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2022-08-05
|
Quadratic Symmetric Polynomials and an analogue of the Davenport Constant
|
In this paper, we define the constant $D(\varphi, p)$, an analogue for the
Davenport constant, for sequences on the finite field $\mathbb{F}_p$, defined
via quadratic symmetric polynomials. Next, we state a series of results
presenting either the exact value of $D(\varphi, p)$, or lower and upper bounds
for this constant.
|
2208.03212v1
|
2022-08-15
|
Liouville Theorem on Ricci shrinkers with constant scalar curvature and its application
|
In this paper we consider harmonic functions on gradient shrinking Ricci
solitons with constant scalar curvature. A Liouville theorem is proved without
using gradient estimate : any bounded harmonic function is constant on gradient
shrinking Ricci solitons with constant scalar curvature. As an application, we
show that the space of harmonic functions with polynomial growth has finite
dimension.
|
2208.07101v1
|
2022-10-24
|
Isoparametric hypersurfaces and hypersurfaces with constant principal curvatures in Finsler spaces
|
In this paper, we study the relationship between isoparametric hypersurfaces
and hypersurfaces
with constant principal curvatures in Finsler spaces. We give some examples
of isoparametric hypersurfaces with (non)constant principal curvatures on
Randers manifolds with nonconstant flag curvatures. Furthermore, we construct
an example of a conformally flat Randers manifold which admits a family of
nonisoparametric hyperplanes with constant principal curvatures.
|
2210.12937v1
|
2022-11-21
|
A Cut-Matching Game for Constant-Hop Expanders
|
This paper provides a cut-strategy that produces constant-hop expanders in
the well-known cut-matching game framework.
Constant-hop expanders strengthen expanders with constant conductance by
guaranteeing that any demand can be (obliviously) routed along constant-hop
paths - in contrast to the $\Omega(\log n)$-hop routes in expanders.
Cut-matching games for expanders are key tools for obtaining
close-to-linear-time approximation algorithms for many hard problems, including
finding (balanced or approximately-largest) sparse cuts, certifying the
expansion of a graph by embedding an (explicit) expander, as well as computing
expander decompositions, hierarchical cut decompositions, oblivious routings,
multi-cuts, and multicommodity flows. The cut-matching game provided in this
paper is crucial in extending this versatile and powerful machinery to
constant-hop expanders. It is also a key ingredient towards close-to-linear
time algorithms for computing a constant approximation of multicommodity-flows
and multi-cuts - the approximation factor being a constant relies on the
expanders being constant-hop.
|
2211.11726v1
|
2023-03-14
|
Some fast convergent series for the mathematical constants $ζ(4)$ and $ζ(5)$
|
Recently, Sun [preprint, arXiv: 2210.07238v7] proposed two conjectural series
for the mathematical constant $\zeta(4)$ and two conjectural series for the
mathematical constant $\zeta(5)$. In terms of the operator method and two
hypergeometric transformations, we prove these four conjectures. Furthermore,
we also find some new series for the two constants in this paper.
|
2303.07887v2
|
2023-05-26
|
Geodesics on metrics of Eguchi-Hanson type
|
Geodesic equations are solved when at least two of $\theta$, $\phi$ and
$\psi$ are constant, or $r$ is constant, on scalar flat metrics of
Eguchi-Hanson type. They can also be solved also on Eguchi-Hanson metrics which
are Ricci flat if only $\phi$ is constant. However, the explicit solution of
the geodesic equations is not available yet if only $\psi$ is constant.
|
2305.18621v1
|
1996-10-28
|
QSO Absorbing Galaxies at z<~1: Deep Imaging and Spectroscopy in the Field of 3C 336
|
We present very deep WFPC2 images and FOS spectroscopy from the Hubble Space
Telescope (HST) together with numerous supporting ground-based observations of
the field of the quasar 3C 336 ($z_{em}=0.927$). The observations are designed
to investigate the nature of galaxies producing metal line absorption systems
in the spectrum of the QSO. Along a single line of sight, we find at least 6
metal line absorption systems (of which 3 are newly discovered) ranging in
redshift from 0.317 to 0.892. Through an extensive program of optical and IR
imaging, QSO spectroscopy, and faint galaxy spectroscopy, we have identified 5
of the 6 metal line absorption systems with luminous (L_K > 0.1 L*_K) galaxies.
These have morphologies ranging from very late-type spiral to S0, and exhibit a
wide range of inclination and position angles with respect to the QSO
sightline. The only unidentified absorber, despite our intensive search, is a
damped Lyman $\alpha$ system at $z_{abs}=0.656$. Analysis of the absorption
spectrum suggests that the metal abundances ([Fe/H]$=-1.2$) in this system are
similar to those in damped systems at $z \sim 2$, and to the two other damped
systems for which abundances have been determined at $z <1$. We have found no
examples of intrinsically faint galaxies ($L < 0.1 L^{\ast}$) at small impact
parameters that might have been missed as absorber candidates in our previous
ground-based imaging and spectroscopic programs on MgII absorbing galaxies.
There are no bright galaxies (L > 0.1 L_K) within 50h^{-1} kpc which do not
produce detectable metal lines (of Mg II 2796, 2803 and/or C IV 1548, 1550) in
the QSO spectrum. All of these results generally support the inferences which
we have previously reached from a larger survey for absorption-selected
galaxies at $z\simlt 1$.
|
9610230v1
|
1996-11-05
|
The nature of intermediate-redshift damped Ly-alpha absorbers
|
We present HST/WFPC2 high-spatial resolution images in the R and B bands of
the close environment of the sightlines to seven quasars which spectra show
either a damped Ly-alpha absorption line, 21cm absorption, or a very strong
MgII/FeII absorption system at intermediate redshifts (0.4 < z < 1). Objects
down to about 0.3", or 2.0 kpc at z=0.6 (H0 = 50 kms/s/Mpc, q0=0), and to a
limiting magnitude m(702, lim)=25.9 could be detected for seven fields
comprising eight absorbers (one at higher redshift z=1.78 towards MC 1331+170)
with high HI column densities of at least 1x10^20 cm^-2. In each case, a
candidate absorber with absolute magnitude Mb =-19.0 or much brighter has been
detected. This small sample of gas-rich galaxies at intermediate redshifts
covers a wide range in morphological types. There are three spiral galaxies of
various sizes and luminosities (towards 3C 196, Q 1209+107 and MC 1331+170),
three compact objects (towards EX 0302-223, PKS 0454+039 and, at high redshift,
MC 1331+170), and two amorphous, low surface brightness galaxies (towards PKS
1229-021 and 3C 286). In the fields around 3C 196, PKS 1229-021 and Q 1209+107,
there is an excess of galaxies in the PC2 images, suggestive of the presence of
a group of galaxies associated with the damped Ly-alpha absorber, or maybe with
the quasar itself for the two z = 1.0 cases. For 3C 196 and 3C 286, the quasar
host galaxies have also tentatively been discovered.
|
9611031v1
|
1997-07-08
|
The Formation of Galactic Disks
|
We study the population of galactic disks expected in current hierarchical
clustering models for structure formation. A rotationally supported disk with
exponential surface density profile is assumed to form with a mass and angular
momentum which are fixed fractions of those of its surrounding dark halo. We
assume that haloes respond adiabatically to disk formation, and that only
stable disks can correspond to real systems. With these assumptions the
predicted population can match both present-day disks and the damped Lyman
alpha absorbers in QSO spectra. Good agreement is found provided: (i) the
masses of disks are a few percent of those of their haloes; (ii) the specific
angular momenta of disks are similar to those of their haloes; (iii)
present-day disks were assembled recently (at z<1). In particular, the observed
scatter in the size-rotation velocity plane is reproduced, as is the slope and
scatter of the Tully-Fisher relation. The zero-point of the TF relation is
matched for a stellar mass-to-light ratio of 1 to 2 h in the I-band, consistent
with observational values derived from disk dynamics. High redshift disks are
predicted to be small and dense, and could plausibly merge together to form the
observed population of elliptical galaxies. In many (but not all) currently
popular cosmogonies, disks with rotation velocities exceeding 200 km/s can
account for a third or more of the observed damped Lyman alpha systems at
z=2.5. Half of the lines-of-sight to such systems are predicted to intersect
the absorber at r>3kpc/h and about 10% at r>10kpc/h. The cross-section for
absorption is strongly weighted towards disks with large angular momentum and
so large size for their mass. The galaxy population associated with damped
absorbers should thus be biased towards low surface brightness systems.
|
9707093v1
|
1997-09-26
|
No C+ emission from the z=3.137 damped Lyman-alpha absorber towards PC1643+4631A
|
We describe a search for redshifted [C II] in a z=3.137 damped Ly-alpha
absorption system that has a large neutral hydrogen column density and which
was controversially reported to be a source of CO emission, indicative of rapid
star-formation (Frayer, Brown & Vanden Bout 1994; Braine, Downes & Guilloteau
1996). There is no sign of [C II] emission in our spectrum, which was obtained
during excellent observing conditions at the James Clerk Maxwell Telescope
(JCMT) and covers 1890 km/s. The upper limit we have placed on the integrated
line intensity (3 sigma(T_MB) < 5.9 K km/s for a profile akin to that of the CO
lines) constrains the [C II]/CO(1-0) line-intensity ratio to 3 sigma < 8300,
based on the line intensity reported by Frayer et al. (1994), or to 3 sigma <
58700 based on the data obtained by Braine et al. (1996). These limits are
consistent with values measured in the Galactic plane and for nearby starburst
nuclei; the former, however, is significantly lower than the ratio found in
low-metallicity systems such as the Large Magellanic Cloud (which might be
expected to have much in common with a damped Ly-alpha absorption system at
high redshift). This can be taken as evidence against the reality of the CO
line detections, with the proviso that a system significantly larger than
present-day disk galaxies would NOT have been fully covered by our small beam
whereas it WOULD have been properly sampled by the Frayer et al. observations.
Finally, we demonstate (as did Ivison et al. 1996) that knitting together
overlapping bands can generate erroneous results - specifically, an emission
feature that has a width, profile and central velocity consistent with the
controversial CO emission lines and which could have drawn us to entirely the
wrong conclusions.
|
9709266v1
|
1998-04-06
|
Three-dimensional waves generated at Lindblad resonances in thermally stratified disks
|
We analyze the linear, 3D response to tidal forcing of a disk that is thin
and thermally stratified in the direction normal to the disk plane. We model
the vertical disk structure locally as a polytrope which represents a disk of
high optical depth. We solve the 3D gas-dynamic equations semi-analytically in
the neighborhood of a Lindblad resonance. These solutions match asymptotically
on to those valid away from resonances and provide solutions valid at all
radii. We obtain the following results. 1) A variety of waves are launched at
resonance. However, the f mode carries more than 95% of the torque exerted at
the resonance. 2) These 3D waves collectively transport exactly the amount of
angular momentum predicted by the 2D torque formula. 3) Near resonance, the f
mode occupies the full vertical extent of the disk. Away from resonance, the f
mode becomes confined near the surface of the disk, and, in the absence of
other dissipation mechanisms, damps via shocks. The radial length scale for
this process is roughly r_L/m (for resonant radius r_L and azimuthal wavenumber
m), independent of the disk thickness H. This wave channeling process is due to
the variations of physical quantities in r and is not due to wave refraction.
4) However, the inwardly propagating f mode launched from an m=2 inner Lindblad
resonance experiences relatively minor channeling.
We conclude that for binary stars, tidally generated waves in highly
optically thick circumbinary disks are subject to strong nonlinear damping by
the channeling mechanism, while those in circumstellar accretion disks are
subject to weaker nonlinear effects. We also apply our results to waves excited
by young planets for which m is approximately r/H and conclude that the waves
are damped on the scale of a few H.
|
9804063v1
|
2000-11-23
|
Near-Infrared Integral Field Spectroscopy of Damped Lyman-alpha Systems
|
We assess the feasibility of detecting star formation in damped Lyman-alpha
systems (DLAs) at z>1 through near-infrared spectroscopy using the forthcoming
integral field units on 8m-class telescopes. Although their relation to
galaxies is not well established, high-z DLAs contain most of the neutral gas
in the Universe, and this reservoir is depleted with time - presumably through
star formation. Line emission should be an indicator of star formation
activity, but searches based on Lyman-alpha are unreliable because of the
selective extinction of this resonant UV line. Using more robust lines such as
H-alpha forces a move to the near-infrared at z>1. For line emission searches,
spectroscopy is more sensitive than imaging, but previous long-slit
spectroscopic searches have been hampered by the likelihood that any star
forming region in the DLA galaxy disk would fall outside the narrow slit. The
new integral field units such as CIRPASS on Gemini will cover sufficient solid
angles to intercept these, even in the extreme case of large galactic disks at
high redshift. On an 8m-class telescope, star formation rates of <1M_sun/yr
will be reached at z~1.4 with H-alpha in the H-band. Such star formation rates
are well below L* for the high-z Lyman-break population, and are comparable
locally to the luminous giant HII complexes in M101. It appears that
instruments such as CIRPASS on Gemini will have both the sensitivity and the
survey area to measure star formation rates in z>1 DLAs. These observations
will probe the nature of damped Lyman-alpha systems and address their relation
to galaxies.
|
0011421v1
|
2001-12-03
|
Euler, Jacobi, and Missions to Comets and Asteroids
|
Whenever a freely spinning body is found in a complex rotational state, this
means that either the body is a recent victim of an impact or a tidal
interaction, or is a fragment of a recently disrupted progenitor. Another
factor (relevant for comets) is outgassing. Due to impacts, tidal forces and
outgassing, the asteroidal and cometary precession must be a generic
phenomenon: while some rotators are in the state of visible tumbling, a much
larger amount of objects must be performing narrow-cone precession not so
easily observable from the Earth. The internal dissipation in a freely
precessing top leads to relaxation (gradual damping of the precession) and
sometimes to spontaneous changes in the rotation axis. Recently developed
theory of dissipative precession of a rigid body reveals that this is a highly
nonlinear process: while the body is precessing at an angular rate $ \omega$,
the precession-caused stresses and strains in the body contain components
oscillating at other frequencies. Dependent upon the spin state, those
frequencies may be higher or, most remarkably, lower than the precession rate.
In many states dissipation at the harmonics is comparable to or even exceeds
that at the principal frequency. For this and other reasons, in many spin
states the damping of asteroidal and cometary wobble happens faster, by several
orders, than believed previously. This makes it possible to measure the
precession-damping rate. The narrowing of the precession cone through the
period of about a year can be registered by the currently available
spacecraft-based observational means. However, in the near-separatrix spin
states a precessing rotator can considerably slow down its relaxation.
|
0112054v3
|
2002-06-17
|
The UCSD HIRES/KECK I Damped Lya Abundance Database: IV. Probing Galactic Enrichment Histories with Nitrogen
|
We present 14 N^0 measurements from our HIRES/Keck database of damped Lya
abundances. These data are combined with measurements from the recent and past
literature to build an homogeneous, uniform set of observations. We examine
photoionization diagnostics like Fe^++ and Ar^0 in the majority of the complete
sample and assess the impact of ionization corrections on N/alpha and alpha/H
values derived from observed ionic column densities of N^0, Si^+, H^0, and S^+.
Our final sample of 19 N/alpha, alpha/H pairs appears bimodal; the majority of
systems show N/alpha values consistent with metal-poor emission regions in the
local universe but a small sub-sample exhibit significantly lower N/alpha
ratios. Contrary to previous studies of N/alpha in the damped systems, our
sample shows little scatter within each sub-sample. We consider various
scenarios to explain the presence of the low N/alpha sightlines and account for
the apparent bimodality. We favor a model where at least some galaxies undergo
an initial burst of star formation with suppressed formation of
intermediate-mass stars. We found a power-law IMF with slope 0.10 or a mass cut
of ~5-8 Msolar would successfully reproduce the observed LN-DLA values. If the
bimodal distribution is confirmed by a larger sample of measurements, this may
present the first observational evidence for a top heavy initial mass function
in some early stellar populations.
|
0206296v1
|
2003-08-11
|
The Nature of Damped Ly-alpha Absorbing Galaxies at z<=1--A Photometric Redshift Survey of Damped Ly-alpha Absorbers
|
We study the nature of damped Lya absorption (DLA) systems at z<=1 using a
sample of 11 DLA galaxies, for which accurate redshift measurements are
available. We demonstrate that the precision of photometric redshifts is
sufficient for identifying DLA galaxies, because DLAs are rare and their
intrinsically high column density implies a small impact parameter of the host
galaxy to the QSO line of sight. We adopt this first large DLA galaxy sample to
study the neutral gas cross section of intermediate-redshift galaxies and
examine the optical properties of DLA galaxies at z<=1. The results of our
study are: (1) the extent of neutral gas around intermediate-redshift galaxies
scales with B-band luminosity as R/R_* = [L_B/L_{B_*}]^{\beta} with R_*=24-30
h^{-1} kpc and \beta = 0.26_{-0.06}^{+0.24} at N(HI)=10^{20} cm^{-2}; (2) the
observed incidence of the DLAs versus the B-band luminosity of the DLA galaxies
is consistent with models derived from adopting a known galaxy B-band
luminosity function and the best-fit scaling relation of the neutral gas cross
section at M_B - 5\log h <= -17; (3) comparison of the observed and predicted
number density of DLAs supports that luminous galaxies can explain most of the
DLAs found in QSO absorption line surveys and a large contribution of dwarfs
(M_B - 5\log h >= -17) to the total neutral gas cross section is not necessary;
(4) of the 11 DLAs studied, 45% are disk dominated, 22% are bulge dominated,
11% are irregular, and 22% are in galaxy groups, indicating that galaxies that
give rise to the DLAs span a wide range of morphological types and arise in a
variety of galaxy environment; (Abridged)
|
0308190v1
|
2004-07-21
|
Discovery of a Primitive Damped Lyman alpha Absorber Near an X-ray Bright Galaxy Group in the Virgo Cluster
|
We present a new UV echelle spectrum of PG1216+069, obtained with HST+STIS,
which reveals damped Lya (DLA) absorption as well as O I, C II, Si II, and Fe
II absorption lines at z(abs) = 0.00632 near the NGC4261 group. The absorber
shows no evidence of highly-ionized gas, which places constraints on "warm-hot"
missing baryons in the NGC4261 group. The well-developed damping wings of the
Lya line tightly constrain the H I column density; we find log N(H I) =
19.32+/-0.03. The metallicity of this sub-DLA is remarkably low, [O/H] =
-1.60^{+0.09}_{-0.11}, which is comparable to many analogous high-redshift
systems, and the iron abundance indicates that this absorber contains little or
no dust. Nitrogen is underabundant; we detect neither N I or N II, and we show
that this is not due to ionization effects but rather indicates that [N/O] <
-0.28 (3sigma). Despite the proximity to NGC4261 group, there are no bright
galaxies close to the sight line at the absorption redshift. The nearest known
galaxy is a sub-L* galaxy with a projected distance rho = 86 kpc; the closest
L* galaxy is NGC4260 at rho = 246 kpc. The low metallicity and [N/O] indicate
that this low-z sub-DLA is a relatively primitive gas cloud. We consider the
nature and origin of the sub-DLA, and we find several possibilities. The
properties of the sub-DLA are similar to those of the interstellar media in
blue compact dwarf galaxies and are also reminiscent of Milky Way HVCs. Or, the
object could simply be a small dark-matter halo, self-enriched by a small
amount of internal star formation but mostly undisturbed since its initial
formation. In this case, the small halo would likely be an ancient building
block of galaxy formation that formed before the epoch of reionization.
|
0407465v2
|
2004-08-27
|
Detection of 21 Centimeter HI Absorption at z = 0.78 in a Survey of Radio Continuum Sources
|
We report the detection of a deep broad HI 21 cm absorption system at z =
0.78 toward the radio source [HB89] 2351+456 (4C+45.51) at z = 1.992. The HI
absorption was identified in a blind spectral line survey conducted at the
Green Bank Telescope spanning 0.63 < z < 1.10 toward a large sample of radio
continuum sources. The HI column density is N(HI) = 2.35 x 10^19 (T_s/f) cm^-2,
where T_s is the spin temperature and f is the continuum covering factor of the
absorbing gas. For T_s/f > 8.5 K, this system is by definition a damped Ly
alpha absorption system (N(HI) >= 2 x 10^20 cm^-2). The line is unusually
broad, with a FWHM of 53 km/s and a full span of 163 km/s, suggesting a
physically extended HI gas structure. Radio surveys identify damped Ly alpha
systems in a manner that bypasses many of the selection effects present in
optical/UV surveys, including dust extinction and the atmospheric cutoff for z
< 1.65. The smooth broad profile of this HI 21 cm absorption system is similar
to the z = 0.89 HI absorption toward PKS 1830-211, which suggests that the
absorber toward [HB89] 2351+456 is also a gravitational lens and a molecular
absorption system. However, very long baseline interferometry and Hubble Space
Telescope observations show little evidence for gravitational lensing, and BIMA
millimeter observations show no HCO+ (1-2) or HCN (1-2) absorption down to tau
= 0.15 (3 sigma) in 5 km/s channels. Although this radio damped Ly alpha
selection technique would include dusty, molecule-rich systems, [HB89] 2351+456
appears to be a ``vanilla'' HI 21 cm absorber.
|
0408531v1
|
2005-03-17
|
The first WIMPy halos
|
Dark matter direct and indirect detection signals depend crucially on the
dark matter distribution. While the formation of large scale structure is
independent of the nature of the cold dark matter (CDM), the fate of
inhomogeneities on sub-galactic scales, and hence the present day CDM
distribution on these scales, depends on the micro-physics of the CDM
particles. We study the density contrast of Weakly Interacting Massive
Particles (WIMPs) on sub-galactic scales. We calculate the damping of the
primordial power spectrum due to collisional damping and free-streaming of
WIMPy CDM and show that free-streaming leads to a CDM power spectrum with a
sharp cut-off at about $10^{-6} M_\odot$. We also calculate the transfer
function for the growth of the inhomogeneities in the linear regime, taking
into account the suppression in the growth of the CDM density contrast after
matter-radiation equality due to baryons and show that our analytic results are
in good agreement with numerical calculations. Combining the transfer function
with the damping of the primordial fluctuations we produce a WMAP normalized
primordial CDM power spectrum, which can serve as an input for high resolution
CDM simulations. We find that the smallest inhomogeneities typically have
co-moving radius of about 1 pc and enter the non-linear regime at a redshift of
$60 \pm 20$. We study the effect of scale dependence of the primordial power
spectrum on these numbers and also use the spherical collapse model to make
simple estimates of the properties of the first generation of WIMP halos to
form. We find that the very first WIMPy halos may have a significant impact on
indirect dark matter searches.
|
0503387v2
|
2005-06-30
|
Molecular Hydrogen in the Damped Ly alpha Absorber of Q1331+170
|
We used HST/STIS to obtain the spectrum of molecular hydrogen associated with
the damped Ly$\alpha$ system at $z_{\rm abs}=1.7765$ toward the quasar
Q1331+170 at $z_{\rm em}=2.084$. Strong ${\rm H}_2$ absorption was detected,
with a total ${\rm H}_2$ column density of $N({\rm H}_2)=(4.45\pm 0.36)\times
10^{19} {\rm cm^{-2}}$.The molecular hydrogen fraction is $f_{{\rm
H}_2}=\frac{2N_{\rm H_2}}{N_{\rm HI}+2N_{\rm H_2}}=(5.6\pm 0.7)%$, which is the
greatest value reported so far in any redshifted damped Ly$\alpha$ system. This
results from the combined effect of a relatively high dust-to-gas ratio, a low
gas temperature, and an extremely low ambient UV radiation field. Based on the
observed population of $J$ states, we estimate the photo-absorption rate to be
$R_{\rm abs}=(7.6\pm 2.4)\times 10^{-13} {\rm s^{-1}}$, corresponding to a
local UV radiation field of $J(1000{\rm \AA})\approx 2.1\times 10^{-3}
J_{1000{\rm \AA},\odot}$, where $J_{1000{\rm \AA},\odot}$ is the UV intensity
at $1000 \AA$ in the solar neighborhood. This is comparable with the
metagalactic UV background intensity at this redshift, and implies an extremely
low star formation rate in the absorber's environment. The observed CO-to-H$_2$
column density ratio is $\frac{N_{\rm CO}}{N_{\rm H_2}}<2.5\times 10^{-7}$,
which is similar to the value measured for diffuse molecular clouds in the
Galactic ISM. Finally, applying the inferred physical conditions to the
observed C I fine structure excitation (Songaila {\it et al.} 1994), we
estimate the cosmic microwave background temperature to be $T_{\rm CMB}=(7.2\pm
0.8) {\rm K}$ at $z=1.77654$, consistent with the predicted value of $7.566
{\rm K}$ from the standard cosmology.
|
0506766v1
|
1997-05-06
|
Resonant Raman Scattering in Antiferromagnets
|
Two-magnon Raman scattering provides important information about electronic
correlations in the insulating parent compounds of high-$T_c$ materials. Recent
experiments have shown a strong dependence of the Raman signal in $B_{1g}$
geometry on the frequency of the incoming photon. We present an analytical and
numerical study of the Raman intensity in the resonant regime. It has been
previously argued by one of us (A.Ch) and D. Frenkel that the most relevant
contribution to the Raman vertex at resonance is given by the triple resonance
diagram. We derive an expression for the Raman intensity in which we
simultaneously include the enhancement due to the triple resonance and a final
state interaction. We compute the two-magnon peak height (TMPH) as a function
of incident frequency and find two maxima at $\omega^{(1)}_{res} \approx
2\Delta + 3J$ and $\omega^{(2)}_{res} \approx 2\Delta + 8J$. We argue that the
high-frequency maximum is cut only by a quasiparticle damping, while the
low-frequency maximum has a finite amplitude even in the absence of damping. We
also obtain an evolution of the Raman profile from an asymmetric form around
$\omega^{(1)}_{res}$ to a symmetric form around $\omega^{(2)}_{res}$. We
further show that the TMPH depends on the fermionic quasiparticle damping, the
next-nearest neighbor hopping term $t^{\prime}$ and the corrections to the
interaction vertex between light and the fermionic current. We discuss our
results in the context of recent experiments by Blumberg et al. on
$Sr_2CuO_2Cl_2$ and $YBa_2Cu_3O_{6.1}$ and R\"{u}bhausen et al. on
$PrBa_2Cu_3O_7$ and show that the triple resonance theory yields a qualitative
and to some extent also quantitative understanding of the experimental data.
|
9705051v1
|
1998-09-15
|
Solid friction at high sliding velocities: an explicit 3D dynamical SPH approach
|
We present realistic 3D numerical simulations of elastic bodies sliding on
top of each other in a regime of velocities ranging from meters to tens of
meters per second using the so-called Smoothed Particle Hydrodynamics (SPH)
method. Our investigations are restricted to regimes of pressure and roughness
where only elastic deformations occur between asperities at the contact surface
between the slider block and the substrate. In this regime, solid friction is
due to the generation of vibrational radiations which are subsequently damped
out. We study periodic commensurate and incommensurate asperities and various
types of disordered surfaces. We report the evidence of a transition from zero
(or non-measurable $\mu < 0.001$) friction to a finite friction as the normal
pressure increases above about $10^6~Pa$. For larger normal pressures (up to
$10^9~Pa$), we find a remarkably universal value for the friction coefficient
$\mu \approx 0.06$, which is independent of the internal dissipation strength
over three order of magnitudes, and independent of the detailled nature of the
slider block-substrate interactions. We find that disorder may either decrease
or increase $\mu$ due to the competition between two effects: disorder detunes
the coherent vibrations of the asperties that occur in the periodic case,
leading to weaker acoustic radiation and thus weaker damping. On the other
hand, large disorder leads to stronger vibration amplitudes at local asperities
and thus stronger damping. Our simulations have confirmed the existence of
jumps over steps or asperities of the slider blocks occurring at the largest
velocities studied ($10~m/s$). These jumps lead to chaotic motions similar to
the bouncing-ball problem. We find a velocity strengthening with a doubling of
the friction coefficient as the velocity increases from $1~m/s$ to $10~m/s$.
|
9809213v1
|
2000-03-10
|
Competing orders and quantum criticality in doped antiferromagnets
|
We use a number of large-N limits to explore the competition between ground
states of square lattice doped antiferromagnets which break electromagnetic
U(1), time-reversal, or square lattice space group symmetries. Among the states
we find are d-, (s+id)-, and (d+id)-wave superconductors, Wigner crystals,
Wigner crystals of hole pairs, orbital antiferromagnets (or staggered-flux
states), and states with spin-Peierls and bond-centered charge stripe order. In
the vicinity of second-order quantum phase transitions between the states, we
go beyond the large-N limit by identifying the universal quantum field theories
for the critical points, and computing the finite temperature, quantum-critical
damping of fermion spectral functions. We identify candidate critical points
for the recently observed quantum-critical behavior in photoemission
experiments on BSCCO by Valla et al. (Science 285, 2110 (1999)). These involve
onset of a charge density wave, or of broken time-reversal symmetry with (d+id)
or (s+id) pairing, in a d-wave superconductor. It is not required (although it
is allowed) that the stable state in the doped cuprates to be anything other
than the d-wave superconductor--the other states need only be stable nearby in
parameter space. At finite temperatures, fluctuations associated with these
nearby states lead to the observed fermion damping in the vicinity of the nodal
points in the Brillouin zone. The cases with broken time-reversal symmetry are
appealing because the order parameter is not required to satisfy any special
commensurability conditions. The observed absence of inelastic damping of
quasiparticles with momenta (pi,k), (k,pi) (with 0 < k < pi) also appears very
naturally for the case of a transition to (d+id) order.
|
0003163v7
|
2001-12-03
|
Theory of proximity effect in superconductor/ferromagnet heterostructures
|
We present a microscopic theory of proximity effect in the
ferromagnet/superconductor/ferromagnet (F/S/F) nanostructures where S is s-wave
low-T_c superconductor and F's are layers of 3d transition ferromagnetic metal.
Our approach is based on the solution of Gor'kov equations for the normal and
anomalous Green's functions together with a self-consistent evaluation of the
superconducting order parameter. We take into account the elastic
spin-conserving scattering of the electrons assuming s-wave scattering in the S
layer and s-d scattering in the F layers. In accordance with the previous
quasiclassical theories, we found that due to exchange field in the ferromagnet
the anomalous Green's function F(z) exhibits the damping oscillations in the
F-layer as a function of distance z from the S/F interface. In the given model
a half of period of oscillations is determined by the length \xi_m^0 = \pi
v_F/E_ex, where v_F is the Fermi velocity and E_ex is the exchange field, while
damping is governed by the length l_0 = (1/l_{\uparrow} +
1/l_{\downarrow})^{-1} with l_{\uparrow} and l_{\downarrow} being
spin-dependent mean free paths in the ferromagnet. The superconducting
transition temperature T_c(d_F) of the F/S/F trilayer shows the damping
oscillations as a function of the F-layer thickness d_F with period \xi_F =
\pi/\sqrt{m E_ex}, where m is the effective electron mass. We show that strong
spin-conserving scattering either in the superconductor or in the ferromagnet
significantly suppresses these oscillations. The calculated T_c(d_F)
dependences are compared with existing experimental data for Fe/Nb/Fe trilayers
and Nb/Co multilayers.
|
0112034v3
|
2007-07-27
|
C IV absorption in damped and sub-damped Lyman-alpha systems: correlations with metallicity and implications for galactic winds at z~2-3
|
We present a study of C IV absorption in a sample of 63 damped Lyman-alpha
(DLA) systems and 11 sub-DLAs in the redshift range 1.75<z_abs<3.61, using a
dataset of high-resolution (6.6 km/s FWHM), high signal-to-noise VLT/UVES
spectra. Narrow and broad C IV absorption line components indicate the presence
of both warm, photoionized and hot, collisionally ionized gas. We report new
correlations between the metallicity (measured in the neutral-phase) and each
of the C IV column density, the C IV total line width, and the maximum C IV
velocity. We explore the effect on these correlations of the sub-DLAs, the
proximate DLAs (defined as those within 5 000 km/s of the quasar), the
saturated absorbers, and the metal line used to measure the metallicity, and we
find the correlations to be robust. There is no evidence for any difference
between the measured properties of DLA C IV and sub-DLA C IV. In 25 DLAs and 4
sub-DLAs, covering 2.5 dex in [Z/H], we directly observe C IV moving above the
escape speed, where v_esc is derived from the total line width of the neutral
gas profiles. These high-velocity C IV clouds, unbound from the central
potential well, can be interpreted as highly ionized outflowing winds, which
are predicted by numerical simulations of galaxy feedback. The distribution of
C IV column density in DLAs and sub-DLAs is similar to the distribution in
Lyman Break galaxies, where winds are directly observed, supporting the idea
that supernova feedback creates the ionized gas in DLAs. The unbound C IV
absorbers show a median mass flow rate of ~22(r/40 kpc) solar masses per year,
where r is the characteristic C IV radius. Their kinetic energy fluxes are
large enough that a star formation rate (SFR) of ~2 solar masses per year is
required to power them.
|
0707.4065v2
|
2008-10-01
|
Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations
|
We study the global stability of non-axisymmetric p-modes (also called
inertial-acoustic modes) trapped in the inner-most regions of accretion discs
around black holes. We show that the lowest-order (highest-frequency) p-modes,
with frequencies $\omega=(0.5-0.7) m\Omega_{\rm ISCO}$, can be overstable due
to general relativistic effects, according to which the radial epicyclic
frequency is a non-monotonic function of radius near the black hole. The mode
is trapped inside the corotation resonance radius and carries a negative
energy. The mode growth arises primarily from wave absorption at the corotation
resonance, and the sign of the wave absorption depends on the gradient of the
disc vortensity. When the mode frequency is sufficiently high, such that the
slope of the vortensity is positive at corotation positive wave energy is
absorbed at the resonance, leading to the growth of mode amplitude. We also
study how the rapid radial inflow at the inner edge of the disc affects the
mode trapping and growth. Our analysis of the behavior of the fluid
perturbations in the transonic flow near the ISCO indicates that, while the
inflow tends to damp the mode, the damping effect is sufficiently small under
some conditions so that net mode growth can still be achieved. We further
clarify the role of the Rossby wave instability and show that it does not
operate for black hole accretion discs with smooth-varying vortensity profiles.
Overstable non-axisymmetric p-modes driven by the corotational instability
provide a plausible explanation for the high-frequency (> 100 Hz)
quasi-periodic oscillations (HFQPOs) observed from a number of black-hole X-ray
binaries in the very high state. The absence of HFQPOs in the soft (thermal)
state may result from mode damping due to the radial infall at the ISCO.
|
0810.0203v2
|
2008-10-03
|
Stochastic excitation of nonradial modes II. Are solar asymptotic gravity modes detectable?
|
Detection of solar gravity modes remains a major challenge to our
understanding of the innerparts of the Sun. Their frequencies would enable the
derivation of constraints on the core physical properties while their
amplitudes can put severe constraints on the properties of the inner convective
region. Our purpose is to determine accurate theoretical amplitudes of solar g
modes and estimate the SOHO observation duration for an unambiguous detection.
We investigate the stochastic excitation of modes by turbulent convection as
well as their damping. Input from a 3D global simulation of the solar
convective zone is used for the kinetic turbulent energy spectrum. Damping is
computed using a parametric description of the nonlocal time-dependent
convection-pulsation interaction. We then provide a theoretical estimation of
the intrinsic, as well as apparent, surface velocity. Asymptotic g-mode
velocity amplitudes are found to be orders of magnitude higher than previous
works. Using a 3D numerical simulation, from the ASH code, we attribute this to
the temporal-correlation between the modes and the turbulent eddies which is
found to follow a Lorentzian law rather than a Gaussian one as previously used.
We also find that damping rates of asymptotic gravity modes are dominated by
radiative losses, with a typical life-time of $3 \times 10^5$ years for the
$\ell=1$ mode at $\nu=60 \mu$Hz. The maximum velocity in the considered
frequency range (10-100 $\mu$Hz) is obtained for the $\ell=1$ mode at $\nu=60
\mu$Hz and for the $\ell=2$ at $\nu=100 \mu$Hz. Due to uncertainties in the
modeling, amplitudes at maximum i.e. for $\ell=1$ at 60 $\mu$Hz can range from
3 to 6 mm s$^{-1}$.
|
0810.0602v2
|
2008-10-20
|
The kinematic signature of damped Lyman alpha systems: Using the D-index to screen for high column density HI absorbers
|
Using a sample of 21 damped Lyman alpha systems (DLAs) and 35 sub-DLAs, we
evaluate the D-index = EW / Delta v x 1000 from high resolution spectra of the
MgII lambda 2796 profile. This sample represents an increase in sub-DLA D-index
statistics by a factor of four over the sample used by Ellison (2006). We
investigate various techniques to define the velocity spread (Delta v) of the
MgII line to determine an optimal D-index for the identification of DLAs. The
success rate of DLA identification is 50 -- 55%, depending on the velocity
limits used, improving by a few percent when the column density of FeII is
included in the D-index calculation. We recommend the set of parameters that is
judged to be most robust, have a combination of high DLA identification rate
(57%) and low DLA miss rate (6%) and most cleanly separate the DLAs and
sub-DLAs (Kolmogorov-Smirnov probability 0.5%). These statistics demonstrate
that the D-index is the most efficient technique for selecting low redshift DLA
candidates: 65% more efficient than selecting DLAs based on the equivalent
widths of MgII and FeII alone. We also investigate the effect of resolution on
determining the N(HI) of sub-DLAs. We convolve echelle spectra of sub-DLA Lya
profiles with Gaussians typical of the spectral resolution of instruments on
the Hubble Space Telescope and compare the best fit N(HI) values at both
resolutions. We find that the fitted HI column density is systematically
over-estimated by ~ 0.1 dex in the moderate resolution spectra compared to the
best fits to the original echelle spectra. This offset is due to blending of
nearby Lya clouds that are included in the damping wing fit at low resolution.
|
0810.3700v1
|
2009-01-24
|
Dynamic migration of rotating neutron stars due to a phase transition instability
|
Using numerical simulations based on solving the general relativistic
hydrodynamic equations, we study the dynamics of a phase transition in the
dense core of isolated rotating neutron stars, triggered by the back bending
instability reached via angular momentum loss. In particular, we investigate
the dynamics of a migration from an unstable configuration into a stable one,
which leads to a mini-collapse of the neutron star and excites sizeable
pulsations in its bulk until it acquires a new stable equilibrium state. We
consider equations of state with softening at high densities, a simple analytic
one with a mixed hadron-quark phase in an intermediate pressure interval and
pure quark matter at very high densities, and a microphysical one that has a
first-order phase transition, originating from kaon condensation. Although the
marginally stable initial models are rigidly rotating, we observe that during
the collapse (albeit little) differential rotation is created. We analyze the
emission of gravitational radiation, which in some models is amplified by mode
resonance effects, and assess its prospective detectability by interferometric
detectors. We expect that the most favorable conditions for dynamic migration
exist in very young magnetars. We find that the damping of the post-migration
pulsations strongly depends on the character of the equation of state
softening. The damping of pulsations in the models with the microphysical
equation of state is caused by dissipation associated with matter flowing
through the density jump at the edge of the dense core. If at work, this
mechanism dominates over all other types of dissipation, like bulk viscosity in
the exotic-phase core, gravitational radiation damping, or numerical viscosity.
|
0901.3819v2
|
2009-02-12
|
New Magellan Inamori Kyocera Echelle Observations of z<1.5 sub-damped Lyman-alpha systems
|
The Damped and sub-Damped Lyman-alpha (DLA and sub-DLA) systems seen in the
spectra of QSOs offer a unique way to study the interstellar medium of high
redshift galaxies. In this paper we report on new abundance determinations in a
sample of 10 new systems, nine of the lesser studied sub-DLAs and one DLA,
along the line of sight to seven QSOs from spectra taken with the MIKE
spectrograph. Lines of Mg I, Mg II, Al II, Al III, Ca II, Mn II, Fe II, and Zn
II were detected. Here, we give the column densities and equivalent widths of
the observed absorption lines, as well as the abundances determined for these
systems. Zn, a relatively undepleted element in the local interstellar medium
(ISM) is detected in one system with a high metallicity of [Zn/H]=+0.27\pm0.18.
In one other system, a high abundance based on the more depleted element Fe is
seen with [Fe/H]=-0.37\pm0.13, although Zn is not detected. The N(HI)-weighted
mean metallicity of these sub-DLA systems based on Fe is <[Fe/H]>=-0.76\pm0.11,
nearly ~0.7 dex higher (a factor of 5) than what is seen in DLAs in this
redshift range. The relative abundance of [Mn/Fe] is also investigated. A clear
trend is visible for these systems as well as systems from the literature, with
[Mn/Fe] increasing with increasing metallicity in good agreement with with
Milky Way stellar abundances.
|
0902.2022v2
|
2009-04-16
|
Revisiting the origin of the high metallicities of sub-damped Lyman-alpha systems
|
Sub-damped Lyman-alpha systems (sub-DLAs) have previously been found to
exhibit a steeper metallicity evolution than the classical damped Lyman-alpha
systems (DLAs), evolving to close to solar metallicity by z~1. From new
high-resolution spectra of 17 sub-DLAs we have increased the number of
measurements of [Fe/H] at z<1.7 by 25% and compiled the most complete
literature sample of sub-DLA and DLA abundances to date. We find that sub-DLAs
are indeed significantly more metal-rich than DLAs, but only at z<1.7; the
metallicity distributions of sub-DLAs and DLAs at z>1.7 are statistically
consistent. We also present the first evidence that sub-DLAs follow a velocity
width-metallicity correlation over the same velocity range as DLAs, but the
relation is offset to higher metallicities than the DLA relation. On the basis
of these results, we revisit the previous explanation that the systematically
higher metallicities observed in sub-DLAs are indicative of higher host galaxy
masses. We discuss the various problems that this interpretation encounters and
conclude that in general sub-DLAs are not uniquely synonymous with massive
galaxies. We rule out physically related sources of bias (dust, environment,
ionization effects) and examine systematics associated with the selection and
analysis of low-redshift sub-DLAs. We propose that the high metallicities of
sub-DLAs at z<1.7 that drives an apparently steep evolution may be due to the
selection of most low-redshift sub-DLAs based on their high MgII equivalent
widths.
|
0904.2531v1
|
2009-05-27
|
A MIKE + UVES survey of Sub-Damped Lyman-Alpha Systems at z<1.5
|
We have combined the results from our recent observations of Damped and
sub-Damped Lyman-alpha systems with the MIKE and UVES spectrographs on the
Magellan Clay and VLT Kueyen telescopes with ones from the literature to
determine the N(HI)-weighted mean metallicity of these systems based both on
Fe, a depleted element in QSO absorbers and the local ISM, and Zn a relatively
undepleted element. In each case, the N(HI)-weighted mean metallicity is higher
and shows faster evolution in sub-DLAs than the classical DLA systems. Large
grids of photoionisation models over the sub-DLA \nhI range with CLOUDY show
that the ionisation corrections to the abundances are in general small, however
the fraction of ionized H can be up to ~90 per cent. The individual spectra
have been shifted to the rest frame of the absorber and averaged together to
determine the average properties of these systems at z<1.5. We find that the
average abundance pattern of the Sub-DLA systems is similar to the gas in the
halo of the Milky Way, with an offset of ~0.3 dex in the overall metallicity.
Both DLAs and Sub-DLAs show similar characteristics in their relative
abundances patterns, although the DLAs have smaller <[Mn/Zn]> as well as higher
<[Ti/Zn]> and <[Cr/Zn]>. We calculate the contribution of sub-DLAs to the metal
budget of the Universe, and find that the sub-DLA systems at z<1.5 contain a
comoving density of metals Omega_met (3.5-15.8)x10^{5} M_sun Mpc^{-3}, at least
twice the comoving density of metals in the DLA systems. The sub-DLAs do
however track global chemical evolution models much more closely than do the
DLAs, perhaps indicating that they are a less dust biased metallicity indicator
of galaxies at high redshifts than the DLA systems.
|
0905.4473v2
|
2009-12-14
|
Using 21cm Absorption in Small Impact Parameter Galaxy-QSO Pairs to Probe Low-Redshift Damped and Sub-Damped Lyman-alpha System
|
To search for low-redshift damped Lyman-alpha (DLA) and sub-DLA quasar
absorbers, we have conducted a 21cm absorption survey of radio-loud quasars at
small impact parameters to foreground galaxies selected from the Sloan Digital
Sky Survey (SDSS). Here we present the first results from this survey based on
observations of SDSS J104257.58+074850.5 ($z_{QSO}$ = 2.66521), a quasar at an
angular separation from a foreground galaxy ($z_{gal}$ = 0.03321) of 2.5" (1.7
kpc in projection). The foreground galaxy is a low-luminosity spiral with
on-going star formation (0.004 M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$) and a
metallicity of $-0.27 \pm 0.05$ dex. We detect 21cm absorption from the galaxy
with the Green Bank Telescope (GBT), the Very Large Array (VLA), and the Very
Long Baseline Array (VLBA). The absorption appears to be quiescent disk gas
corotating with the galaxy and we do not find any evidence for outflowing cold
neutral gas. The width of the main absorption line indicates that the gas is
cold, $T_{k} < 283$ K, and the HI column is surprisingly low given the impact
parameter of 1.7 kpc; we find that N(HI) $\leq 9.6 \times 10^{19}$ cm$^{-2}$
(GBT) and N(HI) $\leq 1.5 \times 10^{20}$ cm$^{-2}$ (VLBA). VLBA marginally
resolves the continuum source and the absorber, and a lower limit of 27.1
$\times$ 13.9 pc is derived for the size of the absorbing cloud. In turn, this
indicates a low density for a cold cloud, n(HI) $<$ 3.5 cm$^{-3}$. We
hypothesize that this galaxy, which is relatively isolated, is becoming
depleted in HI because it is converting its interstellar matter into stars
without a replenishing source of gas, and we suggest future observations to
probe this and similar galaxies.
|
0912.2575v2
|
2010-04-21
|
Kadanoff-Baym description of Hubbard clusters out of equilibrium: performance of many-body schemes, correlation-induced damping and multiple steady states
|
We present in detail a method we recently introduced (PRL. 103, 176404
(2009)) to describe finite systems in and out of equilibrium, where the
evolution in time is performed via the Kadanoff-Baym Equations (KBE) within
Many-Body Perturbation Theory (MBPT). The main property we analyze is the
time-dependent density. We also study is the exchange-correlation potential of
TDDFT, obtained via reverse engineering from the time-dependent density. Our
systems consist of small, strongly correlated clusters, described by a Hubbard
Hamiltonian within the Hartree-Fock, second Born, GW and T-matrix
approximations. We compare the results from the KBE dynamics to those from
exact numerical solutions. The outcome of our comparisons is that, among the
many-body schemes considered, the T-matrix approximation is overall superior at
all electron densities. Such comparisons permit a general assessment of the
whole idea of applying MBPT, in the KBE sense, to finite systems. A striking
outcome of our analysis is that when the system evolves under a strong external
field, the KBE develop a steady-state solution as a consequence of a
correlation-induced damping. This damping is present both in isolated (finite)
systems, where it is purely artificial, as well as in clusters contacted to
(infinite) macroscopic leads. To illustrate this point we present selected
results for a system coupled to contacts within the T-matrix and second Born
approximation. The extensive characterization we performed indicates that this
behavior is present whenever approximate self energies, based upon infinite
partial summations, are used. A second important result is that, for isolated
clusters, the steady state reached is not unique but depends on how one
switches on the external field. This may also true for clusters connected to
leads.
|
1004.3662v1
|
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
|
2013-01-22
|
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
|
2013-05-10
|
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
|
2015-08-27
|
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
|
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
|
2016-04-28
|
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
|
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
|
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
|
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
|
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
|
2017-07-31
|
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
|
2018-05-09
|
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
|
2018-09-05
|
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
|
2019-11-12
|
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
|
2020-04-22
|
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
|
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
|
2021-11-22
|
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
|
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
|
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
|
2018-07-04
|
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
|
2018-10-25
|
Time-retarded damping and magnetic inertia in the Landau-Lifshitz-Gilbert equation self-consistently coupled to electronic time-dependent nonequilibrium Green functions
|
The conventional Landau-Lifshitz-Gilbert (LLG) equation is a widely used tool
to describe dynamics of local magnetic moments, viewed as classical vectors of
fixed length, with their change assumed to take place simultaneously with the
cause. Here we demonstrate that recently developed [M. D. Petrovi\'{c} {\em et
al.}, {\tt arXiv:1802.05682}] self-consistent coupling of the LLG equation to
time-dependent quantum-mechanical description of electrons microscopically
generates time-retarded damping in the LLG equation described by a memory
kernel which is also spatially dependent. For sufficiently slow dynamics of
local magnetic moments, the memory kernel can be expanded to extract the
Gilbert damping (proportional to first time derivative of magnetization) and
magnetic inertia (proportional to second time derivative of magnetization)
terms whose parameters, however, are time-dependent in contrast to
time-independent parameters used in the conventional LLG equation. We use
examples of single or multiple magnetic moments precessing in an external
magnetic field, as well as field-driven motion of a magnetic domain wall (DW),
to quantify the difference in their time evolution computed from conventional
LLG equation vs. TDNEGF+LLG quantum-classical hybrid approach. The faster DW
motion predicted by TDNEGF+LLG approach reveals that important quantum effects,
stemming from finite amount of time which it takes for conduction electron spin
to react to the motion of classical local magnetic moments, are missing from
conventional classical micromagnetics simulations. We also demonstrate large
discrepancy between TDNEGF+LLG-computed numerically exact and, therefore,
nonperturbative result for charge current pumped by a moving DW and the same
quantity computed by perturbative spin motive force formula combined with the
conventional LLG equation.
|
1810.11016v2
|
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
|
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
|
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