentials. For the purpose of
these systematic investigations, I approximate the one-body potential with a
third-order polynomial function based on the YPE model. Calculated fusion cross
sections for the medium-heavy mass systems of $^{64}$Ni + $^{64}$Ni, $^{58}$Ni
+ $^{58}$Ni, and $^{58}$Ni + $^{54}$Fe, the medium-light mass systems of
$^{40}$Ca + $^{40}$Ca, $^{48}$Ca + $^{48}$Ca, and $^{24}$Mg + $^{30}$Si, and
the mass-asymmetric systems of $^{48}$Ca + $^{96}$Zr and $^{16}$O + $^{208}$Pb
are consistent with the experimental data. The astrophysical S factor and
logarithmic derivative representations of these are also in good agreement with
the experimental data. Since the results calculated with the damping factor are
in excellent agreement with the experimental data in all systems, I conclude
that the smooth transition from the sudden to adiabatic processes occurs and
that a coordinate-dependent coupling strength is responsible for the fusion
hindrance. In all systems, the potential energies at the touching point $V_{\rm
Touch}$ strongly correlate with the incident threshold energies for which the
fusion hindrance starts to emerge, except for the medium-light mass systems.
|
1510.00806v1
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2015-10-29
|
Numerical simulations of transverse oscillations in radiatively cooling coronal loops
|
We aim to study the influence of radiative cooling on the standing kink
oscillations of a coronal loop. Using the FLASH code, we solved the 3D ideal
magnetohydrodynamic equations. Our model consists of a straight, density
enhanced and gravitationally stratified magnetic flux tube. We perturbed the
system initially, leading to a transverse oscillation of the structure, and
followed its evolution for a number of periods. A realistic radiative cooling
is implemented. Results are compared to available analytical theory. We find
that in the linear regime (i.e. low amplitude perturbation and slow cooling)
the obtained period and damping time are in good agreement with theory. The
cooling leads to an amplification of the oscillation amplitude. However, the
difference between the cooling and non-cooling cases is small (around 6% after
6 oscillations). In high amplitude runs with realistic cooling, instabilities
deform the loop, leading to increased damping. In this case, the difference
between cooling and non-cooling is still negligible at around 12%. A set of
simulations with higher density loops are also performed, to explore what
happens when the cooling takes place in a very short time (tcool = 100 s). We
strengthen the results of previous analytical studies that state that the
amplification due to cooling is ineffective, and its influence on the
oscillation characteristics is small, at least for the cases shown here.
Furthermore, the presence of a relatively strong damping in the high amplitude
runs even in the fast cooling case indicates that it is unlikely that cooling
could alone account for the observed, flare-related undamped oscillations of
coronal loops. These results may be significant in the field of coronal
seismology, allowing its application to coronal loop oscillations with observed
fading-out or cooling behaviour.
|
1510.08760v1
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2016-04-28
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Single-Particle Dynamics in a Nonlinear Accelerator Lattice: Attaining a Large Tune Spread with Octupoles in IOTA
|
Fermilab is constructing the Integrable Optics Test Accelerator (IOTA) as the
centerpiece of the Accelerator R&D Program towards high-intensity circular
machines. One of the factors limiting the beam intensity in present circular
accelerators is collective instabilities, which can be suppressed by a spread
of betatron frequencies (tunes) through the Landau damping mechanism or by an
external damper, if the instability is slow enough. The spread is usually
created by octupole magnets, which introduce the tune dependence on the
amplitude and, in some cases, by a chromatic spread (tune dependence on
particle's momentum). The introduction of octupoles usually has both the
beneficial (improved Landau damping) and harmful properties, such as a resonant
behavior and a reduction of the dynamic aperture. One of the research goals at
the IOTA ring is to achieve a large betatron tune spread, while retaining a
large dynamic aperture, using conventional octupole magnets in a special but
realistic accelerator configuration. In this paper, we present results of
computer simulations of an electron beam in the IOTA by particle tracking and
the Frequency Map Analysis. The results show that the ring's octupole magnets
can be configured to provide a betatron tune shift of 0.08 (for particles at
large amplitudes) with the dynamical aperture of over 20 beam sigma for a
150-MeV electron beam. The influence of the synchrotron motion, lattice errors,
and magnet imperfections is insignificant for the parameters and levels of
tolerances set by the design of the ring. The described octupole insert could
be beneficial for enhancing Landau damping in high intensity machines.
|
1604.08565v4
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2016-10-12
|
Dipole modes with depressed amplitudes in red giants are mixed modes
|
Seismic observations have shown that a number of evolved stars exhibit
low-amplitude dipole modes, which are referred to as depressed modes. Recently,
these low amplitudes have been attributed to the presence of a strong magnetic
field in the stellar core of those stars.
We intend to study the properties of depressed modes in evolved stars, which
is a necessary condition before concluding on the physical nature of the
mechanism responsible for the reduction of the dipole mode amplitudes.
We perform a thorough characterization of the global seismic parameters of
depressed dipole modes and show that these modes have a mixed character. The
observation of stars showing dipole mixed modes that are depressed is
especially useful for deriving model-independent conclusions on the dipole mode
damping.
Observations prove that depressed dipole modes in red giants are not pure
pressure modes but mixed modes. This result invalidates the hypothesis that the
depressed dipole modes result from the suppression of the oscillation in the
radiative core of the stars. Observations also show that, except for the
visibility, the seismic properties of the stars with depressed modes are
equivalent to those of normal stars.
The mixed nature of the depressed modes in red giants and their unperturbed
global seismic parameters carry strong constraints on the physical mechanism
responsible for the damping of the oscillation in the core. This mechanism is
able to damp the oscillation in the core but cannot fully suppress it.
Moreover, it cannot modify the radiative cavity probed by the gravity component
of the mixed modes. The recent mechanism involving high magnetic field proposed
for explaining depressed modes is not compliant with the observations and
cannot be used to infer the strength and the prevalence of high magnetic fields
in red giants.
|
1610.03872v1
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2016-11-07
|
Three-phonon and four-phonon interaction processes in a pair-condensed Fermi gas
|
We study the interactions among phonons and the phonon lifetime in a
pair-condensed Fermi gas in the BEC-BCS crossover in the collisionless regime.
To compute the phonon-phonon coupling amplitudes we use a microscopic model
based on a generalized BCS Ansatz including moving pairs, which allows for a
systematic expansion around the mean field BCS approximation of the ground
state. We show that the quantum hydrodynamic expression of the amplitudes
obtained by Landau and Khalatnikov apply only on the energy shell, that is for
resonant processes that conserve energy. The microscopic model yields the same
excitation spectrum as the Random Phase Approximation, with a linear (phononic)
start and a concavity at low wave number that changes from upwards to downwards
in the BEC-BCS crossover. When the concavity of the dispersion relation is
upwards at low wave number, the leading damping mechanism at low temperature is
the Beliaev-Landau process 2 phonons $\leftrightarrow$ 1 phonon while, when the
concavity is downwards, it is the Landau-Khalatnikov process 2 phonons
$\leftrightarrow$ 2 phonons. In both cases, by rescaling the wave vectors to
absorb the dependence on the interaction strength, we obtain a universal
formula for the damping rate. This universal formula corrects and extends the
original analytic results of Landau and Khalatnikov [ZhETF {\bf 19}, 637
(1949)] for the $2\leftrightarrow2$ processes in the downward concavity case.
In the upward concavity case, for the Beliaev 1$\leftrightarrow$ 2 process for
the unitary gas at zero temperature, we calculate the damping rate of an
excitation with wave number $q$ including the first correction proportional to
$q^7$ to the $q^5$ hydrodynamic prediction, which was never done before in a
systematic way.
|
1611.01954v3
|
2016-11-20
|
Migration of Planets Into and Out of Mean Motion Resonances in Protoplanetary Disks: Analytical Theory of Second-Order Resonances
|
Recent observations of Kepler multi-planet systems have revealed a number of
systems with planets very close to second-order mean motion resonances (MMRs,
with period ratio $1:3$, $3:5$, etc.) We present an analytic study of resonance
capture and its stability for planets migrating in gaseous disks. Resonance
capture requires slow convergent migration of the planets, with sufficiently
large eccentricity damping timescale $T_e$ and small pre-resonance
eccentricities. We quantify these requirements and find that they can be
satisfied for super-Earths under protoplanetary disk conditions. For planets
captured into resonance, an equilibrium state can be reached, in which
eccentricity excitation due to resonant planet-planet interaction balances
eccentricity damping due to planet-disk interaction. We show that this
"captured" equilibrium can be overstable, leading to partial or permanent
escape of the planets from the resonance. In general, the stability of the
captured state depends on the inner to outer planet mass ratio $q=m_1/m_2$ and
the ratio of the eccentricity damping times. The overstability growth time is
of order $T_e$, but can be much larger for systems close to the stability
threshold. For low-mass planets undergoing type I (non-gap opening) migration,
convergent migration requires $q \lesssim 1$, while the stability of the
capture requires $q\gtrsim 1$. These results suggest that planet pairs stably
captured into second-order MMRs have comparable masses. This is in contrast to
first-order MMRs, where a larger parameter space exists for stable resonance
capture. We confirm and extend our analytical results with $N$-body
simulations, and show that for overstable capture, the escape time from the MMR
can be comparable to the time the planets spend migrating between resonances.
|
1611.06463v2
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2016-11-29
|
Kinetic Field Theory: Effects of momentum correlations on the cosmic density-fluctuation power spectrum
|
In earlier work, we have developed a Kinetic Field Theory (KFT) for
cosmological structure formation and showed that the non-linear
density-fluctuation power spectrum known from numerical simulations can be
reproduced quite well even if particle interactions are taken into account to
first order only. Besides approximating gravitational interactions, we had to
truncate the initial correlation hierarchy of particle momenta at the second
order. Here, we substantially simplify KFT. We show that its central object,
the free generating functional, can be factorized, taking the full hierarchy of
momentum correlations into account. The factors appearing in the generating
functional, which we identify as non-linearly evolved density-fluctuation power
spectra, have a universal form and can thus be tabulated for fast access in
perturbation schemes.
In this paper, we focus on a complete evaluation of the free generating
functional of KFT, not including particle interactions yet. This implies that
the non-linearly evolved power spectra contain a damping term which reflects
that structures are being wiped out at late times by free streaming. Once
particle interactions will be taken into account, they will compensate this
damping. If we suppress this damping in a way suggested by the
fluctuation-dissipation relations of KFT, our results show that the complete
hierarchy of initial momentum correlations is responsible for a large part of
the characteristic non-linear deformation and the mode transport in the
density-fluctuation power spectrum. Without any adjustable parameters, KFT
accurately reproduces the scale at which non-linear evolution sets in.
Finally, we further develop perturbation theory based on the factorization of
the generating functional and propose a diagrammatic scheme for the
perturbation terms.
|
1611.09503v2
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2016-12-08
|
Highly inclined and eccentric massive planets. II. Planet-planet interactions during the disc phase
|
We aim to investigate the influence of the eccentricity and inclination
damping due to planet-disc interactions on the final configurations of the
systems, generalizing previous studies on the combined action of the gas disc
and planet-planet scattering during the disc phase. Instead of the simplistic
$K$-prescription, our n-body simulations adopt the damping formulae for
eccentricity and inclination provided by the hydrodynamical simulations of our
companion paper. We follow the evolution of $11000$ numerical experiments of
three giant planets in the late stage of the gas disc, exploring different
initial configurations, planetary mass ratios and disc masses. The dynamical
evolutions of the planetary systems are studied along the simulations, with
emphasis on the resonance captures and inclination-growth mechanisms. Most of
the systems are found with small inclinations ($\le10^{\circ}$) at the
dispersal of the disc. Even though many systems enter an inclination-type
resonance during the migration, the disc usually damps the inclinations on a
short timescale. Although the majority of the multiple systems in our results
are quasi-coplanar, $\sim5\%$ of them end up with high mutual inclinations
($\ge10^{\circ}$). Half of these highly mutually inclined systems result from
two- or three-body MMR captures, the other half being produced by orbital
instability and/or planet-planet scattering. When considering the long-term
evolution over $100$ Myr, destabilization of the resonant systems is common,
and the percentage of highly mutually inclined systems still evolving in
resonance drops to $30\%$. Finally, the parameters of the final system
configurations are in very good agreement with the semi-major axis and
eccentricity distributions in the observations, showing that planet-planet
interactions during the disc phase could have played an important role in
sculpting planetary systems.
|
1612.02693v1
|
2017-02-22
|
A Model of Energetic Ion Effects on Pressure Driven Tearing Modes in Tokamaks
|
The effects that energetic trapped ions have on linear resistive
magnetohydrodynamic (MHD) instabilities are studied in a reduced model that
captures the essential physics driving or damping the modes through variations
in the magnetic shear. The drift-kinetic orbital interaction of a slowing down
distribution of trapped energetic ions with a resistive MHD instability is
integrated to a scalar contribution to the perturbed pressure, and entered into
an asymptotic matching formalism for the resistive MHD dispersion relation.
Toroidal magnetic field line curvature is included to model trapping in the
particle distribution, in an otherwise cylindrical model. The focus is on a
configuration that is driven unstable to the m/n = 2/1 mode by increasing
pressure, where m is the poloidal mode number and n the toroidal. The particles
and pressure can affect the mode both in the core region where there can be low
and reversed shear and outside the resonant surface in significant positive
shear. The results show that the energetic ions damp and stabilize the mode
when orbiting in significant positive shear, increasing the marginal stability
boundary. However, the inner core region contribution with low and reversed
shear can drive the mode unstable. This effect of shear on the energetic ion
pressure contribution is found to be consistent with the literature. These
results explain the observation that the 2/1 mode was found to be damped and
stabilized by energetic ions in {\delta}f - MHD simulations of tokamak
experiments with positive shear throughout, while the 2/1 mode was found to be
driven unstable in simulations of experiments with weakly reversed shear in the
core. This is also found to be consistent with related experimental
observations of the stability of the 2/1 mode changing significantly with core
shear.
|
1702.06837v2
|
2017-06-16
|
Damping of Rabi oscillations in intensity-dependent photon echoes from exciton complexes in a CdTe/(Cd,Mg)Te single quantum well
|
We study Rabi oscillations detected in the coherent optical response from
various exciton complexes in a 20~nm-thick CdTe/(Cd,Mg)Te quantum well using
time-resolved photon echoes. In order to evaluate the role of exciton
localization and inhomogeneous broadening we use selective excitation with
spectrally narrow ps-pulses. We demonstrate that the transient profile of the
photon echo from the localized trion (X$^-$) and the donor-bound exciton
(D$^0$X) transitions strongly depends on the strength of the first pulse. It
acquires a non-Gaussian shape and experiences significant advancement for pulse
areas larger than $\pi$ due to non-negligible inhomogeneity-induced dephasing
of the oscillators during the optical excitation. Next, we observe that an
increase of the area of either the first (excitation) or the second (rephasing)
pulse leads to a significant damping of the photon echo signal, which is
strongest for the neutral excitons and less pronounced for the donor-bound
exciton complex (D$^0$X). The measurements are analyzed using a theoretical
model based on the optical Bloch equations which accounts for the inhomogeneity
of optical transitions in order to reproduce the complex shape of the photon
echo transients. In addition, the spreading of Rabi frequencies within the
ensemble due to the spatial variation of the intensity of the focused Gaussian
beams and excitation-induced dephasing are required to explain the fading and
damping of Rabi oscillations. By analyzing the results of the simulation for
the X$^-$ and the D$^0$X complexes we are able to establish a correlation
between the degree of localization and the transition dipole moments determined
as $\mu($X$^-$)=73~D and $\mu($D$^0$X)=58~D.
|
1706.05327v1
|
2017-06-28
|
Generating Log-normal Mock Catalog of Galaxies in Redshift Space
|
We present a public code to generate a mock galaxy catalog in redshift space
assuming a log-normal probability density function (PDF) of galaxy and matter
density fields. We draw galaxies by Poisson-sampling the log-normal field, and
calculate the velocity field from the linearised continuity equation of matter
fields, assuming zero vorticity. This procedure yields a PDF of the pairwise
velocity fields that is qualitatively similar to that of N-body simulations. We
check fidelity of the catalog, showing that the measured two-point correlation
function and power spectrum in real space agree with the input precisely. We
find that a linear bias relation in the power spectrum does not guarantee a
linear bias relation in the density contrasts, leading to a cross-correlation
coefficient of matter and galaxies deviating from unity on small scales. We
also find that linearising the Jacobian of the real-to-redshift space mapping
provides a poor model for the two-point statistics in redshift space. That is,
non-linear redshift-space distortion is dominated by non-linearity in the
Jacobian. The power spectrum in redshift space shows a damping on small scales
that is qualitatively similar to that of the well-known Fingers-of-God (FoG)
effect due to random velocities, except that the log-normal mock does not
include random velocities. This damping is a consequence of non-linearity in
the Jacobian, and thus attributing the damping of the power spectrum solely to
FoG, as commonly done in the literature, is misleading.
|
1706.09195v2
|
2017-07-31
|
Investigating quantum wireless multihop teleportation under decoherence
|
This research work scrutinizes quantum routing protocol with multihop
teleportation for wireless mesh backbone networks, in amplitude and phase
damping channels. After analyzing the quantum multihop protocol, we select a
four-qubit cluster state as the quantum channel for the protocol. The quantum
channel linking the intermediate nodes has been established via entanglement
swapping based on four-qubit cluster state. Also, we established the classical
and the quantum route in a distributed manner. We show that from the source
node to the destination node, quantum information can be teleported hop-by-hop
through an amplitude damping channel. We show that the quantum teleportation
could be successful if the sender node performs Bell state measurements (BSM),
and the receiver introduces auxiliary particles, applies positive operative
value measure and then utilizes corresponding unitary transformation to recover
the transmitted state. We scrutinize the success probability of transferring
the quantum state through a noisy channel. We found that optimum probability
would be attained if decoherence rate of amplitude damping channel ($\xi_a$) is
zero or the number of hops ($N$) is above $75$. Our numerical results evince
susceptibility of success probability to $\xi_a$ and $N$. It has been shown
that as the decoherence increases, the fidelity exponentially decays until it
vanishes. This decay is as a consequence of information loss from the system to
the surrounding. However, the fidelity can be enhanced by considering fewer
hops.
|
1708.00087v6
|
2017-11-30
|
Origins of sharp cosmic-ray electron structures and the DAMPE excess
|
Nearby sources may contribute to cosmic-ray electron (CRE) structures at high
energies. Recently, the first DAMPE results on the CRE flux hinted at a narrow
excess at energy ~1.4 TeV. We show that in general a spectral structure with a
narrow width appears in two scenarios: I) "Spectrum broadening" for the
continuous sources with a delta-function-like injection spectrum. In this
scenario, a finite width can develop after propagation through the Galaxy,
which can reveal the distance of the source. Well-motivated sources include
mini-spikes and subhalos formed by dark matter (DM) particles $\chi_{s}$ which
annihilate directly into e+e- pairs. II) "Phase-space shrinking" for burst-like
sources with a power-law-like injection spectrum. The spectrum after
propagation can shrink at a cooling-related cutoff energy and form a sharp
spectral peak. The peak can be more prominent due to the energy-dependent
diffusion. In this scenario, the width of the excess constrains both the power
index and the distance of the source. Possible such sources are pulsar wind
nebulae (PWNe) and supernova remnants (SNRs). We analysis the DAMPE excess and
find that the continuous DM sources should be fairly close within ~0.3 kpc, and
the annihilation cross sections are close to the thermal value. For the
burst-like source, the narrow width of the excess suggests that the injection
spectrum must be hard with power index significantly less than two, the
distance is within ~(3-4) kpc, and the age of the source is ~0.16 Myr. In both
scenarios, large anisotropies in the CRE flux are predicted. We identify
possible candidates of mini-spike (PWN) sources in the current Fermi-LAT 3FGL
(ATNF) catalog. The diffuse gamma-rays from these sources can be well below the
Galactic diffuse gamma-ray backgrounds and less constrained by the Ferm-LAT
data, if they are located at the low Galactic latitude regions.
|
1712.00005v2
|
2017-11-30
|
Bayesian analysis of the break in DAMPE lepton spectra
|
Recently, DAMPE has released its first results on the high-energy cosmic-ray
electrons and positrons (CREs) from about $25$ GeV to $4.6$ TeV, which directly
detect a break at $\sim 1$ TeV. This result gives us an excellent opportunity
to study the source of the CREs excess. In this work, we used the data fo
proton and helium flux (from AMS-02 and CREAM), $\bar{\mathrm{p}}/\mathrm{p}$
ratio (from AMS-02), positron flux (from AMS-02) and CREs flux (from DAMPE
without the peak signal point at $\sim 1.4$ TeV) to do global fitting
simultaneously, which can account the influence from the propagation model, the
nuclei and electron primary source injection and the secondary lepton
production precisely. For extra source to interpret the excess in lepton
spectrum, we consider two separate scenarios (pulsar and dark matter
annihilation via leptonic channels) to construct the bump ($\gtrsim 100$ GeV)
and the break at $\sim 1$ TeV. The result shows: (i) in pulsar scenario, the
spectral index of the injection should be $\nu_{\mathrm{psr}} \sim 0.65$ and
the cut-off should be $R_{c} \sim 650$ GV; (ii) in dark matter scenario, the
dark matter particle's mass is $m_{\chi} \sim 1208$ GeV and the cross section
is $\langle \sigma v \rangle \sim 1.48 \times 10^{-23} \mathrm{cm}^{3}
\mathrm{s}^{-1}$. Moreover, in the dark matter scenario, the $\tau \bar{\tau}$
annihilation channel is highly suppressed, and a DM model is built to satisfy
the fitting results.
|
1712.00372v4
|
2017-12-07
|
Internal alignment and position resolution of the silicon tracker of DAMPE determined with orbit data
|
The DArk Matter Particle Explorer (DAMPE) is a space-borne particle detector
designed to probe electrons and gamma-rays in the few GeV to 10 TeV energy
range, as well as cosmic-ray proton and nuclei components between 10 GeV and
100 TeV. The silicon-tungsten tracker-converter is a crucial component of
DAMPE. It allows the direction of incoming photons converting into
electron-positron pairs to be estimated, and the trajectory and charge (Z) of
cosmic-ray particles to be identified. It consists of 768 silicon micro-strip
sensors assembled in 6 double layers with a total active area of 6.6 m$^2$.
Silicon planes are interleaved with three layers of tungsten plates, resulting
in about one radiation length of material in the tracker. Internal alignment
parameters of the tracker have been determined on orbit, with non-showering
protons and helium nuclei. We describe the alignment procedure and present the
position resolution and alignment stability measurements.
|
1712.02739v2
|
2018-01-24
|
Modelling redshift-space distortion in the post-reionization ${\rm HI}$ 21-cm power spectrum
|
The post-reionization ${\rm HI}$ 21-cm signal is an excellent candidate for
precision cosmology, this however requires accurate modelling of the expected
signal. Sarkar et al. (2016) have simulated the real space ${\rm HI}$ 21-cm
signal, and have modelled the ${\rm HI}$ power spectrum as $P_{{\rm HI}}(k)=b^2
P(k)$ where $P(k)$ is the dark matter power spectrum and $b(k)$ is a (possibly
complex) scale dependent bias for which fitting formulas have been provided.
This paper extends these simulations to incorporate redshift space distortion
and predict the expected redshift space ${\rm HI}$ 21-cm power spectrum
$P^s_{{\rm HI}}(k_{\perp},k_{\parallel})$ using two different prescriptions for
the ${\rm HI}$ distributions and peculiar velocities. We model $P^s_{{\rm
HI}}(k_{\perp},k_{\parallel})$ assuming that it is the product of $P_{{\rm
HI}}(k)=b^2 P(k)$ with a Kaiser enhancement term and a Finger of God (FoG)
damping which has $\sigma_p$ the pair velocity dispersion as a free parameter.
Considering several possibilities for the bias and the damping profile, we find
that the models with a scale dependent bias and a Lorentzian damping profile
best fit the simulated $P^s_{{\rm HI}}(k_{\perp},k_{\parallel})$ over the
entire range $1 \le z \le 6$. The best fit value of $\sigma_p$ falls
approximately as $(1+z)^{-m}$ with $m=2$ and $1.2$ respectively for the two
different prescriptions. The model predictions are consistent with the
simulations for $k < 0.3 \, {\rm Mpc}^{-1}$ over the entire $z$ range for the
monopole $P^s_0(k)$, and at $z \le 3$ for the quadrupole $P^s_2(k)$. At $z \ge
4$ the models underpredict $P^s_2(k)$ at large $k$, and the fit is restricted
to $k < 0.15 \, {\rm Mpc}^{-1}$.
|
1801.07868v1
|
2018-02-16
|
Quantitative Constraints on the Reionization History from the IGM Damping Wing Signature in Two Quasars at z > 7
|
During reionization, neutral hydrogen in the intergalactic medium (IGM)
imprints a damping wing absorption feature on the spectrum of high-redshift
quasars. A detection of this signature provides compelling evidence for a
significantly neutral Universe, and enables measurements of the hydrogen
neutral fraction $x_{\rm HI}(z)$ at that epoch. Obtaining reliable quantitative
constraints from this technique, however, is challenging due to stochasticity
induced by the patchy inside-out topology of reionization, degeneracies with
quasar lifetime, and the unknown unabsorbed quasar spectrum close to rest-frame
Ly$\alpha$. We combine a large-volume semi-numerical simulation of reionization
topology with 1D radiative transfer through high-resolution hydrodynamical
simulations of the high-redshift Universe to construct models of quasar
transmission spectra during reionization. Our state-of-the-art approach
captures the distribution of damping wing strengths in biased quasar halos that
should have reionized earlier, as well as the erosion of neutral gas in the
quasar environment caused by its own ionizing radiation. Combining this
detailed model with our new technique for predicting the quasar continuum and
its associated uncertainty, we introduce a Bayesian statistical method to
jointly constrain the neutral fraction of the Universe and the quasar lifetime
from individual quasar spectra. We apply this methodology to the spectra of the
two highest redshift quasars known, ULAS J1120+0641 and ULAS J1342+0928, and
measured volume-averaged neutral fractions $\langle x_{\rm HI}
\rangle(z=7.09)=0.48^{+0.26}_{-0.26}$ and $\langle x_{\rm HI}
\rangle(z=7.54)=0.60^{+0.20}_{-0.23}$ (posterior medians and 68% credible
intervals) when marginalized over quasar lifetimes of $10^3 \leq t_{\rm q} \leq
10^8$ years.
|
1802.06066v1
|
2018-02-16
|
Landau Damping in a strong magnetic field: Dissociation of Quarkonia
|
We have investigated the effects of strong magnetic field on the properties
of quarkonia immersed in a thermal medium of quarks and gluons and studied its
quasi-free dissociation due to the Landau-damping. Thermalizing the Schwinger
propagator in the lowest Landau levels for quarks and the Feynman propagator
for gluons in real-time formalism, we have calculated the resummed retarded and
symmetric propagators, which in turn give the real and imaginary components of
dielectric permittivity, respectively. The magnetic field affects the
large-distance interaction more than the short-distance interaction, as a
result, the real part of potential becomes more attractive and the magnitude of
imaginary part too becomes larger, compared to the thermal medium in absence of
strong magnetic field. As a consequence the average size of $J/\psi$'s and
$\psi^\prime$'s are increased but $\chi_c$'s get shrunk. Similarly the magnetic
field affects the binding of $J/\psi$'s and $\chi_c$'s discriminately, i.e. it
decreases the binding of $J/\psi$ and increases for $\chi_c$. However, the
further increase in magnetic field results in the decrease of binding energies.
On contrary the magnetic field increases the width of the resonances, unless
the temperature is sufficiently high. We have finally studied how the presence
of magnetic field affects the dissolution of quarkonia in a thermal medium due
to the Landau damping, where the dissociation temperatures are found to
increase compared to the thermal medium in absence of magnetic field. However,
further increase of magnetic field decreases the dissociation temperatures. For
example, $J/\psi$'s and $\chi_c$'s are dissociated at higher temperatures at 2
$T_c$ and 1.1 $T_c$ at a magnetic field $eB \approx 6~{\rm{and}}~4~m_\pi^2$,
respectively, compared to the values 1.60 $T_c$ and 0.8 $T_c$ in the absence of
magnetic field, respectively.
|
1802.06874v1
|
2018-05-09
|
Amplitude and lifetime of radial modes in red giant star spectra observed by Kepler
|
Context: the space-borne missions CoRoT and Kepler have provided photometric
observations of unprecedented quality. The study of solar-like oscillations
observed in red giant stars by these satellites allows a better understanding
of the different physical processes occurring in their interiors. In
particular, the study of the mode excitation and damping is a promising way to
improve our understanding of stellar physics that has, so far, been performed
only on a limited number of targets. Aims: the recent asteroseismic
characterization of the evolutionary status for a large number of red giants
allows us to study the physical processes acting in the interior of red giants
and how they are modify during stellar evolution. In this work, we aim to
obtain information on the excitation and damping of pressure modes through the
measurement of the stars' pressure mode widths and amplitudes and to analyze
how they are modified with stellar evolution. The objective is to bring
observational constraints on the modeling of the physical processes behind mode
excitation and damping. Methods: we fit the frequency spectra of red giants
with well defined evolutionary status using Lorentzians functions to derive the
pressure mode widths and amplitudes. To strengthen our conclusions, we used two
different fitting techniques. Results: pressure mode widths and amplitudes were
determined for more than 5000 red giants. With a stellar sample two orders of
magnitude larger than previous results, we confirmed that the mode width
depends on stellar evolution and varies with stellar effective temperature. In
addition, we discovered that the mode width depends on stellar mass. We also
confirmed observationally the influence of the stellar metallicity on the mode
amplitudes, as predicted by models.
|
1805.03690v1
|
2018-05-31
|
Impact of bias and redshift-space modelling for the halo power spectrum: Testing the effective field theory of large-scale structure
|
We study the impact of different bias and redshift-space models on the halo
power spectrum, quantifying their effect by comparing the fit to a subset of
realizations taken from the WizCOLA suite. These provide simulated power
spectrum measurements between $k_{\rm min}$ = 0.03 h/Mpc and $k_{\rm max}$ =
0.29 h/Mpc, constructed using the comoving Lagrangian acceleration method. For
the bias prescription we include (i) simple linear bias; (ii) the McDonald &
Roy model and (iii) its coevolution variant introduced by Saito et al.; and
(iv) a very general model including all terms up to one-loop and corrections
from advection. For the redshift-space modelling we include the Kaiser formula
with exponential damping and the power spectrum provided by (i) tree-level
perturbation theory and (ii) the Halofit prescription; (iii) one-loop
perturbation theory, also with exponential damping; and (iv) an effective field
theory description, also at one-loop, with damping represented by the EFT
subtractions. We quantify the improvement from each layer of modelling by
measuring the typical improvement in chi-square when fitting to a member of the
simulation suite. We attempt to detect overfitting by testing for compatibility
between the best-fit power spectrum per realization and the best-fit over the
entire WizCOLA suite. For both bias and the redshift-space map we find that
increasingly permissive models yield improvements in chi-square but with
diminishing returns. The most permissive models show modest evidence for
overfitting. Accounting for model complexity using the Bayesian Information
Criterion, we argue that standard perturbation theory up to one-loop, or a
related model such as that of Taruya, Nishimichi & Saito, coupled to the
coevolution bias model, is likely to provide a good compromise for near-future
galaxy surveys operating with comparable $k_{\rm max}$.
|
1805.12394v3
|
2018-06-10
|
Non-damping oscillations at flaring loops
|
Context. QPPs are usually detected as spatial displacements of coronal loops
in imaging observations or as periodic shifts of line properties in
spectroscopic observations. They are often applied for remote diagnostics of
magnetic fields and plasma properties on the Sun. Aims. We combine imaging and
spectroscopic measurements of available space missions, and investigate the
properties of non-damping oscillations at flaring loops. Methods. We used the
IRIS to measure the spectrum over a narrow slit. The double-component Gaussian
fitting method was used to extract the line profile of Fe XXI 1354.08 A at "O
I" window. The quasi-periodicity of loop oscillations were identified in the
Fourier and wavelet spectra. Results. A periodicity at about 40 s is detected
in the line properties of Fe XXI, HXR emissions in GOES 1-8 A derivative, and
Fermi 26-50 keV. The Doppler velocity and line width oscillate in phase, while
a phase shift of about Pi/2 is detected between the Doppler velocity and peak
intensity. The amplitudes of Doppler velocity and line width oscillation are
about 2.2 km/s and 1.9 km/s, respectively, while peak intensity oscillate with
amplitude at about 3.6% of the background emission. Meanwhile, a quasi-period
of about 155 s is identified in the Doppler velocity and peak intensity of Fe
XXI, and AIA 131 A intensity. Conclusions. The oscillations at about 40 s are
not damped significantly during the observation, it might be linked to the
global kink modes of flaring loops. The periodicity at about 155 s is most
likely a signature of recurring downflows after chromospheric evaporation along
flaring loops. The magnetic field strengths of the flaring loops are estimated
to be about 120-170 G using the MHD seismology diagnostics, which are
consistent with the magnetic field modeling results using the flux rope
insertion method.
|
1806.03573v1
|
2018-07-04
|
Constraints on reionisation from the z=7.5 QSO ULASJ1342+0928
|
The recent detection of ULASJ1342+0928, a bright QSO at $z=7.54$, provides a
powerful probe of the ionisation state of the intervening intergalactic medium,
potentially allowing us to set strong constraints on the epoch of reionisation
(EoR). Here we quantify the presence of Ly$\alpha$ damping wing absorption from
the EoR in the spectrum of ULASJ1342+0928. Our Bayesian framework
simultaneously accounts for uncertainties on: (i) the intrinsic QSO emission
(obtained from reconstructing the Ly$\alpha$ profile from a covariance matrix
of emission lines) and (ii) the distribution of HII regions during reionisation
(obtained from three different 1.6$^3$ Gpc$^3$ simulations spanning the range
of plausible EoR morphologies). Our analysis is complementary to that in the
discovery paper (Ba\~nados et al.) and the accompanying method paper (Davies et
al.) as it focuses solely on the damping wing imprint redward of Ly$\alpha$
($1218 < \lambda < 1230$\AA), and uses a different methodology for (i) and
(ii). We recover weak evidence for damping wing absorption. Our intermediate
EoR model yields a volume-weighted neutral hydrogen fraction at $z=7.5$ of
$\bar{x}_{\rm HI} = 0.21\substack{+0.17 \\ -0.19}$ (68 per cent). The
constraints depend weakly on the EoR morphology. Our limits are lower than
those presented previously, though they are consistent at ~1-1.5$\sigma$. We
attribute this difference to: (i) a lower amplitude intrinsic Ly$\alpha$
profile obtained from our reconstruction pipeline, driven by correlations with
other high-ionisation lines in the spectrum which are relatively weak; and (ii)
only considering transmission redward of Ly$\alpha$ when computing the
likelihood, which reduces the available constraining power but makes the
results less model-dependent. Our results are consistent with previous
estimates of the EoR history, and support the picture of a moderately extended
EoR.
|
1807.01593v1
|
2018-07-04
|
Stellar masses, metallicity gradients and suppressed star formation revealed in a new sample of absorption selected galaxies
|
Context. Absorbing galaxies are selected via the detection of characteristic
absorption lines which their gas-rich media imprint in the spectra of distant
light-beacons. The proximity of the typically faint foreground absorbing
galaxies to bright background sources makes it challenging to robustly identify
these in emission, and hence to characterise their relation to the general
galaxy population. Aims. We search for emission to confirm and characterise ten
galaxies hosting damped, metal-rich quasar absorbers at redshift z < 1.
Methods. We identify the absorbing galaxies by matching spectroscopic
absorption -and emission redshifts and from projected separations. Combining
emission-line diagnostics with existing absorption spectroscopy and photometry
of quasar-fields hosting metal-rich, damped absorbers, we compare our new
detections with reference samples and place them on scaling relations. Results.
We spectroscopically confirm seven galaxies harbouring damped absorbers (a 70%
success-rate). Our results conform to the emerging picture that neutral gas on
scales of tens of kpc in galaxies is what causes the characteristic Hi
absorption. Our key results are: (I) Absorbing galaxies with $\log _{10}
[M_\star ~(M_\odot)] \gtrsim 10$ have star formation rates that are lower than
predicted for the main sequence of star formation. (II) The distribution of
impact parameter with Hi column density and with absorption-metallicity for
absorbing galaxies at $z\sim 2-3$ extends to $z\sim 0.7$ and to lower Hi column
densities. (III) A robust mean metallicity gradient of $\langle \Gamma \rangle
= 0.022 \pm 0.001~[dex~kpc^{-1}]$. (IV) By correcting absorption metallicities
for $\langle \Gamma \rangle$ and imposing a truncation-radius at
$12~\mathrm{kpc}$, absorbing galaxies fall on top of predicted mass-metallicity
relations, with a statistically significant decrease in scatter.
|
1807.01755v1
|
2018-07-05
|
Stationarity and energy transfer in out-of-equilibrium systems
|
We define a characteristic energy density based on the measurement of the two
first moments of the extrinsic injected power smoothed over time. Using the
stationarity, we show that this definition characterizes an energy per degrees
freedom of the intrinsic dissipation. Our framework can be applied to systems
in contact with thermostats put out of equilibrium by an external driving but
it holds also for intrinsically dissipative macroscopic systems that go at rest
when the forcing is stopped. Moreover, we are not concerned about the
fluctuations around zero of the smoothed injected power that can be extremely
rare and difficult to catch experimentally. Then we show that the
characteristic energy density we defined, reduces to the kinetic energy of a
Brownian-like particle described by a set of Langevin equations with a viscous
damping term. The particle can be either in contact with a thermostat or
intrinsically dissipative and driven by a random force. In the first case, we
recover the result obtained in the framework of the fluctuation relation but
extended to a correlated thermal noise. Our characteristic energy density is
measured in an experimental system of nonlinear waves generated by a large
shaker in a thin elastic plate. A smaller shaker attached to the moving plate
is used as a probe to measure the energy exchanged with the plate excited by
the large shaker. For both, the proportionality of our characteristic energy
density with the kinetic energy is demonstrated. It is a consequence of the
viscous damping driving the dissipation in this system. Another system with
nonlinear frictional dissipation is investigated numerically model. In this
case, our definition of energy density deduced from fluctuations of injected
power still characterizes the dissipation but is no more proportional to the
kinetic energy because the dissipative process is not a viscous damping.
|
1807.01856v1
|
2018-09-05
|
Nonlinear Mixed Modes in Red Giants
|
Turbulent motions in the convective envelope of red giants excite a rich
spectrum of solar-like oscillation modes. Observations by CoRoT and Kepler have
shown that the mode amplitudes increase dramatically as the stars ascend the
red giant branch, i.e., as the frequency of maximum power, $\nu_\mathrm{max}$,
decreases. Most studies nonetheless assume that the modes are well described by
the linearized fluid equations. We investigate to what extent the linear
approximation is justified as a function of stellar mass $M$ and
$\nu_\mathrm{max}$, focusing on dipole mixed modes with frequency near
$\nu_\mathrm{max}$. A useful measure of a mode's nonlinearity is the product of
its radial wavenumber and its radial displacement, $k_r \xi_r$ (i.e., its
shear). We show that $k_r \xi_r \propto \nu_\mathrm{max}^{-9/2}$, implying that
the nonlinearity of mixed modes increases significantly as a star evolves. The
modes are weakly nonlinear ($k_r \xi_r > 10^{-3}$) for $\nu_\mathrm{max}
\lesssim 150 \, \mu\mathrm{Hz}$ and strongly nonlinear ($k_r \xi_r > 1$) for
$\nu_\mathrm{max} \lesssim 30 \, \mu\mathrm{Hz}$, with only a mild dependence
on $M$ over the range we consider ($1.0 - 2.0 M_\odot$). A weakly nonlinear
mixed mode can excite secondary waves in the stellar core through the
parametric instability, resulting in enhanced, but partial, damping of the
mode. By contrast, a strongly nonlinear mode breaks as it propagates through
the core and is fully damped there. Evaluating the impact of nonlinear effects
on observables such as mode amplitudes and linewidths requires large mode
network simulations. We plan to carry out such calculations in the future and
investigate whether nonlinear damping can explain why some red giants exhibit
dipole modes with unusually small amplitudes, known as depressed modes.
|
1809.01727v2
|
2018-12-14
|
Probing neutron star structure via f-mode oscillations and damping in dynamical spacetime models
|
Gravitational wave and electromagnetic observations can provide new insights
into the nature of matter at supra-nuclear densities inside neutron stars.
Improvements in electromagnetic and gravitational wave sensing instruments
continue to enhance the accuracy with which they can measure the masses, radii,
and tidal deformability of neutron stars. These better measurements place
tighter constraints on the equation of state of cold matter above nuclear
density. In this article, we discuss a complementary approach to get insights
into the structure of neutron stars by providing a model prediction for
non-linear fundamental eigenmodes (f-modes) and their decay over time, which
are thought to be induced by time-dependent tides in neutron star binaries.
Building on pioneering studies that relate the properties of f-modes to the
structure of neutron stars, we systematically study this link in the
non-perturbative regime using models that utilize numerical relativity. Using a
suite of fully relativistic numerical relativity simulations of oscillating TOV
stars, we establish blueprints for the numerical accuracy needed to accurately
compute the frequency and damping times of f-mode oscillations, which we expect
to be a good guide for the requirements in the binary case. We show that the
resulting f-mode frequencies match established results from linear perturbation
theory, but the damping times within numerical errors depart from linear
predictions. This work lays the foundation for upcoming studies aimed at a
comparison of theoretical models of f-mode signatures in gravitational waves,
and their uncertainties with actual gravitational wave data, searching for
neutron star binaries on highly eccentric orbits, and probing neutron star
structure at high densities.
|
1812.06126v1
|
2019-05-06
|
Proximate Molecular Quasar Absorbers: Excess of damped H2 systems at zabs~zQSO in SDSS DR14
|
We present results from a search for strong H2 absorption systems proximate
to quasars (zabs~zem) in the Sloan Digital Sky Survey (SDSS) Data Release 14.
The search is based on the Lyman-Werner band signature of damped H2 absorption
lines without any prior on the associated metal or neutral hydrogen content.
This has resulted in the detection of 81 systems with log N(H2)~19-20 located
within a few thousand km/s from the quasar. Compared to a control sample of
intervening systems, this implies an excess of proximate H2 systems by about a
factor of 4 to 5. The incidence of H2 systems increases steeply with decreasing
relative velocity, reaching an order of magnitude higher than expected from
intervening statistics at Delta_v<1000 km/s. The most striking feature of the
proximate systems compared to the intervening ones is the presence of Ly-alpha
emission in the core of the associated damped HI absorption line in about half
of the sample. This puts constraints on the relative projected sizes of the
absorbing clouds to those of the quasar line emitting regions. Using the SDSS
spectra, we estimate the HI, metal and dust content of the systems, which are
found to have typical metallicities of one tenth Solar, albeit with a large
spread among individual systems. We observe trends between the fraction of
leaking Ly-alpha emission and the relative absorber-quasar velocity as well as
with the excitation of several metal species, similar to what has been seen in
metal-selected proximate DLAs. With the help of theoretical HI-H2 transition
relations, we show that the presence of H2 helps to break the degeneracy
between density and strength of the UV field as main sources of excitation and
hence provides unique constraints on the possible origin and location of the
absorbing clouds. We suggest that most of these systems originate from galaxies
in the quasar group. [truncated]
|
1905.02040v1
|
2019-07-17
|
Sub-damped Lyman alpha systems in the XQ-100 survey I -- Identification and contribution to the cosmological HI budget
|
Sub-damped Lyman alpha systems (subDLAs; HI column densities of
19.0<=logN(HI)<20.3) are rarely included in the cosmic HI census performed at
redshifts z>=1.5, yet are expected to contribute significantly to the overall
HI mass budget of the Universe. In this paper, we present a blindly selected
sample of 155 subDLAs found along 100 quasar sightlines (with a redshift path
length X=475) in the XQ-100 survey to investigate the contribution of subDLAs
to the HI mass density of the Universe. The impact of X-Shooter's spectral
resolution on sub-DLA identification is evaluated, and found to be sufficient
for reliably finding absorbers with logN(HI)>=18.9. We compared the
implications of searching for subDLAs solely using HI absorption versus the use
of additional metal lines to confirm the identification, and found that
metal-selection techniques would have missed 75 subDLAs. Using a
bootstrap-Monte Carlo simulation, we computed the column density distribution
function (f(N,X)) and the cosmological HI mass density of subDLAs and compared
with our previous work based on the XQ-100 damped Lyman alpha systems. We do
not find any significant redshift evolution in f(N,X) or cosmological HI mass
density for subDLAs. However, subDLAs contribute 10-20 per cent of the total
cosmological HI mass density measured at redshifts 2<z<5 (agreeing with
previous measurements), and thus have a small but significant contribution to
the HI budget of the Universe.
|
1907.07703v1
|
2019-10-28
|
Investigating the damping rate of phase-mixed Alfven waves
|
Context: This paper investigates the effectiveness of phase mixing as a
coronal heating mechanism. A key quantity is the wave damping rate, $\gamma$,
defined as the ratio of the heating rate to the wave energy.
Aims: We investigate whether or not laminar phase-mixed Alfv\'en waves can
have a large enough value of $\gamma$ to heat the corona. We also investigate
the degree to which the $\gamma$ of standing Alfv\'en waves which have reached
steady-state can be approximated with a relatively simple equation. Further
foci of this study are the cause of the reduction of $\gamma$ in response to
leakage of waves out of a loop, the quantity of this reduction, and how
increasing the number of excited harmonics affects $\gamma$.
Results: We find that at observed frequencies $\gamma$ is too small to heat
the corona by approximately three orders of magnitude. Therefore, we believe
that laminar phase mixing is not a viable stand-alone heating mechanism for
coronal loops. We show that $\gamma$ is largest at resonance. We find our
simple equation provides a good estimate for the damping rate (within
approximately 10% accuracy) for resonant field lines. However, away from
resonance, the equation provides a poor estimate, predicting $\gamma$ to be
orders of magnitude too large. We find that leakage acts to reduce $\gamma$ but
plays a negligible role if $\gamma$ is of the order required to heat the
corona. If the wave energy follows a power spectrum with slope -5/3 then
$\gamma$ grows logarithmically with the number of excited harmonics. If the
number of excited harmonics is increased by much more than 100, then the
heating is mainly caused by gradients that are parallel to the field rather
than perpendicular to it. Therefore, in this case, the system is not heated
mainly by phase mixing.
|
1910.12510v1
|
2019-11-12
|
Self Sustained Thermally Induced Gas-Damped Oscillations of Bimetal Cantilevers with Application to the Design of a New Pyroelectric Micro Energy Harvester
|
Low efficiency is the main drawback of many MEMS thermal energy harvesters.
Recently, energy harvesting micro-devices that operate using the pyroelectric
effect gained attention due to their potential superior performance. Operation
of these devices is based on the cyclic motion of a pyroelectric capacitor that
operates between a high temperature and a low temperature reservoirs. In this
paper, we investigate the dynamics of oscillations of a pyroelectric capacitor
self sustained by thermally actuated bimetal micro-cantilevers, a topic which
is so far under investigated. In addition to highlighting key thermodynamic
aspects of the operation, we explore conditions for self-sustained oscillations
and discuss the viability of operation at the mechanical resonance frequency.
The analysis is presented for a new design inspired by the device proposed in
Refs.\cite{2011,2012}, where in contrast, our proposed design boasts the
following features: The pyroelectric capacitor remains parallel to the heat
reservoirs, by virtue of its symmetric support by two bimetallic cantilever
beams; In addition, the cyclic operation of the device does not require
physical contact, thus lowering the risk of mechanical failure; To adjust the
damping force imparted by the surrounding gas, the thermal reservoirs are
equipped with trenches. To study the dynamic operation of the device, we
developed a physically based reduced order, yet accurate, model that accounts
for the heat transfer between and within the different components, and for the
various forces including the gas damping force. The model is embedded within an
optimization algorithm to produce optimal designs over the range 26-38 C of
temperature difference between the two reservoirs. The corresponding range of
harvested power density is 0.4-0.65 mW/cm2.
|
1911.04823v1
|
2020-03-01
|
Positivity and nonadditivity of quantum capacities using generalized erasure channels
|
We consider various forms of a process, which we call {\em gluing}, for
combining two or more complementary quantum channel pairs
$(\mathcal{B},\mathcal{C})$ to form a composite. One type of gluing combines a
perfect channel with a second channel to produce a \emph{generalized erasure
channel} pair $(\mathcal{B}_g,\mathcal{C}_g)$. We consider two cases in which
the second channel is (i) an amplitude-damping, or (ii) a phase-damping qubit
channel; (ii) is the \emph{dephrasure channel} of Leditzky et al. For both (i)
and (ii), $(\mathcal{B}_g,\mathcal{C}_g)$ depends on the damping parameter
$0\leq p\leq 1$ and a parameter $0 \leq \lambda \leq 1$ that characterizes the
gluing process. In both cases we study $Q^{(1)}(\mathcal{B}_g)$ and
$Q^{(1)}(\mathcal{C}_g)$, where $Q^{(1)}$ is the channel coherent information,
and determine the regions in the $(p,\lambda)$ plane where each is zero or
positive, confirming previous results for (ii). A somewhat surprising result
for which we lack any intuitive explanation is that $Q^{(1)}(\mathcal{C}_g)$ is
zero for $\lambda \leq 1/2$ when $p=0$, but is strictly positive (though
perhaps extremely small) for all values of $\lambda> 0$ when $p$ is positive by
even the smallest amount. In addition we study the nonadditivity of
$Q^{(1)}(\mathcal{B}_g)$ for two identical channels in parallel. It occurs in a
well-defined region of the $(p,\lambda)$ plane in case (i). In case (ii) we
have extended previous results for the dephrasure channel without, however,
identifying the full range of $(p,\lambda)$ values where nonadditivity occurs.
Again, an intuitive explanation is lacking.
|
2003.00583v2
|
2020-04-22
|
A Significantly Neutral Intergalactic Medium Around the Luminous z=7 Quasar J0252-0503
|
Luminous $z\ge7$ quasars provide direct probes of the evolution of
supermassive black holes (SMBHs) and the intergalactic medium (IGM) during the
epoch of reionization (EoR). The Ly$\alpha$ damping wing absorption imprinted
by neutral hydrogen in the IGM can be detected in a single EoR quasar spectrum,
allowing the measurement of the IGM neutral fraction towards that line of
sight. However, damping wing features have only been detected in two $z>7$
quasars in previous studies. In this paper, we present new high quality optical
and near-infrared spectroscopy of the $z=7.00$ quasar DES J025216.64--050331.8
obtained with Keck/NIRES and Gemini/GMOS. By using the MgII single-epoch virial
method, we find that it hosts a $\rm (1.39\pm0.16) \times10^{9} ~M_\odot$ SMBH
accreting at an Eddington ratio of $\lambda_{\rm Edd}=0.7\pm0.1$, consistent
with the values seen in other luminous $z\sim 7$ quasars. Furthermore, the
Ly$\alpha$ region of the spectrum exhibits a strong damping wing absorption
feature. The lack of associated metal absorption in the quasar spectrum
indicates that this absorption is imprinted by a neutral IGM. Using a
state-of-the-art model developed by Davies et al., we measure a volume-averaged
neutral hydrogen fraction at $z=7$ of $\langle x_{\rm HI} \rangle =
0.70^{+0.20}_{-0.23} (^{+0.28}_{-0.48})$ within 68% (95%) confidence intervals
when marginalizing over quasar lifetimes of $10^3\le t_{\rm Q}\le10^8$ yr. This
is the highest IGM neutral fraction yet measured using reionization-era quasar
spectra.
|
2004.10877v1
|
2020-09-28
|
Comparison of proton shower developments in the BGO calorimeter of the Dark Matter Particle Explorer between GEANT4 and FLUKA simulations
|
The DArk Matter Particle Explorer (DAMPE) is a satellite-borne detector for
high-energy cosmic rays and $\gamma$-rays. To fully understand the detector
performance and obtain reliable physical results, extensive simulations of the
detector are necessary. The simulations are particularly important for the data
analysis of cosmic ray nuclei, which relies closely on the hadronic and nuclear
interactions of particles in the detector material. Widely adopted simulation
softwares include the GEANT4 and FLUKA, both of which have been implemented for
the DAMPE simulation tool. Here we describe the simulation tool of DAMPE and
compare the results of proton shower properties in the calorimeter from the two
simulation softwares. Such a comparison gives an estimate of the most
significant uncertainties of our proton spectral analysis.
|
2009.13036v1
|
2021-01-19
|
Sub-damped Lyman alpha systems in the XQ-100 survey II -- Chemical evolution at 2.4<z<4.3
|
We present the measured gas-phase metal column densities in 155 sub-damped
Lyman alpha systems (subDLAs) with the aim to investigate the contribution of
subDLAs to the chemical evolution of the Universe. The sample was identified
within the absorber-blind XQ-100 quasar spectroscopic survey over the redshift
range 2.4<=z<=4.3. Using all available column densities of the ionic species
investigated (mainly CIV, SiII, MgII, SiIV, AlII, FeII, CII, and OI; in order
of decreasing detection frequency), we estimate the ionization-corrected
gas-phase metallicity of each system using Markov Chain Monte Carlo techniques
to explore a large grid of Cloudy ionization models. Without accounting for
ionization and dust depletion effects, we find that the HI-weighted gas-phase
metallicity evolution of subDLAs are consistent with damped Lyman alpha systems
(DLAs). When ionization corrections are included, subDLAs are systematically
more metal-poor than DLAs (between ~0.5 sigma and ~3 sigma significance) by up
to ~1.0 dex over the redshift range 3<=z<=4.3. The correlation of gas-phase
[Si/Fe] with metallicity in subDLAs appears to be consistent with that of DLAs,
suggesting that the two classes of absorbers have a similar relative dust
depletion pattern. As previously seen for Lyman limit systems, the gas-phase
[C/O] in subDLAs remains constantly solar for all metallicities indicating that
both subDLAs and Lyman limit systems could trace carbon-rich ejecta,
potentially in circumgalactic environments.
|
2101.07821v1
|
2021-02-01
|
On a Possible Solution to the Tidal Realignment Problem for Hot Jupiters
|
Hot stars with hot Jupiters have a wide range of obliquities, while cool
stars with hot Jupiters tend to have low obliquities. An enticing explanation
for this pattern is tidal realignment of the cool host stars, although this
explanation assumes that obliquity damping occurs faster than orbital decay, an
assumption that needs further exploration. Here we revisit this tidal
realignment problem, building on previous work identifying a low-frequency
component of the time-variable tidal potential that affects the obliquity but
not the orbital separation. We adopt a recent empirically-based model for the
stellar tidal quality factor and its sharp increase with forcing frequency.
This leads to enhanced dissipation at low frequencies, and efficient obliquity
damping. We model the tidal evolution of 46 observed hot Jupiters orbiting cool
stars. A key parameter is the stellar age, which we determine in a homogeneous
manner for the sample, taking advantage of Gaia DR2 data. We explore a variety
of tidal histories and futures for each system, finding in most cases that the
stellar obliquity is successfully damped before the planet is destroyed. A
testable prediction of our model is that hot-Jupiter hosts with orbital periods
shorter than 2--3 days should have obliquities much smaller than $1^\circ$.
With the possible exception of WASP-19b, the predicted future lifetimes of the
planets range from $10^8$\,yr to more than $10^{10}$\,yr. Thus, our model
implies that these hot Jupiters are probably not in immediate danger of being
devoured by their host stars while they are on the main sequence.
|
2102.01081v2
|
2021-02-22
|
Slow-Mode Magnetoacoustic Waves in Coronal Loops
|
Rapidly decaying long-period oscillations often occur in hot coronal loops of
active regions associated with small (or micro-) flares. This kind of wave
activity was first discovered with the SOHO/SUMER spectrometer from Doppler
velocity measurements of hot emission lines, thus also often called "SUMER"
oscillations. They were mainly interpreted as global (or fundamental mode)
standing slow magnetoacoustic waves. In addition, increasing evidence has
suggested that the decaying harmonic type of pulsations detected in light
curves of solar and stellar flares are likely caused by standing slow-mode
waves. The study of slow magnetoacoustic waves in coronal loops has become a
topic of particular interest in connection with coronal seismology. We review
recent results from SDO/AIA and Hinode/XRT observations that have detected both
standing and reflected intensity oscillations in hot flaring loops showing the
physical properties (e.g., oscillation periods, decay times, and triggers) in
accord with the SUMER oscillations. We also review recent advances in theory
and numerical modeling of slow-mode waves focusing on the wave excitation and
damping mechanisms. MHD simulations in 1D, 2D and 3D have been dedicated to
understanding the physical conditions for the generation of a reflected
propagating or a standing wave by impulsive heating. Various damping mechanisms
and their analysis methods are summarized. Calculations based on linear theory
suggest that the non-ideal MHD effects such as thermal conduction, compressive
viscosity, and optically thin radiation may dominate in damping of slow-mode
waves in coronal loops of different physical conditions. Finally, an overview
is given of several important seismological applications such as determination
of transport coefficients and heating function.
|
2102.11376v1
|
2021-04-24
|
Compressive oscillations in hot coronal loops: Are sloshing oscillations and standing slow waves independent?
|
Employing high-resolution EUV imaging observations from SDO/AIA, we analyse a
compressive plasma oscillation in a hot coronal loop triggered by a C-class
flare near one of its foot points as first studied by Kumar et al. We
investigate the oscillation properties in both the 131{\,}{\AA} and 94{\,}{\AA}
channels and find that what appears as a pure sloshing oscillation in the
131{\,}{\AA} channel actually transforms into a standing wave in the
94{\,}{\AA} channel at a later time. This is the first clear evidence of such
transformation confirming the results of a recent numerical study which
suggests that these two oscillations are not independent phenomena. We
introduce a new analytical expression to properly fit the sloshing phase of an
oscillation and extract the oscillation properties. For the AIA 131{\,}{\AA}
channel, the obtained oscillation period and damping time are 608$\pm$4{\,}s
and 431$\pm$20{\,}s, respectively during the sloshing phase. The corresponding
values for the AIA 94{\,}{\AA} channel are 617$\pm$3{\,}s and 828$\pm$50{\,}s.
During the standing phase that is observed only in the AIA 94{\,}{\AA} channel,
the oscillation period and damping time have increased to 791$\pm$5{\,}s and
1598$\pm$138{\,}s, respectively. The plasma temperature obtained from the DEM
analysis indicates substantial cooling of the plasma during the oscillation.
Considering this, we show that the observed oscillation properties and the
associated changes are compatible with damping due to thermal conduction. We
further demonstrate that the absence of a standing phase in the 131{\,}{\AA}
channel is a consequence of cooling plasma besides the faster decay of
oscillation in this channel.
|
2104.12038v1
|
2021-08-05
|
Small-scale clumping at recombination and the Hubble tension
|
Despite the success of the standard $\Lambda$CDM model of cosmology, recent
data improvements have made tensions emerge between low- and high-redshift
observables, most importantly in determinations of the Hubble constant $H_0$
and the (rescaled) clustering amplitude $S_8$. The high-redshift data, from the
cosmic microwave background (CMB), crucially relies on recombination physics
for its interpretation. Here we study how small-scale baryon inhomogeneities
(i.e., clumping) can affect recombination and consider whether they can relieve
both the $H_0$ and $S_8$ tensions. Such small-scale clumping, which may be
caused by primordial magnetic fields or baryon isocurvature below kpc scales,
enhances the recombination rate even when averaged over larger scales, shifting
recombination to earlier times. We introduce a flexible clumping model,
parametrized via three spatial zones with free densities and volume fractions,
and use it to study the impact of clumping on CMB observables. We find that
increasing $H_0$ decreases both $\Omega_m$ and $S_8$, which alleviates the
$S_8$ tension. On the other hand, the shift in $\Omega_m$ is disfavored by the
low-$z$ baryon-acoustic-oscillations measurements. We find that the clumping
parameters that can change the CMB sound horizon enough to explain the $H_0$
tension also alter the damping tail, so they are disfavored by current Planck
2018 data. We test how the CMB damping-tail information rules out changes to
recombination by first removing $\ell>1000$ multipoles in Planck data, where we
find that clumping could resolve the $H_0$ tension. Furthermore, we make
predictions for future CMB experiments, as their improved damping-tail
precision can better constrain departures from standard recombination. Both the
Simons Observatory and CMB-S4 will provide decisive evidence for or against
clumping as a resolution to the $H_0$ tension.
|
2108.02747v3
|
2021-08-09
|
Synchronization of Power Systems under Stochastic Disturbances
|
The synchronization of power generators is an important condition for the
proper functioning of a power system, in which the fluctuations in frequency
and the phase angle differences between the generators are sufficiently small
when subjected to stochastic disturbances. Serious fluctuations can prompt
desynchronization, which may lead to widespread power outages. Here, we model
the stochastic disturbance by a Brownian motion process in the linearized
system of the non-linear power systems and characterize the fluctuations by the
variances of the frequency and the phase angle differences in the invariant
probability distribution. We propose a method to calculate the variances of the
frequency and the phase angle differences. For the system with uniform
disturbance-damping ratio, we derive explicit formulas for the variance
matrices of the frequency and the phase angle differences. It is shown that the
fluctuation of the frequency at a node depends on the disturbance-damping ratio
and the inertia at this node only, and the fluctuations of the phase angle
differences in the lines are independent of the inertia. In particular, the
synchronization stability is related to the cycle space of the network. We
reveal the influences of constructing new lines and increasing capacities of
lines on the fluctuations in the phase angle differences in the existing lines.
The results are illustrated for the transmission system of Shandong Province of
China. For the system with non-uniform disturbance-damping ratio, we further
obtain bounds of the variance matrices.
|
2108.04667v2
|
2021-10-12
|
Two-body collapse model for self-gravitating flow of dark matter and generalized stable clustering hypothesis for pairwise velocity
|
Analytical tools are extremely hard to find for non-linear gravitational
collpase. Only a few simple but powerful tools exist so far. Two examples are
the spherical collapse model (SCM) and stable clustering hypothesis (SCH). We
present a new analytical tool, a two-body collapse model (TBCM), that plays the
same fundamental role as harmonic oscillator in dynamics. For convenience, TBCM
is formulated for gravity with any potential exponent $n$ in a static
background with a fixed damping ($n$=-1 for Newtonian gravity). The competition
between gravity, expanding background (or damping), and angular momentum
classifies two-body collapse into: 1) free fall collapse, where free fall time
is greater if same system starts to collapse at earlier time; 2) equilibrium
collapse that persists longer in time, whose perturbative solutions lead to
power-law evolution of system energy and momentum. Two critical values
$\beta_{s1}=1$ and $\beta_{s2}=1/3\pi$ are identified that quantifies the
competition between damping and gravity. Value $\beta_{s2}$ only exists for
discrete values of potential exponent $n=(2-6m)/(1+3m)=$ -1,-10/7... for
integer $m$. Critical density ratio ($\Delta_c=18\pi^2$) is obtained for $n$=-1
that is consistent with SCM. TBCM predicts angular velocity $\propto Hr^{-3/2}$
for two-body system of size $r$. The isothermal density is a result of
extremely fast mass accretion. TBCM is able to demonstrate SCH, i.e. mean
pairwise velocity (first moment) $\langle\Delta u\rangle=-Hr$. A generalized
SCH is developed for higher order moments $\langle\Delta
u^{2m+1}\rangle=-(2m+1)\langle\Delta u^{2m}\rangle Hr$ that is validated by
N-body simulation. Energy evolution in TBCM is independent of particle mass and
energy equipartition does not apply. TBCM can be considered as a non-radial
SCM. Both models predict the same critical density ratio, while TBCM contains
much richer information.
|
2110.05784v2
|
2021-10-25
|
Capillary gravity water waves linearized at monotone shear flows: eigenvalues and inviscid damping
|
This paper is concerned with the eigenvalues and linear inviscid damping of
the 2D capillary gravity water waves of finite depth $x_2\in(-h,0)$ linearized
at a monotone shear flow $U(x_2)$. Unlike the linearized Euler equation in a
fixed channel where eigenvalues exist only in low horizontal wave number $k$,
we first prove the linearized capillary gravity wave has two branches of
eigenvalues $-ikc^\pm(k)$, where the wave speeds $c^\pm(k)=O(\sqrt{|k|})$ for
$|k|\gg1$ have the same asymptotics as the those of the linear irrotational
capillary gravity waves. Under the additional assumption of $U"\ne0$, we obtain
the complete continuation of these two branches, which are all the eigenvalues
in this (and some other) case(s). Particularly $-ikc^-(k)$ could bifurcate into
unstable eigenvalues at $c^-(k)=U(-h)$. The bifurcation of unstable eigenvalues
from inflection values of $U$ is also proved. Assuming no singular modes, i.e.
no embedded eigenvalues for any wave number $k$, linear solutions
$(v(t,x),\eta(t,x_1))$ are studieded in both periodic-in-$x_1$ and $x_1\in R$
cases, where $v$ is the velocity and $\eta$ the surface profile. Solutions can
be split into $(v^p,\eta^p)$ and $(v^c,\eta^c)$ whose $k$-th Fourier mode in
$x_1$ correspond to the eigenvalues and the continuous spectra of wave number
$k$, respectively. The component $(v^p,\eta^p)$ is governed by a (possibly
unstable) dispersion relation given by the eigenvalues, which are simply
$k\to-ikc^\pm(k)$ in the case of $x_1\in R$. The other component $(v^c,\eta^c)$
satisfies the inviscid damping as fast as
$|v_1^c|_{L_x^2},|\eta^c|_{L_x^2}=O(|t|^{-1})$ and $|v_2^c|_{L_x^2}=O(t^{-2})$
as $|t|\gg1$. Additional decay of $tv_1^c,t^2v_2^c$ in $L_x^2L_t^q$,
$q\in(2,\infty]$, is obtained after leading asymptotic terms are removed, which
are in the forms of $t$-dependent translations in $x_1$ of certain functions of
$x$.
|
2110.12604v3
|
2021-11-22
|
Recent Developments in Quantum-Circuit Refrigeration
|
We review the recent progress in direct active cooling of the
quantum-electric degrees freedom in engineered circuits, or quantum-circuit
refrigeration. In 2017, the invention of a quantum-circuit refrigerator (QCR)
based on photon-assisted tunneling of quasiparticles through a
normal-metal--insulator--superconductor junction inspired a series of
experimental studies demonstrating the following main properties: (i) the
direct-current (dc) bias voltage of the junction can change the QCR-induced
damping rate of a superconducting microwave resonator by orders of magnitude
and give rise to non-trivial Lamb shifts, (ii) the damping rate can be
controlled in nanosecond time scales, and (iii) the dc bias can be replaced by
a microwave excitation, the amplitude of which controls the induced damping
rate. Theoretically, it is predicted that state-of-the-art superconducting
resonators and qubits can be reset with an infidelity lower than $10^{-4}$ in
tens of nanoseconds using experimentally feasible parameters. A QCR-equipped
resonator has also been demonstrated as an incoherent photon source with an
output temperature above one kelvin yet operating at millikelvin. This source
has been used to calibrate cryogenic amplification chains. In the future, the
QCR may be experimentally used to quickly reset superconducting qubits, and
hence assist in the great challenge of building a practical quantum computer.
|
2111.11234v1
|
2021-12-01
|
Damped Ly-alpha Absorbers in Star-forming Galaxies at z < 0.15 Detected with the Hubble Space Telescope and Implications for Galaxy Evolution
|
We report {\it HST} COS spectroscopy of 10 quasars with foreground
star-forming galaxies at 0.02$<$$z$$<$ 0.14 within impact parameters of
$\sim$1-7 kpc. We detect damped/sub-damped Ly$\alpha$ absorption in 100$\%$ of
cases where no higher-redshift Lyman-limit systems extinguish the flux at the
expected wavelength of Ly$\alpha$ absorption, obtaining the largest targeted
sample of DLA/sub-DLAs in low-redshift galaxies. We present absorption
measurements of neutral hydrogen and metals. Additionally, we present GBT 21-cm
emission measurements for 5 of the galaxies (including 2 detections). Combining
our sample with the literature, we construct a sample of 115 galaxies
associated with DLA/sub-DLAs spanning 0$<$$z$$<$4.4, and examine trends between
gas and stellar properties, and with redshift. The H~I column density is
anti-correlated with impact parameter and stellar mass. More massive galaxies
appear to have gas-rich regions out to larger distances. The specific SFR
(sSFR) of absorbing galaxies increases with redshift and decreases with
$M^{\ast}$, consistent with evolution of the star-formation main sequence
(SFMS). However, $\sim$20$\%$ of absorbing galaxies lie below the SFMS,
indicating that some DLA/sub-DLAs trace galaxies with longer-than-typical
gas-depletion time-scales. Most DLA/sub-DLA galaxies with 21-cm emission have
higher H I masses than typical galaxies with comparable $M^{\ast}$. High
$M_{\rm H I}/M^{\ast}$ ratios and high sSFRs in DLA/sub-DLA galaxies with
$M^{\ast}$$<$$10^{9}$$M_{\odot}$ suggest these galaxies may be gas-rich because
of recent gas accretion rather than inefficient star formation. Our study
demonstrates the power of absorption and emission studies of DLA/sub-DLA
galaxies for extending galaxy-evolution studies to previously under-explored
regimes of low $M^{\ast}$ and low SFR.
|
2112.00870v1
|
2022-01-06
|
Parameter-free quantum hydrodynamic theory for plasmonics: Electron density-dependent damping rate and diffusion coefficient
|
Plasmonics is a rapid growing field, which has enabled both fundamental
science and inventions of various quantum optoelectronic devices. An accurate
and efficient method to calculate the optical response of metallic structures
with feature size in the nanoscale plays an important role. Quantum
hydrodynamic theory (QHT) provides an efficient description of the
free-electron gas, where quantum effects of nonlocality and spill-out are taken
into account. In this work, we introduce a general QHT that includes diffusion
to account for the broadening, which is a key problem in practical applications
of surface plasmon. We will introduce a density-dependent diffusion coefficient
to give very accurate linewidth. It is a self-consistent method, in which both
the ground and excited states are solved by using the same energy functional,
with the kinetic energy described by the Thomas-Fermi and von Weizs\"{a}cker
(vW) formalisms. In addition, our QHT method is stable by introduction of an
electron density-dependent damping rate. For sodium nanosphere of various
sizes, the plasmon energy and broadening by our QHT method are in excellent
agreement with those by density functional theory and Kreibig formula. By
applying our QHT method to sodium jellium nanorods, we clearly show that our
method enables a parameter-free simulation, i.e. without resorting to any
empirical parameter, such as size-dependent damping rate and diffusing
coefficient. It is found that there exists a perfect linear relation between
the resonance wavelength and aspect radio. The width decreases with increasing
aspect ratio and height. The calculations show that our QHT method provides an
explicit and unified way to account for size-dependent frequency shifts and
broadening of arbitrarily shaped geometries. It is reliable and robust with
great predicability, and hence provides a general and efficient platform to
study plasmonics.
|
2201.03426v3
|
2022-01-12
|
Neutrino effective potential and damping in a fermion and scalar background in the resonance region
|
We consider the propagation of a neutrino or an antineutrino in a medium
composed of fermions $f$ and scalars $\phi$ interacting via a Yukawa-type
coupling of the form $\bar f\nu\phi$, for neutrino energies at which the
processes like $\nu + \phi \leftrightarrow f$ or $\nu + \bar f \leftrightarrow
\bar\phi$, and the corresponding ones for the antineutrino, are kinematically
accessible. The relevant energy values are around $|m^2_\phi - m^2_f|/2m_\phi$
or $|m^2_\phi - m^2_f|/2m_f$, where $m_\phi$ and $m_f$ are the masses of $\phi$
and $f$, respectively. We refer to either one of these regions as a resonance
energy range. Near these points, the one-loop formula for the neutrino
self-energy has a singularity. From a technical point of view, that feature is
indicative that the self-energy acquires an imaginary part, which is associated
with damping effects and cannot be neglected, while the integral formula for
the real part must be evaluated using the principal value of the integral. We
carry out the calculations explicitly for some cases that allow us to give
analytic results. Writing the dispersion relation in the form $\omega = \kappa
+ V_{\text{eff}} - i\gamma/2$, we give the explicit formula for
$V_{\text{eff}}$ and $\gamma$ for the cases considered. When the neutrino
energy is either much larger or much smaller than the resonance energy,
$V_{\text{eff}}$ reduces to the effective potential that has been already
determined in the literature in the high or low momentum regime, respectively.
The virtue of the formula we give for $V_{\text{eff}}$ is that it is valid also
in the \emph{resonance energy range}, which is outside the two limits
mentioned. As a guide to possible applications we give the relevant formulas
for $V_{\text{eff}}$ and $\gamma$, and consider the solution to the oscillation
equations including the damping term, in a simple two-generation case.
|
2201.04661v2
|
2022-01-19
|
Transverse Coronal-Loop Oscillations Induced by the Non-radial Eruption of a Magnetic Flux Rope
|
We investigate the transverse coronal-loop oscillations induced by the
eruption of a prominence-carrying flux rope on 7 December 2012. The flux rope
originating from NOAA Active Region (AR) 11621 was observed in EUV wavelengths
by the SDO/AIA and in H$\alpha$ line center by the ground-based telescope at
the BBSO. The early evolution of the flux rope is divided into two steps: a
slow rise phase at a speed of $\approx$230\,km\,s$^{-1}$ and a fast rise phase
at a speed of $\approx$706\,km\,s$^{-1}$. The eruption generates a C5.8 flare
and the onset of the fast rise is consistent with the HXR peak time of the
flare. The embedded prominence has a lower speed of $\approx$452\,km\,s$^{-1}$.
During the early eruption of the flux rope, the nearby coronal loops are
disturbed and experience independent kink-mode oscillations in the horizontal
and vertical directions. The oscillation in the horizontal direction has an
initial amplitude of $\approx$3.1\,Mm, a period of $\approx$294\,seconds, and a
damping time of $\approx$645\,seconds. It is most striking in 171\,{\AA} and
lasts for three to four cycles. The oscillations in the vertical directions are
observed mainly in 171, 193, and 211\,{\AA}. The initial amplitudes lie in the
range of 3.4\,--\,5.2\,Mm, with an average value of 4.5\,Mm. The periods are
between 407\,seconds and 441\,seconds, with an average value of 423\,seconds.
The oscillations are damping and last for nearly four cycles. The damping times
lie in the range of 570\,--\,1012\,seconds, with an average value of
741\,seconds. Assuming a semi-circular shape of the vertically oscillating
loops, we calculate the loop lengths according to their heights. Using the
observed periods, we carry out coronal seismology and estimate the internal
Alfv\'{e}n speeds (988\,--\,1145\,km\,s$^{-1}$) and the magnetic-field
strengths (12\,--\,43\,G) of the oscillating loops.
|
2201.07389v1
|
2022-03-16
|
Snowmass Whitepaper AF6: Plasma-Based Particle Sources
|
High-brightness beams generated by particle sources based on advanced
accelerator concepts have the potential to become an essential part of future
accelerator technology. High-gradient accelerators can generate and rapidly
accelerate particle beams to relativistic energies while minimizing
irreversible detrimental effects to the beam brightness that occur at low beam
energies. Due to the high accelerating gradients, these novel accelerators are
also significantly more compact than conventional technology. The beam
parameters of these particle sources are largely determined by the injection
and subsequent acceleration processes. While there has been significant
progress crucial parameters that are required for a future collider or more
near-term applications, including X-ray free-electron lasers (XFELs), such as a
sufficiently small energy spread and small emittance for bunches with a high
charge and at high pulse repetition rate. Major research and development
efforts are required to realize these approaches for a front-end injector for a
future collider in order to address these limitations. In particular, this
includes methods to control and manipulate the phase-space and spin
degrees-of-freedom of ultrashort LWFA electron bunches with high accuracy,
methods that increase the laser-to-electron beam efficiency and increased
repetition rate. This also includes the development of high-resolution
diagnostics, such as full 6D phase-space measurements, beam polarimetry and
high-fidelity simulation tools. A further increase in beam luminosity can be
achieve through emittance damping. For future colliders, the damping rings
might be replaced by a substantially more compact plasma-based approach. Here,
plasma wigglers are used to achieve similar damping performance but over a two
orders of magnitude reduced length.
|
2203.08379v2
|
2022-04-04
|
Staring at the Shadows of Archaic Galaxies: Damped Ly$α$ and Metal Absorbers toward a Young $z \sim 6$ Weak-line Quasar
|
We characterize the Ly$\alpha$ halo and absorption systems toward PSO
J083+11, a unique $z=6.3401$ weak-line quasar, using Gemini/GNIRS,
Magellan/FIRE, and VLT/MUSE data. Strong absorptions by hydrogen and several
metal lines (e.g., CII, MgII, and OI) are discovered in the spectrum, which
indicates the presence of: (i) a proximate sub-damped Ly$\alpha$ (sub-DLA)
system at $z=6.314$ and (ii) a MgII absorber at $z=2.2305$. To describe the
observed damping wing signal, we model the Ly$\alpha$ absorption with a
combination of a sub-DLA with the neutral hydrogen column density of $\log
N_\mathrm{HI} = 20.03 \pm 0.30$ cm$^{-2}$ and absorption from the intergalactic
medium with a neutral fraction of around 10 percent. The sub-DLA toward PSO
J083+11 has an abundance ratio of [C/O] $=-0.04 \pm 0.33$ and metallicity of
[O/H] $=-2.19 \pm 0.44$, similar to those of low-redshift metal-poor DLAs.
These measurements suggest that the sub-DLA might truncate PSO J083+11's
proximity zone size and complicate the quasar lifetime measurement. However,
this quasar shows no sign of a Ly$\alpha$ halo in the MUSE data cube, where the
estimated $1\sigma$ limit of surface brightness is $2.76 \times 10^{-18}$ erg
s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ at aperture size of 1 arcsecond, or equivalent
to a Ly$\alpha$ luminosity of $\leq 43.46$ erg s$^{-1}$. This non-detection,
while being only weak independent evidence on its own, is at least consistent
with a young quasar scenario, as expected for a quasar with a short accretion
timescale.
|
2204.01245v2
|
2022-04-28
|
Viscous inertial modes on a differentially rotating sphere: Comparison with solar observations
|
In a previous paper we studied the effect of latitudinal rotation on solar
equatorial Rossby modes in the beta-plane approximation. Since then, a rich
spectrum of inertial modes has been observed on the Sun, which is not limited
to the equatorial Rossby modes and includes high-latitude modes.
Here we extend the computation of toroidal modes in 2D to spherical geometry,
using realistic solar differential rotation and including viscous damping. The
aim is to compare the computed mode spectra with the observations and to study
mode stability.
At fixed radius, we solve the eigenvalue problem numerically using a
spherical harmonics decomposition of the velocity stream function. Due to the
presence of viscous critical layers, the spectrum consists of four different
families: Rossby modes, high-latitude modes, critical-latitude modes, and
strongly damped modes. For each longitudinal wavenumber m<4, up to three
Rossby-like modes are present on the sphere, in contrast to the equatorial beta
plane where only the equatorial Rossby mode is present. The least damped modes
in the model have eigenfrequencies and eigenfunctions that resemble the
observed modes; the comparison improves when the radius is taken in the lower
half of the convection zone. For radii above 0.75R and Ekman numbers E<10^{-4},
at least one mode is unstable. For either m=1 or m=2, up to two Rossby modes
are unstable when the radial dependence of the Ekman number follows a quenched
diffusivity model (E=2. 10^{-5} at the base of the convection zone). For m=3,
up to two Rossby modes can be unstable, including the equatorial Rossby mode.
Although the 2D model discussed here is highly simplified, the spectrum of
toroidal modes appears to include many of the observed solar inertial modes.
The self-excited modes in the model have frequencies close to those of the
observed modes with the largest amplitudes.
|
2204.13412v1
|
2022-05-29
|
Modeling the Dynamics of the Coronavirus SARS-CoV-2 Pandemic using Modified SIR Model with the 'Damped-Oscillator' Dynamics of the Effective Reproduction Number
|
The COVID-19 pandemic has been a great catastrophe that upended human lives
and caused millions of deaths all over the world. The rapid spread of the
virus, with its early-stage exponential growth and subsequent 'waves', caught
many medical professionals and decision-makers unprepared. Even though
epidemiological models have been known for almost a century (since the 'Spanish
Influenza' pandemic of 1918-20), the real-life spread of the SARS-CoV-2 virus
often confounded the modelers. While the general framework of epidemiological
models like SEIR (susceptible-exposed-infected-recovered) or SIR
(susceptible-exposed-infected) was not in question, the behavior of model
parameters turned out to be unpredictable and complicated. In particular, while
the 'basic' reproduction number, R0, can be considered a constant (for the
original SARS-CoV-2 virus, prior to the emergence of variants, R0 is between
2.5 and 3.0), the 'effective' reproduction number, R(t), was a complex function
of time, influenced by human behavior in response to the pandemic (e.g.,
masking, lockdowns, transition to remote work, etc.) To better understand these
phenomena, we model the first year of the pandemic (between February 2020 and
February 2021) for a number of localities (fifty US states, as well as several
countries) using a simple SIR model. We show that the evolution of the pandemic
can be described quite successfully by assuming that R(t) behaves in a
'viscoelastic' manner, as a sum of two or three 'damped oscillators' with
different natural frequencies and damping coefficients. These oscillators
likely correspond to different sub-populations having different reactions to
proposed mitigation measures. The proposed approach can offer future data
modelers new ways to fit the reproduction number evolution with time (as
compared to the purely data-driven approaches most prevalent today).
|
2205.14747v1
|
2022-07-27
|
Determination of Thickness-dependent Damping Constant and Plasma Frequency for Ultrathin Ag and Au Films: Nanoscale Dielectric Function
|
There is an ever increasing interest in the development of plasmonic 2D
nanomaterials, with widespread applications in optoelectronics, high resolution
microscopy, imaging and sensing, among others. With the current ability of
ultrathin noble metal film deposition down to a few monolayers in thickness,
there is a need for an analytical expression of the thickness dependent complex
dielectric function for predicting optical properties for arbitrary
thicknesses. The free and bound electron contributions to the dielectric
function are dealt with independently, since their influences affect separate
wavelengths ranges. The former is dealt within the Drude model framework for
large wavelengths with appropriately addressed damping constant and plasma
frequency parameters to account for thickness dependence. Applying our
previously developed method, we determine these parameters for specific film
thicknesses, based on refractive index experimental values for Ag and Au thin
films. Fitting separately each one of these parameters allowed us to find an
analytical expression for their dependence on arbitrary film thickness and
consequently for the free electron contribution. Concerning bound electrons, it
is seen that its contribution for small wavelengths is the same for all
analyzed thicknesses and may be set equal to the bulk bound contribution.
Taking all these facts into account, the complex dielectric function can be
rewritten analytically, in terms of the bulk dielectric function plus
corrective film thickness dependent terms. In particular, the fitting process
for the damping constant allows us to determine that the electron scattering at
the film boundary is mainly diffusive (inelastic) for both silver and gold thin
films. It is also shown that, in accordance with theoretical studies, plasma
frequency shows a red shift as the film thickness decreases.
|
2207.13580v1
|
2022-10-18
|
Magnetohydrodynamic instabilities in a self-gravitating rotating cosmic plasma
|
The generation of magnetohydrodynamic (MHD) waves and their instabilities are
studied in galactic gaseous rotating plasmas with the effects of the magnetic
field, the self gravity, the diffusion-convection of cosmic rays as well as the
gas and cosmic-ray pressures. The coupling of the Jeans, Alfv{\'e}n and
magnetosonic waves, and the conditions of damping or instability are studied in
three different cases, namely when the propagation direction is perpendicular,
parallel and oblique to the static magnetic field, and are shown to be
significantly modified by the effects of the Coriolis force due to the rotation
of cosmic fluids and the cosmic-ray diffusion. The coupled modes can be damped
or anti-damped depending on the wave number is above or below the Jeans
critical wave number that is reduced by the effects of the Coriolis force and
the cosmic-ray pressure. It is found that the deviation of the axis of rotation
from the direction of the static magnetic field gives rise to the coupling
between the Alfv{\'e}n wave and the classical Jeans mode which otherwise
results into the modified slow and fast Alfv{\'e}n waves as well as the
modified classical Jeans modes. Furthermore, due to the effects of the cosmic
rays diffusion, there appears a new wave mode (may be called the fast Jeans
mode) in the intermediate frequency regimes of the slow and fast Alfv{\'e}n
waves, which seems to be dispersionless in the long-wavelength propagation and
has a lower growth rate of instability in the high density regimes of galaxies.
The dispersion properties and the instabilities of different kinds of MHD waves
reported here can play pivotal roles in the formation of various galactic
structures at different length scales.
|
2210.09595v1
|
2023-04-17
|
Theoretical study of the Alfven Eigenmode stability in CFETR steady state discharges
|
The aim of this study is to analyze the stability of Alfven Eigenmodes (AE)
in the China Fusion Engineering Test Reactor (CFETR) plasma for steady state
operations. The analysis is done using the gyro-fluid code FAR3d including the
effect of the acoustic modes, EP Finite Larmor radius damping effects and
multiple energetic particle populations. Two high poloidal beta scenarios are
studied with respect to the location of the internal transport barrier (ITB) at
r/a = 0.45 (case A) and r/a = 0.6 (case B). Both operation scenarios show a
narrow TAE gap between the inner-middle plasma region and a wide EAE gap all
along the plasma radius. The AE stability of CFETR plasmas improves if the ITB
is located inwards, case A, showing AEs with lower growth rates with respect to
the case B. The AEs growth rate is smaller in the case A because the modes are
located in the inner-middle plasma region where the stabilizing effect of the
magnetic shear is stronger with respect to the case B. Multiple EP populations
effects (NBI driven EP + alpha articles) are negligible for the case A,
although the simulations for the case B show a stabilizing effect of the NBI EP
on the n=1 BAE caused by alpha particles during the thermalization process. If
the FLR damping effects are included in the simulations, the growth rate of the
EAE/NAE decreases up to 70 %, particularly for n > 3 toroidal families. Low n
AEs (n<6) show the largest growth rates. On the other hand, high $n$ modes (n=6
to 15) are triggered in the frequency range of the NAE, strongly damped by the
FLR effects.
|
2304.08412v1
|
2023-05-21
|
Small-amplitude Compressible Magnetohydrodynamic Turbulence Modulated by Collisionless Damping in Earth's Magnetosheath: Observation Matches Theory
|
Plasma turbulence is a ubiquitous dynamical process that transfers energy
across many spatial and temporal scales and affects energetic particle
transport. Recent advances in the understanding of compressible
magnetohydrodynamic (MHD) turbulence demonstrate the important role of damping
in shaping energy distributions on small scales, yet its observational evidence
is still lacking. This study provides the first observational evidence of
substantial collisionless damping (CD) modulation on small-amplitude
compressible MHD turbulence cascade in Earth's magnetosheath using four Cluster
spacecraft. Based on an improved compressible MHD decomposition algorithm,
turbulence is decomposed into three eigenmodes: incompressible Alfv\'en modes,
and compressible slow and fast (magnetosonic) modes. Our observations
demonstrate that CD enhances the anisotropy of compressible MHD modes because
CD has a strong dependence on wave propagation angle. The wavenumber
distributions of slow modes are mainly stretched perpendicular to the
background magnetic field ($\mathbf{B_0}$) and weakly modulated by CD. In
contrast, fast modes are subjected to a more significant CD modulation. Fast
modes exhibit a weak, scale-independent anisotropy above the CD truncation
scale. Below the CD truncation scale, the anisotropy of fast modes enhances as
wavenumbers increase. As a result, fast mode fractions in the total energy of
compressible modes decrease with the increase of perpendicular wavenumber (to
$\mathbf{B_0}$) or wave propagation angle. Our findings reveal how the
turbulence cascade is shaped by CD and its consequences to anisotropies in the
space environment.
|
2305.12507v3
|
2023-07-14
|
PIC simulations of stable surface waves on a subcritical fast magnetosonic shock front
|
We study with particle-in-cell (PIC) simulations the stability of fast
magnetosonic shocks. They expand across a collisionless plasma and an
orthogonal magnetic field that is aligned with one of the directions resolved
by the 2D simulations. The shock speed is 1.6 times the fast magnetosonic speed
when it enters a layer with a reduced density of mobile ions, which decreases
the shock speed by up to 15\% in 1D simulations. In the 2D simulations, the
density of mobile ions in the layer varies sinusoidally perpendicularly to the
shock normal. We resolve one sine period. This variation only leads to small
changes in the shock speed evidencing a restoring force that opposes a shock
deformation. As the shock propagates through the layer, the ion density becomes
increasingly spatially modulated along the shock front and the magnetic field
bulges out where the mobile ion density is lowest. The perturbed shock
eventually reaches a steady state. Once it leaves the layer, the perturbations
of the ion density and magnetic field oscillate along its front at a frequency
close to the lower-hybrid frequency; the shock is mediated by a standing wave
composed of obliquely propagating lower-hybrid waves. We perform three 2D
simulations with different box lengths along the shock front. The shock front
oscillations are aperiodically damped in the smallest box with the fastest
variation of the ion density, strongly damped in the intermediate one, and
weakly damped in the largest box. The shock front oscillations perturb the
magnetic field in a spatial interval that extends by several electron skin
depths upstream and downstream of the shock front and could give rise to
Whistler waves that propagate along the shock's magnetic field overshoot.
Similar waves were observed in hybrid and PIC simulations and by the MMS
satellite mission.
|
2307.07435v1
|
2023-08-03
|
Part I: Rebuttal to "Uniform stabilization for the Timoshenko beam by a locally distributed damping"
|
A paper, entitled "Uniform stabilization for the Timoshenko beam by a locally
distributed damping" was published in 2003, in the journal Electronic Journal
of Differential Equations. Its title concerns exclusively its Section 3,
devoted to the case of equal speeds of propagation and to its main theorem,
namely Theorem 3.1. It states that the solutions of the Timoshenko system (see
(1.3) in [1]) decays exponentially when the damping coefficient b is locally
distributed. The proof of Theorem 3.1 is crucially based on Lemma 3.6, which
states the existence of a strict Lyapunov function along which the solutions of
(1.3) decay when the speeds of propagation are equal. This rebuttal shows the
major gap and flaws in the proof of Lemma 3.6, which invalidate the proofs of
Lemma 3.6 and Theorem 3.1. Lemma 3.6 is stated at the top of page 12. The main
part of its proof is given in the pages 12 and 13. In the last eight lines of
page 13, eight inequalities are requested to hold together for the proof of
Lemma 3.6. They don't appear in the statements of Lemma 3.6. The subsequent
flaws come from the evidence that several of them are contradictory either
between them or with claims in the title of the article. We also point in this
rebuttal other flaws, or gaps in the proofs of Theorem 2.2 related to strong
stability and non uniform stability for the case of distinct speeds of
propagation. In [3], we correct and complete the proof of strong stability. We
also correct, set up the missing functional frames, fill the gaps in the proof
of non uniform stability in the cases of different speeds of propagation, and
complete a missing argument in the proof of Theorem A in [4] (see Remark 4.3),
the result of Theorem A being used in the paper [1] on which this rebuttal is
mainly devoted.
|
2308.01611v1
|
2023-08-05
|
Modulating Spin Current Induced Effective Damping in $β-W/Py$ Heterostructures by a Systematic Variation in Resistivity of the Sputtered Deposited $β-W$ films
|
Utilizing the spin-induced pumping from a ferromagnet (FM) into a heavy metal
(HM) under the ferromagnetic resonance (FMR) condition, we report an
enhancement in effective damping in $\beta$- W/Py bilayers by systematically
varying resistivity ($\rho_{W}$) of $\beta$-W films. Different resistivity
ranging from 100 $\mu\Omega$-cm to 1400 $\mu\Omega$-cm with a thickness of 8 nm
can be achieved by varying the argon pressure ($P_{Ar}$) during the growth by
the method of sputtering. The coefficient of effective damping $\alpha_{eff}$
is observed to increase from 0.010 to 0.025 with $\rho_{W}$, which can be
modulated by $P_{Ar}$. We observe a modest dependence of $\alpha_{eff}$ on the
sputtering power ($p_{S}$) while keeping the $P_{Ar}$ constant. $\alpha_{eff}$
dependence on both $P_{Ar}$ and $p_{S}$ suggests that there exists a strong
correlation between $\alpha_{eff}$ and $\rho_{W}$. It is thus possible to
utilize $\rho_{W}$ as a tuning parameter to regulate the $\alpha_{eff}$, which
can be advantageous for faster magnetization dynamics switching. The thickness
dependence study of Py in the aforementioned bilayers manifests a higher spin
mixing conductance ($g^{\uparrow\downarrow}_{eff}$) which suggests a strong
spin pumping from Py into the $\beta$-W layer. The effective spin current
($J_{S(eff)}$) is also evaluated by considering the spin-back flow in this
process. Intrinsic spin mixing conductance ($g^{\uparrow\downarrow}_{W}$) and
spin diffusion length ($\lambda_{SD}$) of $\beta$-W are additionally
investigated using thickness variations in $\beta$-W. Furthermore, the
low-temperature study in $\beta$-W/Py reveals an intriguing temperature
dependence in $\alpha_{eff}$ which is quite different from $\alpha_{b}$ of
single Py layer and the enhancement in $\alpha_{eff}$ at low temperature can be
attributed to the spin-induced pumping from Py layer into $\beta$-W.
|
2308.02939v1
|
2023-09-01
|
f-mode oscillations of anisotropic neutron stars in full general relativity
|
We investigate f-mode oscillations of static anisotropic stable neutron stars
within the framework of full general relativity. We present equations governing
unperturbed stellar structures and oscillations with an ansatz to account for
the anisotropy. We solve those equations for two different equations of states.
We see that, moderately anisotropic neutron stars with the tangential pressure
larger than the radial pressure can give more massive neutron stars than the
isotropic or very anisotropic ones. We find that the frequency of the f-mode
exhibits a linear relationship with the square root of the average density of
the stars and the slope of the fit depends on the anisotropic strength. For any
given value of the anisotropic strength, the frequency increases with the
increase of the mass of the neutron star, linearly for lower masses, and
rapidly at higher masses. However, this non-linear rise in the frequency with
the mass is not prominent when the radial pressure is larger than the
tangential pressure. For a fixed value of a small mass, higher anisotropy leads
to a larger value of the frequency, but when the fixed mass is above a
threshold value, higher anisotropy leads to a smaller value of the frequency.
The nature of the variation in the frequency with the change in the anisotropic
strength is similar for the two equations of state, but for a fixed mass and
the same amount of the anisotropy, the softer equations of state gives higher
frequency. We also find that the damping time of the f-mode oscillation
decreases as the mass of the neutron star increases for all values of the
anisotropic strength. For a fixed mass of the neutron star and for the same
amount of the anisotropy, the value of the damping time is lower for the softer
equation of state, but the nature of the variation in the damping time with the
change in the anisotropic strength is similar.
|
2309.00439v2
|
2023-09-10
|
Stability and Regularity for Double Wall Carbon Nanotubes Modeled as Timoshenko Beams with Thermoelastic Effects and Intermediate Damping
|
This research studies two systems composed by the Timoshenko beam model for
double wall carbon nanotubes, coupled with the heat equation governed by
Fourier's law. For the first system, the coupling is given by the speed the
rotation of the vertical filament in the beam $\beta\psi_t$ from the first beam
of Tymoshenko and the Laplacian of temperature $\delta\theta_{xx}$, where we
also consider the damping terms fractionals
$\gamma_1(-\partial_{xx})^{\tau_1}\phi_t$, $\gamma_2(-\partial_{xx})^{\tau_2}
y_t$ and $\gamma_3(-\partial_{xx})^{\tau_3} z_t$, where $(\tau_1, \tau_2,
\tau_3) \in [0,1]^3$. For this first system we proved that the semigroup
$S_1(t)$ associated to system decays exponentially for all $(\tau_1 , \tau_2 ,
\tau_3 ) \in [0,1]^3$. The second system also has three fractional damping
$\gamma_1(-\partial_{xx})^{\beta_1}\phi_t$, $\gamma_2(-\partial_{xx})^{\beta_2}
y_t$ and $\gamma_3(-\partial_{xx})^{\beta_3} z_t$, with $(\beta_1, \beta_2,
\beta_3) \in [0,1]^3$. Furthermore, the couplings between the heat equation and
the Timoshenko beams of the double wall carbon nanotubes for the second system
is given by the Laplacian of the rotation speed of the vertical filament in the
beam $\beta\psi_{xxt}$ of the first beam of Timoshenko and the Lapacian of the
temperature $\delta\theta_{xx}$. For the second system, we prove the
exponential decay of $S_2(t)$ for $(\beta_1, \beta_2, \beta_3) \in [0,1]^3$ and
also show that $S_2(t)$ admits Gevrey classes $s>(\phi+1)/(2\phi)$ for
$\phi=\min\{\beta_1,\beta_2,\beta_3\}, \forall (\beta_1,\beta_2,\beta_3)\in
(0,1)^3$, and proving that $S_2(t)$ is analytic when the parameters $(\beta_1,
\beta_2, \beta_3) \in [1/2,1]^3$. One of the motivations for this research was
the work; Ramos et al. \cite{Ramos2023CNTs}, whose partial results are part of
our results obtained for the first system for $(\tau_1, \tau_2, \tau_3) = (0,
0, 0)$.
|
2309.04906v1
|
2023-11-03
|
Probing the disc-jet coupling in S4 0954+65, PKS 0903-57, & 4C +01.02 with $γ$-rays
|
We present a comprehensive variability study on three blazars, S4 0954+65,
PKS 0903-57, and 4C +01.02 covering a mass range of log(M/M$_{\odot}$) = 8--9,
by using $\sim$15 years-long $\gamma$-ray light curves from \textit{Fermi}-LAT.
The variability level is characterized by the fractional variability amplitude
which is higher for $\gamma$-rays compared to optical/UV and X-rays emissions.
A power spectral density (PSD) study and damped random walk (DRW) modeling are
done to probe the characteristic timescale. The PSD is fitted with a single
power-law (PL) and bending power-law models and the corresponding success
fraction was estimated. In the case of PKS 0903-57, We observed a break in the
$\gamma$-ray PSD at 256 days which is comparable to the viscous timescale in
the accretion disc suggesting a possible disk-jet coupling. The non-thermal
damping timescale from the DRW modeling is compared with the thermal damping
timescale for AGNs including our three sources. Our sources lie on the best-fit
of the $\mathrm{\tau^{rest}_{damping}} - M_{BH}$ plot derived for AGN
suggesting a possible accretion disc-jet connection. If the jet's variability
is linked to the disc's variability, we expect a log-normal flux distribution,
often connected to the accretion disc's multiplicative processes. Our study
observed a double log-normal flux distribution, possibly linked to long and
short-term variability from the accretion disk and the jet. In summary, PSD and
DRW modeling results for these three sources combined with blazars and AGNs
studied in literature favor a disc-jet coupling scenario. However, more such
studies are needed to refine this understanding.
|
2311.01738v1
|
2023-11-30
|
Compton scattering of electrons in the intergalactic medium
|
This paper investigates the distribution and implications of cosmic ray
electrons within the intergalactic medium (IGM). Utilizing a synthesis model of
the extragalactic background, we evolve the spectrum of Compton-included cosmic
rays. The energy density distribution of cosmic ray electrons peaks at redshift
$z \approx2$, and peaks in the $\sim$MeV range. The fractional contribution of
cosmic ray pressure to the general IGM pressure progressively increases toward
lower redshift. At mean density, the ratio of cosmic ray electron to thermal
pressure in the IGM $ P_{\rm CRe} / P_{\rm th}$ is 0.3% at $z=2$, rising to
1.0% at $z=1$, and 1.8% at $z=0.1$ (considering only the cosmic rays produced
locally by Compton scattering). We compute the linear Landau damping rate of
plasma oscillations in the IGM caused by the $\sim$MeV cosmic ray electrons,
and find it to be of order $\sim 10^{-6}\,\rm s^{-1}$ for wavenumbers
$1.2\lesssim ck/\omega_{\rm p}\lesssim 5$ at $z=2$ and mean density (where
$\omega_{\rm p}$ is the plasma frequency). This strongly affects the fate of
TeV $e^+e^-$ pair beams produced by blazars, which are potentially unstable to
oblique instabilities involving plasma oscillations with wavenumber
$ck/\omega_{\rm p}\approx\sec\theta$ ($\theta$ being the angle between the beam
and wave vector). Linear Landau damping is at least thousands of times faster
than either pair beam instability growth or collisional effects; it thus turns
off the pair beam instability except for modes with very small $\theta$
($ck/\omega_{\rm p}\rightarrow 1$, where linear Landau damping is kinematically
suppressed). This leaves open the question of whether the pair beam instability
is turned off entirely, or can still proceed via the small-$\theta$ modes.
|
2311.18721v2
|
2023-12-26
|
Observation of Magnon Damping Minimum Induced by Kondo Coupling in a van der Waals Ferromagnet Fe$_{3-x}$GeTe$_{2}$
|
In heavy-fermion systems with $f$ electrons, there is an intricate interplay
between Kondo screening and magnetic correlations, which can give rise to
various exotic phases. Recently, similar interplay appears to also occur in
$d$-electron systems, but the underlying mechanism remains elusive. Here, using
inelastic neutron scattering, we investigate the temperature evolution of the
low-energy spin waves in a metallic van der Waals ferromagnet
Fe$_{3-x}$GeTe$_{2}$ (Curie temperature $T_{\rm C}\sim160$ K), where the
Kondo-lattice behavior emerges in the ferromagnetic phase below a
characteristic temperature $T^*\sim90$ K. We observe that the magnon damping
constant diverges at both low and high temperatures, exhibiting a minimum
coincidentally around $T^*$. Such an observation is analogous to the
resistivity minimum as due to the single-impurity Kondo effect. This unusual
behavior is described by a formula that combines logarithmic and power terms,
representing the dominant contributions from Kondo screening and thermal
fluctuations, respectively. Furthermore, we find that the magnon damping
increases with momentum below $T_{\rm C}$. These findings can be explained by
considering spin-flip electron-magnon scattering, which serves as a magnonic
analog of the Kondo-impurity scattering, and thus provides a measure of the
Kondo coupling through magnons. Our results provide critical insights into how
Kondo coupling manifests itself in a system with magnetic ordering and shed
light on the coexistence of and interplay between magnetic order and Kondo
effect in itinerant 3$d$-electron systems.
|
2312.15961v1
|
2024-01-18
|
Chronicling the reionization history at $6\lesssim z \lesssim 7$ with emergent quasar damping wings
|
The spectra of high-redshift ($z\gtrsim 6$) quasars contain valuable
information on the progression of the Epoch of Reionization (EoR). At redshifts
$z<6$, the observed Lyman-series forest shows that the intergalactic medium
(IGM) is nearly ionized, while at $z>7$ the observed quasar damping wings
indicate high neutral gas fractions. However, there remains a gap in neutral
gas fraction constraints at $6\lesssim z \lesssim 7$ where the Lyman series
forest becomes saturated but damping wings have yet to fully emerge. In this
work, we use a sample of 18 quasar spectra at redshifts $6.0<z<7.1$ to close
this gap. We apply neural networks to reconstruct the quasars' continuum
emission around the partially absorbed Lyman $\alpha$ line to normalize their
spectra, and stack these continuum-normalized spectra in three redshift bins.
To increase the robustness of our results, we compare the stacks to a grid of
models from two hydrodynamical simulations, ATON and CROC, and we measure the
volume-averaged neutral gas fraction, $\bar{x}_{\rm HI}$, while jointly fitting
for the mean quasar lifetime, $t_{\rm Q}$, for each stacked spectrum. We
chronicle the evolution of neutral gas fraction using the ATON (CROC) models as
follows: $\bar{x}_{\rm HI} = 0.21_{-0.07}^{+0.17}$ ($\bar{x}_{\rm HI} =
0.10_{<10^{-4}}^{+0.73}$) at $\langle z \rangle =6.10$, $\bar{x}_{\rm HI} =
0.21_{-0.07}^{+0.33}$ ($\bar{x}_{\rm HI} =0.57_{-0.47}^{+0.26}$) at $\langle z
\rangle =6.46$, and $\bar{x}_{\rm HI} = 0.37_{-0.17}^{+0.17}$ ($\bar{x}_{\rm
HI} =0.57_{-0.21}^{+0.26}$) at $\langle z \rangle =6.87$. At the same time we
constrain the average quasar lifetime to be $t_{\rm Q} \lesssim 7\ {\rm Myr}$
across all redshift bins, in good agreement with previous studies.
|
2401.10328v1
|
2024-03-04
|
Exploring Standing and Reflected Slow-mode Waves in Flaring Coronal Loops: A Parametric Study Using 2.5D MHD Modeling
|
Recent observations of reflected propagating and standing slow-mode waves in
hot flaring coronal loops have spurred our investigation into their underlying
excitation and damping mechanisms. To understand these processes, we conduct
2.5D magnetohydrodynamic (MHD) simulations using an arcade active region model
that includes a hot and dense loop. Our simulations allow for in-depth
parametric investigations complementing and expanding our previous 3D MHD
modeling results. We excite these waves in two distinct models as motivated by
observations from the SDO/AIA. Model 1 incorporates classical compressive
viscosity coefficient, while Model 2 adopts a 10-times enhanced viscosity
coefficient. We find that: (1) Our 2.5D MHD simulations reinforce previous
conclusions derived from 1D and 3D MHD models that significantly enhanced
viscosity is crucial for the rapid excitation of standing slow waves with
damping times consistent with observations by Wang et al. (2015). (2) We
uncover that nonlinearity in Model 1 delays the conversion of a reflected
propagating wave into a standing wave. In contrast, Model 2 exhibits a much
weak influence of nonlinearity. (3) Our results reveal that the transverse
temperature structure holds more influence on wave behavior than the density
structure. In Model 1, increased loop temperature contrast significantly
enhances wave trapping within the structure, mitigating the impact of
temperature-dependent viscous damping. Conversely, in Model 2, the impact of
temperature structure on wave behavior weakens in comparison to the effect of
viscosity. (4) Model 1 displays evident nonlinear coupling to the fast and kink
magnetoacoustic waves and pronounced wave leakage into the corona. However,
analyzing three observed wave events by SDO/AIA aligns with Model 2
predictions, providing further support for the substantial viscosity increase.
|
2403.02464v1
|
2000-02-17
|
Constraints on Cosmological Parameters from Future Galaxy Cluster Surveys
|
We study the expected redshift evolution of galaxy cluster abundance between
0 < z < 3 in different cosmologies, including the effects of the cosmic
equation of state parameter w=p/rho. Using the halo mass function obtained in
recent large scale numerical simulations, we model the expected cluster yields
in a 12 deg^2 Sunyaev-Zeldovich Effect (SZE) survey and a deep 10^4 deg^2 X-ray
survey over a wide range of cosmological parameters. We quantify the
statistical differences among cosmologies using both the total number and
redshift distribution of clusters. Provided that the local cluster abundance is
known to a few percent accuracy, we find only mild degeneracies between w and
either Omega_m or h. As a result, both surveys will provide improved
constraints on Omega_m and w. The Omega_m-w degeneracy from both surveys is
complementary to those found either in studies of CMB anisotropies or of
high-redshift Supernovae (SNe). As a result, combining these surveys together
with either CMB or SNe studies can reduce the statistical uncertainty on both w
and Omega_m to levels below what could be obtained by combining only the latter
two data sets. Our results indicate a formal statistical uncertainty of about
3% (68% confidence) on both Omega_m and w when the SZE survey is combined with
either the CMB or SN data; the large number of clusters in the X-ray survey
further suppresses the degeneracy between w and both Omega_m and h. Systematics
and internal evolution of cluster structure at the present pose uncertainties
above these levels. We briefly discuss and quantify the relevant systematic
errors. By focusing on clusters with measured temperatures in the X-ray survey,
we reduce our sensitivity to systematics such as non-standard evolution of
internal cluster structure.
|
0002336v2
|
2000-03-03
|
J-Band Infrared Spectroscopy of a Sample of Brown Dwarfs Using Nirspec on Keck II
|
Near-infrared spectroscopic observations of a sample of very cool, low-mass
objects are presented with higher spectral resolution than in any previous
studies. Six of the objects are L-dwarfs, ranging in spectral class from L2 to
L8/9, and the seventh is a methane or T-dwarf. These new observations were
obtained during commissioning of NIRSPEC, the first high-resolution
near-infrared cryogenic spectrograph for the Keck II 10-meter telescope on
Mauna Kea, Hawaii. Spectra with a resolving power of R=2500 from 1.135 to 1.360
microns (approximately J-band) are presented for each source. At this
resolution, a rich spectral structure is revealed, much of which is due to
blending of unresolved molecular transitions. Strong lines due to neutral
potassium (K I), and bands due to iron hydride (FeH) and steam (H2O) change
significantly throughout the L sequence. Iron hydride disappears between L5 and
L8, the steam bands deepen and the K I lines gradually become weaker but wider
due to pressure broadening. An unidentified feature occurs at 1.22 microns
which has a temperature dependence like FeH but has no counterpart in the
available FeH opacity data. Because these objects are 3-6 magnitudes brighter
in the near-infrared compared to the I-band, spectral classification is
efficient. One of the objects studied (2MASSW J1523+3014) is the coolest
L-dwarf discovered so far by the 2-Micron All-Sky Survey (2MASS), but its
spectrum is still significantly different from the methane-dominated objects
such as Gl229B or SDSS 1624+0029.
|
0003035v1
|
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