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2024-01-30
|
Poynting-Robertson damping of laser beam driven lightsails
|
Lightsails using Earth-based lasers for propulsion require passive
stabilization to stay within the beam. This can be achieved through the sail's
scattering properties, creating optical restoring forces and torques. Undamped
restoring forces produce uncontrolled oscillations, which could jeopardize the
mission, but it is not obvious how to achieve damping in the vacuum of space.
Using a simple two-dimensional model we show that the Doppler effect and
relativistic aberration of the propelling laser beam create damping terms in
the optical forces and torques. The effect is similar to the Poynting-Robertson
effect causing loss of orbital momentum of dust particles around stars, but can
be enhanced by design of the sail's geometry.
|
2401.16924v1
|
2024-02-29
|
The Equation of Motion for Taut-Line Buzzers
|
Equations of motion are developed for the oscillatory rotation of a disk
suspended between twisted strings kept under tension by a hanging mass, to
which additional forces may be applied. In the absence of forcing, damped
harmonic oscillations are observed to decay with an exponential time envelope
for two different string types. This is consistent with damping caused by
string viscosity, rather than air turbulence, and may be quantified in terms of
a quality factor. To test the proposed equation of motion and model for viscous
damping within the string, we measure both the natural oscillation frequency
and the quality factor for widely varied values of string length, string
radius, disk moment of inertia, and hanging mass. The data are found to scale
in good accord with predictions. A variation where rotational kinetic energy is
converted back and forth to spring potential energy is also discussed.
|
2402.19285v1
|
2024-03-08
|
A design methodology for nonlinear oscillator chains enabling energy localization tuning and soliton stability enhancement with optimal damping
|
In this paper, the vibration energy localization in coupled nonlinear
oscillators is investigated, based on the creation of standing solitons. The
main objective is to establish a design methodology for mechanical lattices
using the Nonlinear Schr\"odinger Equation (NLSE) as a guide strategy, even in
the presence of damping. A three-dimensional diagram is used to illustrate
stable parameter regions for damped stationary solitons. Moreover, an analysis
of the influence of the number of oscillators in the system, and a numerical
investigation regarding the stability of solitonic behavior is done. Through
numerical analyses, it is observed that the developed algorithm not only has
the capability to locate the highest amplitudes in the chain of oscillators,
but also to control the intensity at which these amplitudes are located
according to design requirements. The outcomes of the proposed methodology
elucidate the impact that the coupling stiffness has on the stabilization of
the NLSE, as well as the influence of the number of oscillators on the
continuity hypothesis. The developed algorithm holds potential for practical
applications in mechanical engineering since the NLSE is used as a design line
rather than as a consequence of the phenomenon description.
|
2403.05176v1
|
2024-03-08
|
Damping Obliquities of Hot Jupiter Hosts by Resonance Locking
|
When orbiting hotter stars, hot Jupiters are often highly inclined relative
to their host star equator planes. By contrast, hot Jupiters orbiting cooler
stars are more aligned. Prior attempts to explain this correlation between
stellar obliquity and effective temperature have proven problematic. We show
how resonance locking -- the coupling of the planet's orbit to a stellar
gravity mode (g mode) -- can solve this mystery. Cooler stars with their
radiative cores are more likely to be found with g-mode frequencies increased
substantially by core hydrogen burning. Strong frequency evolution in resonance
lock drives strong tidal evolution; locking to an axisymmetric g mode damps
semi-major axes, eccentricities, and as we show for the first time,
obliquities. Around cooler stars, hot Jupiters evolve into spin-orbit alignment
and avoid engulfment. Hotter stars lack radiative cores, and therefore preserve
congenital spin-orbit misalignments. We focus on resonance locks with
axisymmetric modes, supplementing our technical results with simple physical
interpretations, and show that non-axisymmetric modes also damp obliquity.
|
2403.05616v1
|
1996-05-06
|
A Keck HIRES Investigation of the Metal Abundances and Kinematics of Three Damped Lya Systems Toward Q2206-199
|
We present high resolution, high SNR spectra of the QSO Q2206-199 obtained
with HIRES on the 10m W.M. Keck Telescope. Our analysis focuses on the two
previously identified damped \lya systems found at $z=1.920$ and $z=2.076$. For
each system, we measure accurate abundances. The $z=1.920$ system exhibits the
highest metallicity we have measured for a damped \lya system. We report the
first confident ($>5 \sigma$) detection of Ti in a QSO absorption line system.
By contrast the $z=2.076$ system is the most metal poor we have analyzed,
showing absorption features for only the strongest transitions. We find no
positive evidence for the presence of dust in either system. The two damped
systems exhibit significantly different kinematic characteristics, yet we
contend the two systems are consistent with one physical description: that of a
thick, rotating disk.
We investigate a very strong Mg II system at $z=0.752$ which is very likely
yet a third damped \lya system. The very weak Mn II and Ti II transitions have
been positively measured and imply $\log \N{HI} > 19.0$. We analyze the
abundance ratios [Mn/Fe] and [Ti/Fe] and their values are inconsistent with
dust depletion, yet consistent with the abundance pattern detected for halo
stars in the Galaxy (see Lu et al. 1996a).
Finally, we identify a C IV system at $z=2.014$ that shows a very narrow
feature in Si IV and C IV absorption. The corresponding $b$ values (5.5 \kms
and 8.9 \kms for Si IV and C IV) for this component suggest a temperature of
$4.7 \sci{4} \rm K$. Because collisional ionization can explain the observed
abundances only for $T > 8 \sci{4} \rm K$, we contend these ions must have
formed through a different physical process (e.g. photoionization).
|
9605021v2
|
1996-09-09
|
The Population of Damped Lyman-alpha and Lyman Limit Systems in the Cold Dark Matter Model
|
Lyman limit and damped Lyman-alpha absorption systems probe the distribution
of collapsed, cold gas at high redshift. Numerical simulations that incorporate
gravity and gas dynamics can predict the abundance of such absorbers in
cosmological models. We develop a semi-analytical method to correct the
numerical predictions for the contribution of unresolved low mass halos, and we
apply this method to the Katz et al. (1996) simulation of the standard cold
dark matter model ($\Omega=1$, $h=0.5$, $\Omega_b=0.05$, $\sigma_8=0.7$). Using
this simulation and higher resolution simulations of individual low mass
systems, we determine the relation between a halo's circular velocity $v_c$ and
its cross section for producing Lyman limit or damped absorption. We combine
this relation with the Press-Schechter formula for the abundance of halos to
compute the number of absorbers per unit redshift. The resolution correction
increases the predicted abundances by about a factor of two at z=2, 3, and 4,
bringing the predicted number of damped absorbers into quite good agreement
with observations. Roughly half of the systems reside in halos with circular
velocities $v_c>100\kms$ and half in halos with $35\kms<v_c<100\kms$. Halos
with $v_c>150\kms$ typically harbor two or more systems capable of producing
damped absorption. Even with the resolution correction, the predicted abundance
of Lyman limit systems is a factor of three below observational estimates,
signifying either a failure of standard CDM or a failure of these simulations
to resolve the systems responsible for most Lyman limit absorption. By
comparing simulations with and without star formation, we find that depletion
of the gas supply by star formation affects absorption line statistics at
$z>=2$ only for column densities exceeding $N_{HI}=10^{22} cm^{-2}$.
|
9609072v1
|
1998-05-22
|
Protogalactic Disk Models of Damped Lya Kinematics
|
We present new observational results on the kinematics of the damped lya
systems. Our full sample is now comprised of 31 low-ion profiles and exhibits
similar characteristics to the sample from Paper I. The primary exception is
that the new distribution of velocity widths includes values out to a maximum
of nearly 300 km/s, approx 100 km/s greater than the previous maximum. These
high velocity width systems will significantly leverage models introduced to
explain the damped lya systems. Comparing the characteristics from low-redshift
and high-redshift sub-samples, we find no evidence for significant evolution in
the kinematic properties of protogalaxies from z = 2.0 - 3.3.
The new observations give greater statistical significance to the main
conclusions of our first paper. In particular, those models inconsistent with
the damped lya observations in Paper I are ruled out at even higher levels of
confidence. At the same time, the observations are consistent with a population
of rapidly rotating, thick disks (the TRD model) at high redshift.
Buoyed by the success of the TRD model, we investigate it more closely by
considering more realistic disk properties. Our goal is to demonstrate the
statistical power of the damped lya observations by investigating the
robustness of the TRD model. In particular, we study the effects of warping,
realistic rotation curves, and photoionization on the kinematics of disks in
the TRD model. The principal results are: (1) disk warping has only minimal
effect on the kinematic results, primarily influencing the effective disk
thickness, (2) the TRD model is robust to more realistic rotation curves; (3)
the effects of photoionization require thicker disks to give consistent
velocity width distributions. [abridged]
|
9805293v1
|
2000-05-05
|
UVES observations of QSO 0000-2620: oxygen and zinc abundances in the Damped Ly-alpha galaxy at z_abs=3.3901
|
Observations of the QSO 0000-2620 with UVES spectrograph at the 8.2m ESO
KUEYEN telescope are used for abundance analysis of the damped Ly-alpha system
at z_{abs}=3.3901. Several Oxygen lines are identified in the Ly_alpha forest
and a measure for the oxygen abundance is obtained at [O/H]=-1.85 +/- 0.1 by
means of the unsaturated OI 925 A and OI 950 A lines. This represents the most
accurate O measurement in a damped Ly_alpha galaxy so far. We have also
detected ZnII 2026 A and CrII 2056, 2062 A redshifted at about 8900 A and found
abundances [Zn/H] = -2.07 +/- 0.1 and [Cr/H]=-1.99 +/- 0.1. Furthermore,
previous measurements of Fe, Si, Ni and N have been refined yielding
[Fe/H]=-2.04 +/- 0.1, [Si/H]=-1.90 +/- 0.1, [Ni/H]=-2.27 +/- 0.1, and
[N/H]=-2.68 +/- 0.1. The abundance of the non-refractory element zinc is the
lowest among the damped Ly-alpha systems showing that the associated
intervening galaxy is indeed in the early stages of its chemical evolution. The
fact that the Zn abundance is identical to that of the refractory elements Fe
and Cr suggests that dust grains have not formed yet. In this Damped Ly-alpha
system the observed [O,S,Si/Zn,Fe,Cr] ratios, in whatever combination are
taken, are close to solar (i.e 0.1-0.2 dex) and do not show the
[alpha-element/Fe] enhancement observed in Milky Way stars of comparable
metallicity. The observed behavior supports a galaxy evolution model
characterized by either episodic or low star formation rate rather than a
Milky-Way-type evolutionary model.
|
0005098v1
|
2002-02-06
|
The UCSD HIRES/KeckI Damped Lya Abundance Database III. An Empirical Study of Photoionization in the Damped Lya System Toward GB1759+7539
|
We investigate the ionization state of the damped Lya system at z=2.62 toward
GB1759+7539 through an analysis of ionic ratios sensitive to photoionization:
ArI/SII, FeIII/FeII, NII/NI, AlIII/AlII. Approximately half of the metals arise
in a mostly neutral velocity component with HI/H > 0.9, based on FeIII/FeII <
0.013. In contrast, the remaining half exhibits FeIII/FeII~0.3 indicative of a
partially ionized medium with HI/H~0.5. These conclusions are supported by the
observed NII/NI, AlIII/AlII, and ArI/SII ratios.
We assess ionization corrections for the observed column densities through
photoionization models derived from the CLOUDY software package. In the neutral
gas, the ionization corrections are negligible except for ArI. However for the
partially ionized gas, element abundance ratios differ from the ionic ratios by
0.1-0.3 dex for (SiII, SII, NiII, AlII)/FeII ratios and more for (NI,
ArI)/FeII. Independent of the shape of the photoionizing spectrum and
assumptions on the number of ionization phases, these ionization corrections
have minimal impact (<0.1dex) on the total metallicity inferred for this damped
Lya system. Measurements on the relative elemental abundances of the partially
ionized gas, however, have a greater than ~0.15 dex uncertainty which hides the
effects of nucleosynthesis and dust depletion.
We caution the reader that this damped system is unusual for a number of
reasons (e.g. a very low ArI/SII ratio) and we believe its ionization
properties are special but not unique. Nevertheless, it clearly shows the value
of examining photoionization diagnostics like FeIII/FeII in a larger sample of
systems.
|
0202140v1
|
2009-09-26
|
Damped and sub-damped Lyman-? absorbers in z > 4 QSOs
|
We present the results of a survey for damped (DLA, log N(H I) > 20.3) and
sub-damped Lyman-? systems (19.5 < log N(H I) < 20.3) at z > 2.55 along the
lines-of-sight to 77 quasars with emission redshifts in the range 4 < zem <
6.3. Intermediate resolution (R ? 4300) spectra have been obtained with the
Echellette Spectrograph and Imager (ESI) mounted on the Keck telescope. A total
of 100 systems with log N(H I) > 19.5 are detected of which 40 systems are
damped Lyman-? systems for an absorption length of ?X = 378. About half of the
lines of sight of this homogeneous survey have never been investigated for
DLAs. We study the evolution with redshift of the cosmological density of the
neutral gas and find, consis- tently with previous studies at similar
resolution, that ?DLA,H I decreases at z > 3.5. The overall cosmological
evolution of ?HI shows a peak around this redshift. The H I column density
distribution for log N(H I) ? 20.3 is ?tted, consistently with previous
surveys, with a single power-law of index ? ? -1.8$\pm$0.25. This power-law
overpredicts data at the high-end and a second, much steeper, power-law (or a
gamma function) is needed. There is a flattening of the function at lower H I
column densities with an index of ? ? ?1.4 for the column density range log N(H
I) = 19.5?21. The fraction of H I mass in sub-DLAs is of the order of 30%. The
H column density distribution does not evolve strongly from z ? 2.5 to z ? 4.5.
|
0909.4839v2
|
2011-05-19
|
Tidal dissipation compared to seismic dissipation: in small bodies, in earths, and in superearths
|
While the seismic quality factor and phase lag are defined solely by the bulk
properties of the mantle, their tidal counterparts are determined both by the
bulk properties and self-gravitation of a body as a whole. For a qualitative
estimate, we model the body with a homogeneous sphere and express the tidal
phase lag through the lag in a sample of material. Although simplistic, our
model is sufficient to understand that the lags are not identical. The
difference emerges because self-gravitation pulls the tidal bulge down. At low
frequencies, this reduces strain and makes tidal damping less efficient in
larger bodies. At high frequencies, competition between self-gravitation and
rheology becomes more complex, though for sufficiently large superearths the
same rule works: the larger the body, the weaker tidal damping in it. Being
negligible for small terrestrial planets and moons, the difference between the
seismic and tidal lagging (and likewise between the seismic and tidal damping)
becomes very considerable for superearths. In those, it is much lower than what
one might expect from using a seismic quality factor. The tidal damping rate
deviates from the seismic damping rate especially in the zero-frequency limit,
and this difference takes place for bodies of any size. So the equal in
magnitude but opposite in sign tidal torques, exerted on one another by the
primary and the secondary, go smoothly through zero as the secondary crosses
the synchronous orbit. We describe the mantle rheology with the Andrade model,
allowing it to lean towards the Maxwell model at the lowest frequencies. To
implement this additional flexibility, we reformulate the Andrade model by
endowing it with a free parameter which is the ratio of the anelastic timescale
to the viscoelastic Maxwell time of the mantle. Some uncertainty in this
parameter's frequency-dependence does not influence our principal conclusions.
|
1105.3936v12
|
2014-10-07
|
The Effect of Nonlinear Landau Damping on Ultrarelativistic Beam Plasma Instabilities
|
Very-high energy gamma-rays from extragalactic sources pair-produce off of
the extragalactic background light, yielding an electron-positron pair beam.
This pair beam is unstable to various plasma instabilities, especially the
"oblique" instability, which can be the dominant cooling mechanism for the
beam. However, recently, it has been claimed that nonlinear Landau damping
renders it physically irrelevant by reducing the effective damping rate to a
low level. Here, we show with numerical calculations that the effective damping
rate is $8\times 10^{-4}$ of the growth rate of the linear instability, which
is sufficient for the "oblique" instability to be the dominant cooling
mechanism of these pair beams. In particular, we show that previous estimates
of this rate ignored the exponential cutoff in the scattering amplitude at
large wavenumber and assumed that the damping of scattered waves entirely
depends on collisions, ignoring collisionless processes. We find that the total
wave energy eventually grows to approximate equipartition with the beam by
increasingly depositing energy into long wavelength modes. As we have not
included the effect of nonlinear wave-wave interactions on these long
wavelength modes, this scenario represents the "worst-case" scenario for the
oblique instability. As it continues to drain energy from the beam at a faster
rate than other processes, we conclude that the "oblique" instability is
sufficiently strong to make it the physically dominant cooling mechanism for
high-energy pair beams in the intergalactic medium.
|
1410.3797v2
|
2017-01-24
|
Influence of interlayer coupling on the spin torque driven excitations in a spin torque oscillator
|
The influence of dynamic interlayer interactions on the spin torque driven
and damped excitations are illustrated for a three layer macrospin model system
that corresponds to a standard spin-torque oscillator. The free layer and a
synthetic antiferromagnetic (SyF) pinned layer of the spin-torque oscillator
are in-plane magnetized. In order to understand experimental results, numerical
simulations have been performed considering three types of interlayer
interactions: exchange interaction between the two magnetic layers of the SyF,
mutual spin torque between the top layer of the SyF and the free layer and
dipolar interaction between all three magnetic layers. It will be shown that
the dynamic dipolar coupling plays a predominant role. First, it leads to a
hybridization of the free layer and the SyF linear modes and through this gives
rise to a strong field dependence of the critical current. In particular, there
is a field range of enhanced damping in which much higher current is required
to drive the modes into steady state. This results in a gap in the excitation
spectrum. Second, the dynamic dipolar interaction is also responsible for the
non-linear interaction between the current driven steady state mode and the
damped modes of the system. Here one can distinguish: (i) a resonant
interaction that leads to a kink in the frequency-field and frequency-current
dispersions accompanied by a small hysteresis and a reduction of the linewidth
of the steady state mode and (ii) a non-resonant interaction that leads to a
strong frequency redshift of the damped mode. The results underline the strong
impact of interlayer coupling on the excitation spectra of spin-torque
oscillators and illustrate in a simple three mode model system how in the
non-linear regime the steady state and damped modes influence each other.
|
1701.06787v1
|
2017-03-21
|
Using rf voltage induced ferromagnetic resonance to study the spin-wave density of states and the Gilbert damping in perpendicularly magnetized disks
|
We study how the shape of the spinwave resonance lines in rf-voltage induced
FMR can be used to extract the spinwave density of states and the damping
within the precessing layer in nanoscale tunnel junctions that possess
perpendicular anisotropy. We work with a field applied along the easy axis to
preserve the uniaxial symmetry of the system. We describe the set-up to study
the susceptibility contributions of the spin waves in the field-frequency
space. We then identify the maximum device size above which the spinwaves can
no longer be studied in isolation as the linewidths of their responses make
them overlap. The rf-voltage induced signal is the sum of two voltages that
have comparable magnitudes: a first voltage that originates from the transverse
susceptibility and rectification by magnetoresistance and a second voltage that
arises from the non-linear longitudinal susceptibility and the resultant
time-averaged change of the micromagnetic configuration. The transverse and
longitudinal susceptibility signals have different dc bias dependences such
that they can be separated by measuring how the device rectifies the rf voltage
at different dc bias voltages. The transverse and longitudinal susceptibility
signals have different lineshapes; their joint studies can yield the Gilbert
damping of the free layer of the device with a degree of confidence that
compares well with standard FMR. Our method is illustrated on FeCoB-based free
layers in which the individual spin-waves can be sufficiently resolved only for
disk diameters below 200 nm. The resonance line shapes on devices with 90 nm
diameters are consistent with a Gilbert damping of 0.011. This damping of 0.011
exceeds the value of 0.008 measured on the unpatterned films, which indicates
that device-level measurements are needed for a correct evaluation of
dissipation.
|
1703.07310v2
|
2017-04-07
|
Global Alfven Eigenmodes in the H-1 heliac
|
Recent upgrades in H-1 power supplies have enabled the operation of the H-1
experiment at higher heating powers than previously attainable. A heating power
scan in mixed hydrogen/helium plasmas reveals a change in mode activity with
increasing heating power. At low power (<50 kW) modes with beta-induced Alfven
eigenmode (BAE) frequency scaling are observed. At higher power modes
consistent with an analysis of nonconventional Global Alfven Eigenmodes (GAEs)
are observed, the subject of this work. We have computed the mode continuum,
and identified GAE structures using the ideal MHD solver CKA and the
gyrokinetic code EUTERPE. An analytic model for ICRH-heated minority ions is
used to estimate the fast ion temperature from the hydrogen species. Linear
growth rate scans using a local flux surface stability calculation, LGRO, are
performed. These studies demonstrate growth from circulating particles whose
speed is significantly less than the Alfven speed, and are resonant with the
mode through harmonics of the Fourier decomposition of the strongly-shaped
heliac magnetic field. They reveal drive is possible with a small, hot
energetic tail of the hydrogen species. Local linear growth rate scans are also
complemented with global calculations from CKA and EUTERPE. These qualitatively
confirm the findings from the LGRO study, and show that the inclusion of finite
Larmor radius effects can reduce the growth rate by a factor of three, but do
not affect marginal stability. Finally, a study of damping of the global mode
with the thermal plasma is conducted, computing continuum, and the damping
arising from parallel electric fields. We find that continuum damping is of
order 0.1% for the configuration studied. The inclusion of resistivity lifts
the damping to 19%. Such large damping is consistent with experimental
observations that in absence of drive the mode decays rapidly (~0.1 ms).
|
1704.02089v1
|
2017-11-30
|
Scalar dark matter interpretation of the DAMPE data with U(1) gauge interactions
|
Recently, DAMPE experiment released the new measurement of the total cosmic
$e^+e^-$ flux between 25 GeV and 4.6 TeV which indicates a spectral softening
at around 0.9 TeV and a tentative peak at around 1.4 TeV. We utilize the scalar
dark matter (DM) annihilation scenario to explain the DAMPE peak by extending
$G_{SM}\equiv SU(3)_C \times SU(2)_L \times U(1)_Y$ with additional $U(1)$
gauge symmetries while keeping anomaly free to generate $\chi \chi \to Z^\prime
Z^\prime \to \ell\bar{\ell}\ell^\prime\overline{\ell^\prime}$, where $\chi,
Z^\prime, \ell^{(^\prime)}$ denote the scalar DM, the new gauge boson and
$\ell^{(^\prime)}=e,\mu,\tau$, respectively, with $m_\chi \sim m_{Z^\prime}
\sim 2 \times 1.5$ (TeV). We first illustrate that the minimal framework
$G_{SM} \times U(1)_{Y^\prime}$ with the above mass choices can explain the
DAMPE excess but has been excluded by LHC constraints from the $Z^\prime$
searches. Then we study a non-minimal framework $G_{SM} \times U(1)_{Y^\prime}
\times U(1)_{Y^{\prime \prime}}$ in which $U(1)_{Y^{\prime \prime}}$ mixes with
$U(1)_{Y^\prime}$. We show that such a framework can interpret the DAMPE data
while passing other constraints including the DM relic abundance, DM direct
detection and collider bounds. We also investigate the predicted $e^+e^-$
spectrum in this framework and find that the mass splitting $\Delta m = m_\chi
- m_{Z'}$ should be less than about 17 GeV to produce the peak-like structure.
|
1711.11452v2
|
2017-12-14
|
Scalar dark matter explanation of the DAMPE data in the minimal Left-Right symmetric model
|
Left-Right symmetric model (LRSM) has been an attractive extension of the
Standard Model (SM) which can address the origin of parity violation in the SM
electroweak (EW) interactions, generate tiny neutrino masses, accommodate dark
matter (DM) candidates and provide a natural framework for baryogenesis through
leptogenesis. In this work we utilize the minimal LRSM to study the recently
reported DAMPE results of cosmic $e^+e^-$ spectrum which exhibits a tentative
peak around 1.4 TeV, while satisfying the current neutrino data. We propose to
explain the DAMPE peak with a complex scalar DM $\chi$ in two scenarios: 1)
$\chi\chi^* \to H_1^{++}H_1^{--} \to \ell_i^+\ell_i^+\ell_j^-\ell_j^-$; 2)
$\chi\chi^* \to H_{k}^{++}H_{k}^{--} \to \ell_i^+\ell_i^+\ell_j^-\ell_j^-$
accompanied by $\chi\chi^* \to H_1^+ H_1^- \to \ell_i^+ \nu_{\ell_i} \ell_j^-
\nu_{\ell_j}$ with $\ell_{i,j}=e,\mu,\tau$ and $k=1,2$. We fit the theoretical
prediction on $e^+e^-$ spectrum to relevant experimental data to determine the
scalar mass spectrum favored by the DAMPE excess. We also consider various
constraints from theoretical principles, collider experiments as well as DM
relic density and direct search experiments. We find that there are ample
parameter space which can interpret the DAMPE data while passing the
constraints. Our explanations, on the other hand, usually imply the existence
of other new physics at the energy scale ranging from $10^7 {\rm GeV}$ to
$10^{11} {\rm GeV}$. Collider tests of our explanations are also discussed.
|
1712.05351v3
|
2018-02-20
|
The chemical connection between damped Lyman-α systems and Local Group dwarf galaxies
|
Abundances of the volatile elements S and Zn have now been measured in around
80 individual stars in the Sculptor dwarf spheroidal galaxy, covering the
metallicity range $-2.4\leq\text{[Fe/H]}\leq-0.9$. These two elements are of
particular interest as they are not depleted onto dust in gas, and their ratio,
[S/Zn], has thus commonly been used as a proxy for [$\alpha$/Fe] in Damped
Lyman-$\alpha$ systems. The S abundances in Sculptor are similar to other
$\alpha$-elements in this galaxy, consistent with S being mainly created in
core-collapse supernovae, but also having some contribution from supernovae
Type Ia. However, our results show that Zn and Fe do not trace all the same
nucleosynthetic production channels. In particular, (contrary to Fe) Zn is not
significantly produced by supernovae Type Ia. Thus, [S/Zn] cannot be reliably
used as a proxy for [$\alpha$/Fe]. We propose [O/S] as a function of [S/H] as a
possible alternative. At higher metallicities, the values of [S/Zn] measured in
Damped Lyman-$\alpha$ systems are inconsistent with those in local dwarf
galaxies, and are more compatible with the Milky Way disk. Low-metallicity
Damped Lyman-$\alpha$ systems are, however, consistent with the most metal-poor
stars in Local Group dwarf spheroidal galaxies. Assuming that the dust
depletions of S and Zn are negligible, our comparison indicates that the star
formation histories of Damped Lyman-$\alpha$ systems are on average different
from both the Milky Way and the Sculptor dwarf spheroidal galaxy.
|
1802.07325v5
|
2019-01-12
|
GW170817 implications on the frequency and damping time of f-mode oscillations of neutron stars
|
Within a minimum model for neutron stars consisting of nucleons, electrons
and muons at $\beta$-equilibrium using about a dozen Equation of States (EOSs)
from microscopic nuclear many-body theories and 40,000 EOSs randomly generated
using an explicitly isospin-dependent parametric EOS model for high-density
neutron-rich nucleonic matter within its currently known uncertainty range, we
study correlations among the f-mode frequency, its damping time and the tidal
deformability as well as the compactness of neutron stars. Except for quark
stars, both the f-mode frequency and damping time of canonical neutron stars
are found to scale with the tidal deformability independent of the EOSs used.
Applying the constraint on the tidal deformability of canonical neutron stars
$\Lambda_{1.4}=190^{+390}_{-120}$ extracted by the LIGO+VIRGO Collaborations
from their improved analyses of the GW170817 event, the f-mode frequency and
its damping time of canonical neutron stars are limited to 1.67 kHz - 2.18 kHz
and 0.155 s - 0.255 s, respectively, providing a useful guidance for the
ongoing search for gravitational waves from the f-mode oscillations of isolated
neutron stars. Moreover, assuming either or both the f-mode frequency and its
damping time will be measured precisely in future observations with advanced
gravitational wave detectors, we discuss how information about the mass and/or
radius as well as the still rather elusive nuclear symmetry energies at
supra-saturation densities may be extracted.
|
1901.03779v2
|
2019-01-27
|
An introductory guide to fluid models with anisotropic temperatures Part 2 -- Kinetic theory, Padé approximants and Landau fluid closures
|
In Part 2 of our guide to collisionless fluid models, we concentrate on
Landau fluid closures. These closures were pioneered by Hammett and Perkins and
allow for the rigorous incorporation of collisionless Landau damping into a
fluid framework. It is Landau damping that sharply separates traditional fluid
models and collisionless kinetic theory, and is the main reason why the usual
fluid models do not converge to the kinetic description, even in the
long-wavelength low-frequency limit. We start with a brief introduction to
kinetic theory, where we discuss in detail the plasma dispersion function
$Z(\zeta)$, and the associated plasma response function $R(\zeta)=1+\zeta
Z(\zeta)=-Z'(\zeta)/2$. We then consider a 1D (electrostatic) geometry and make
a significant effort to map all possible Landau fluid closures that can be
constructed at the 4th-order moment level. These closures for parallel moments
have general validity from the largest astrophysical scales down to the Debye
length, and we verify their validity by considering examples of the (proton and
electron) Landau damping of the ion-acoustic mode, and the electron Landau
damping of the Langmuir mode. We proceed by considering 1D closures at
higher-order moments than the 4th-order, and as was concluded in Part 1, this
is not possible without Landau fluid closures. We show that it is possible to
reproduce linear Landau damping in the fluid framework to any desired
precision, thus showing the convergence of the fluid and collisionless kinetic
descriptions. We then consider a 3D (electromagnetic) geometry in the
gyrotropic (long-wavelength low-frequency) limit and map all closures that are
available at the 4th-order moment level. In the Appendix A, we provide
comprehensive tables with Pad\'e approximants of $R(\zeta)$ up to the 8th-pole
order, with many given in an analytic form.
|
1901.09360v2
|
2019-01-28
|
Revisit of non-linear Landau damping for electrostatic instability driven by blazar-induced pair beams
|
We revisit the effect of non-linear Landau (NL) damping on the electrostatic
instability of blazar-induced pair beams, using a realistic pair-beam
distribution. We employ a simplified 2D model in ${\bf k}$-space to study the
evolution of the electric-field spectrum and to calculate the relaxation time
of the beam. We demonstrate that the 2D model is an adequate representation of
the 3D physics. We find that non-linear Landau damping, once it operates
efficiently, transports essentially the entire wave energy to small wavenumbers
where wave driving is weak or absent. The relaxation time also strongly depends
on the IGM temperature, $T_\mathrm{IGM}$, and for $T_\mathrm{IGM}\ll10$ eV, and
in the absence of any other damping mechanism, the relaxation time of the pair
beam is longer than the inverse Compton (IC) scattering time. The weak
late-time beam energy losses arise from the accumulation of wave energy at
small $k$, that non-linearly drains the wave energy at the resonant
$\mathbf{k}$ of the pair-beam instability. Any other dissipation process
operating at small $k$ would reduce that wave-energy drain and hence lead to
stronger pair-beam energy losses. As an example, collisions reduce the
relaxation time by an order of magnitude, although their rate is very small.
Other non-linear processes, such as the modulation instability, could provide
additional damping of the non-resonant waves and dramatically reduce the
relaxation time of the pair beam. An accurate description of the spectral
evolution of the electrostatic waves is crucial for calculating the relaxation
time of the pair beam.
|
1901.09640v3
|
2019-08-08
|
Annihilation of topological solitons in magnetism with spin wave burst finale: The role of nonequilibrium electrons causing nonlocal damping and spin pumping over ultrabroadband frequency range
|
We not only reproduce burst of short-wavelength spin waves (SWs) observed in
recent experiment [S. Woo et al., Nat. Phys. 13, 448 (2017)] on
magnetic-field-driven annihilation of two magnetic domain walls (DWs) but,
furthermore, we predict that this setup additionally generates highly unusual}
pumping of electronic spin currents in the absence of any bias voltage. Prior
to the instant of annihilation, their power spectrum is ultrabroadband, so they
can be converted into rapidly changing in time charge currents, via the inverse
spin Hall effect, as a source of THz radiation of bandwidth $\simeq 27$ THz
where the lowest frequency is controlled by the applied magnetic field. The
spin pumping stems from time-dependent fields introduced into the quantum
Hamiltonian of electrons by the classical dynamics of localized magnetic
moments (LMMs) comprising the domains. The pumped currents carry spin-polarized
electrons which, in turn, exert backaction on LMMs in the form of nonlocal
damping which is more than twice as large as conventional local Gilbert
damping. The nonlocal damping can substantially modify the spectrum of emitted
SWs when compared to widely-used micromagnetic simulations where conduction
electrons are completely absent. Since we use fully microscopic (i.e.,
Hamiltonian-based) framework, self-consistently combining time-dependent
electronic nonequilibrium Green functions with the Landau-Lifshitz-Gilbert
equation, we also demonstrate that previously derived phenomenological formulas
miss ultrabroadband spin pumping while underestimating the magnitude of
nonlocal damping due to nonequilibrium electrons.
|
1908.03194v5
|
2019-11-22
|
Role of Element-Specific Damping on the Ultrafast, Helicity-Independent All-Optical Switching Dynamics in Amorphous (Gd,Tb)Co Thin Films
|
Ultrafast control of the magnetization in ps timescales by fs laser pulses
offers an attractive avenue for applications such as fast magnetic devices for
logic and memory. However, ultrafast helicity-independent all-optical switching
(HI-AOS) of the magnetization has thus far only been observed in Gd-based,
ferrimagnetic amorphous (\textit{a}-) rare earth-transition metal
(\textit{a}-RE-TM) systems, and a comprehensive understanding of the reversal
mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic
\textit{a}-Gd$_{22-x}$Tb$_x$Co$_{78}$ thin films, from x = 0 to x = 18, and
elucidate the role of Gd in HI-AOS in \textit{a}-RE-TM alloys and multilayers.
Increasing Tb content results in increasing perpendicular magnetic anisotropy
and coercivity, without modifying magnetization density, and slower
remagnetization rates and higher critical fluences for switching but still
shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on
the two-temperature model reproduce these results qualitatively and predict
that the lower damping on the RE sublattice arising from the small spin-orbit
coupling of Gd (with $L = 0$) is instrumental for the faster dynamics and lower
critical fluences of the Gd-rich alloys. Annealing
\textit{a}-Gd$_{10}$Tb$_{12}$Co$_{78}$ leads to slower dynamics which we argue
is due to an increase in damping. These simulations strongly indicate that
acounting for element-specific damping is crucial in understanding HI-AOS
phenomena. The results suggest that engineering the element specific damping of
materials can open up new classes of materials that exhibit low-energy,
ultrafast HI-AOS.
|
1911.09803v3
|
2020-09-14
|
Large field-like torque in amorphous Ru2Sn3 originated from the intrinsic spin Hall effect
|
We investigated temperature dependent current driven spin-orbit torques in
magnetron sputtered Ru2Sn3 (4 and 10 nm) /Co20Fe60B20 (5 nm) layered structures
with in-plane magnetic anisotropy. The room temperature damping-like and
field-like spin torque efficiencies of the amorphous Ru2Sn3 films were measured
to be 0.14 +- 0.008 (0.07 +- 0.012) and -0.03 +- 0.006 (-0.20 +- 0.009), for
the 4 (10 nm) films respectively, by utilizing the second harmonic Hall
technique. The large field-like torque in the relatively thicker Ru2Sn3 (10 nm)
thin film is unique compared to the traditional spin Hall materials interfaced
with thick magnetic layers with in-plane magnetic anisotropy which typically
have dominant damping-like and negligible field-like torques. Additionally, the
observed room temperature field-like torque efficiency in Ru2Sn3 (10 nm)/CoFeB
(5 nm) is up to three times larger than the damping-like torque (-0.20 +- 0.009
and 0.07 +- 0.012, respectively) and thirty times larger at 50 K (-0.29 +-
0.014 and 0.009 +- 0.017, respectively). The temperature dependence of the
field-like torques show dominant contributions from the intrinsic spin Hall
effect while the damping-like torques show dominate contributions from the
extrinsic spin Hall effects, skew scattering and side jump. Through macro-spin
calculations, we found that including field-like torques on the order or larger
than the damping-like torque can reduce the switching critical current and
decrease magnetization procession for a perpendicular ferromagnetic layer.
|
2009.06711v2
|
2021-01-12
|
Phase Retrieval using Expectation Consistent Signal Recovery Algorithm based on Hypernetwork
|
Phase retrieval (PR) is an important component in modern computational
imaging systems. Many algorithms have been developed over the past
half-century. Recent advances in deep learning have introduced new
possibilities for a robust and fast PR. An emerging technique called deep
unfolding provides a systematic connection between conventional model-based
iterative algorithms and modern data-based deep learning. Unfolded algorithms,
which are powered by data learning, have shown remarkable performance and
convergence speed improvement over original algorithms. Despite their
potential, most existing unfolded algorithms are strictly confined to a fixed
number of iterations when layer-dependent parameters are used. In this study,
we develop a novel framework for deep unfolding to overcome existing
limitations. Our development is based on an unfolded generalized expectation
consistent signal recovery (GEC-SR) algorithm, wherein damping factors are left
for data-driven learning. In particular, we introduce a hypernetwork to
generate the damping factors for GEC-SR. Instead of learning a set of optimal
damping factors directly, the hypernetwork learns how to generate the optimal
damping factors according to the clinical settings, thereby ensuring its
adaptivity to different scenarios. To enable the hypernetwork to adapt to
varying layer numbers, we use a recurrent architecture to develop a dynamic
hypernetwork that generates a damping factor that can vary online across
layers. We also exploit a self-attention mechanism to enhance the robustness of
the hypernetwork. Extensive experiments show that the proposed algorithm
outperforms existing ones in terms of convergence speed and accuracy and still
works well under very harsh settings, even under which many classical PR
algorithms are unstable.
|
2101.04348v2
|
2024-03-22
|
Investigating the Relationship between Simulation Parameters and Flow Variables in Simulating Atmospheric Gravity Waves in Wind Energy Applications
|
Wind farms, particularly offshore clusters, are becoming larger than ever
before. Besides influencing wind farms and local meteorology downstream, large
wind farms can trigger atmospheric gravity waves in the inversion layer and the
free atmosphere aloft. Wind farm-induced gravity waves can cause adverse
pressure gradients upstream of the wind farm, that contribute to the global
blockage effect, and favorable pressure gradients above and downstream of the
wind farm that enhance wake recovery.
Numerical modeling is a powerful means of studying wind farm-induced
atmospheric gravity waves, but it comes with the challenge of handling spurious
reflections of these waves from domain boundaries. Approaches like radiation
boundary conditions and forcing zones are used to avoid the reflections.
However, the simulation setup heavily relies on ad-hoc processes. For instance,
the widely used Rayleigh damping method requires ad-hoc tuning to acquire a
setup only applicable to a particular case. To surmount this hurdle, we conduct
a systematic LES study for flow over a 2D hill and through wind farm canopies
that explores the dependence of domain size and damping layer setup on
parameters driving linearly stratified atmospheric flows.
Mainly the internal waves in the free atmosphere reflect from the boundaries,
therefore by simulation linearly stratified conditions we focus on internal
waves only. The Froude number drives most of the internal wave properties, such
as wavelengths, amplitude, and direction. Therefore, the domain sizing and
Rayleigh damping layer setup mainly depends on the Froude number. We
anticipated the effective wavelengths to be the correct length scale to size
the domain and damping layer thickness. Also, the damping coefficient is scaled
with Brunt-V\"ais\"al\"a frequency.
|
2403.18863v1
|
2012-03-21
|
On a New Method of Storing a Variable Size Array
|
This paper introduces a new data structure, log_vector, with the following
properties: constant time random access to individual elements; constant time
element addition to the end; constant time element removal from the end;
constant time empty data structure creation; amortized constant space per
individual elements; constant additional space used.
|
1203.4836v1
|
2022-10-24
|
The AFLT $q$-Morris constant term identity
|
It is well-known that the Selberg integral is equivalent to the Morris
constant term identity. More generally, Selberg type integrals can be turned
into constant term identities for Laurent polynomials. In this paper, by
extending the Gessel--Xin method of the Laurent series proof of constant term
identities, we obtain an AFLT type $q$-Morris constant term identity. That is a
$q$-Morris type constant term identity for a product of two Macdonald
polynomials.
|
2210.13245v1
|
2010-01-10
|
Non-expanding universe: a cosmological system of units
|
The product of two empirical constants, the dimensionless fine structure
constant and the von Klitzing constant (an electrical resistance), turns out to
be an exact dimensionless number. Then the accuracy and cosmological time
variation (if any) of these two constants are tied. Also this product defines a
natural unit of electrical resistance, the inverse of a quantum of conductance.
When the speed of light c is taken away from the fine structure constant, as
has been shown elsewhere, its constancy implies the constancy of the ratio e2/h
(the inverse of the von Klitzing constant), e the charge of the electron and h
Planck constant. This forces the charge of the electron e to be constant as
long as the action h (an angular momentum) is a true constant too. From the
constancy of the Rydberg constant the Compton wavelength, h/mc, is then a true
constant and consequently there is no expansion at the quantum mechanical
level. The momentum mc is also a true constant and then general relativity
predicts that the universe is not expanding, as shown elsewhere. The time
variation of the speed of light explains the observed Hubble red shift. And
there is a mass-boom effect. From this a coherent cosmological system of
constant units can be defined.
|
1001.1561v1
|
2010-08-10
|
Linear Size Optimal q-ary Constant-Weight Codes and Constant-Composition Codes
|
An optimal constant-composition or constant-weight code of weight $w$ has
linear size if and only if its distance $d$ is at least $2w-1$. When $d\geq
2w$, the determination of the exact size of such a constant-composition or
constant-weight code is trivial, but the case of $d=2w-1$ has been solved
previously only for binary and ternary constant-composition and constant-weight
codes, and for some sporadic instances.
This paper provides a construction for quasicyclic optimal
constant-composition and constant-weight codes of weight $w$ and distance
$2w-1$ based on a new generalization of difference triangle sets. As a result,
the sizes of optimal constant-composition codes and optimal constant-weight
codes of weight $w$ and distance $2w-1$ are determined for all such codes of
sufficiently large lengths. This solves an open problem of Etzion.
The sizes of optimal constant-composition codes of weight $w$ and distance
$2w-1$ are also determined for all $w\leq 6$, except in two cases.
|
1008.1611v1
|
2007-02-15
|
Constant Mean Curvature n-noids with Platonic Symmetries
|
Constant Mean Curvature n-noids with Platonic Symmetries
|
0702469v1
|
2016-07-08
|
Alternative set of defining constants for redefinition of four SI units
|
We discuss different sets of defining constants, fixed values of which are
considered in connection with the transition to new definitions of four SI
units (the kilogram, the mole, the ampere, and the kelvin). The notion of
constant's order in a given system of units is suggested. We propose an
alternative set of fixed constants applicable for new definitions of the four
SI units. We analyse and discuss in detail the set, which consists of the
Planck constant, the Avogadro constant, the Boltzmann constant and the magnetic
constant.
|
1607.02280v2
|
2021-11-16
|
New geometric constants of isosceles orthogonal type
|
Based on the parallelogram law and isosceles orthogonality, we define a new
orthogonal geometric constant. We first discuss some basic properties of this
new constant. Next, we consider the relation between the constant and the
uniformly non-square property. Moreover, a generalized constant is also
introduced and some basic properties are presented. It is shown that, for a
normed space, the constant value is equal to 1 if and only if the norm can be
induced by the inner product. Finally, we verify that this constant is closely
related to the well-known geometric constants through some inequalities.
|
2111.08392v2
|
2021-12-11
|
Inscribed triangles in the unit sphere and a new class of geometric constants
|
We will introduce a new geometric constant GL(X) based on the constant H(X)
proposed by Gao. We first further survey the constant H(X) and discuss some of
the properties of this constant that have not yet been discovered. Next, we
focus on a new constant GL(X) along with some of its basic properties. In
addition, we show some relations between the well-known geometric constants and
GL(X) through some inequalities. Finally, we characterize some generalized
forms of the constant GL(X).
|
2112.05922v1
|
2004-10-12
|
Atomic Clocks and Constraints on Variations of Fundamental Constants
|
We consider an application of precision frequency measurements to searches
for possible time variations of the fundamental physical constants. Current
laboratory constraints on variations of the fine structure constant alpha and
other fundamental constants are presented.
|
0410074v1
|
2009-11-04
|
On the best constant of Hardy-Sobolev Inequalities
|
We obtain the sharp constant for the Hardy-Sobolev inequality involving the
distance to the origin. This inequality is equivalent to a limiting
Caffarelli-Kohn-Nirenberg inequality. In three dimensions, in certain cases the
sharp constant coincides with the best Sobolev constant.
|
0911.0948v1
|
2010-01-01
|
On Apery's Constant and Catalan's Constant
|
In this paper, Riemann's Zeta function with odd positive integer argument is
represented as an infinite summation of integer powers of $\pi$ with rational
coefficients. Specific values for Apery's Constant and Catalan's Constant are
then derived.
|
1001.0248v2
|
2019-04-23
|
Two-field constant roll inflation
|
Starting from the idea of realising constant roll inflation in string theory
we develop the constant roll formalism for two scalar fields. We derive the
two-field potential which is compatible with a constant roll regime and discuss
possible applications to string-models.
|
1904.10241v1
|
2023-07-16
|
When do Fermat constants completely determine Clairaut constants for branching geodesics on a surface of revolution?
|
We prove that Fermat constants do not completely determine Clairaut constants
for three branching geodesics that meet at the weighted Fermat-Torricelli point
on a surface of revolution, except the case of a standard sphere in
$\mathbb{R}^{3}.$
|
2310.07717v1
|
1994-10-06
|
Damped Lyman Alpha Systems vs. Cold + Hot Dark Matter
|
Damped Ly$\alpha$ systems provide possibly the most significant evidence for
early structure formation, and thus a stringent constraint on the Cold + Hot
Dark Matter (CHDM) cosmology. Using the numbers of halos in N-body simulations
to normalize Press-Schechter (PS) estimates of the number densities of
protogalaxies as a function of redshift, we find that CHDM with
$\Omega_c/\Omega_\nu/\Omega_b = 0.6/0.3/0.1$ is compatible with the damped
Ly$\alpha$ data only at $z<2.5$, but that it is probably incompatible with data
at $z>3$. The predictions of CHDM are quite sensitive to the neutrino fraction.
We find that $\Omega_c/\Omega_\nu/\Omega_b = 0.725/0.20/0.075$ is compatible
with the $z>3$ data. With one massive neutrino species, this corresponds to
lowering the neutrino mass from 7.0 to 4.7 eV, for $H_0=50\kmsMpc$ and
$T=2.726$ K. By analysing our numerical simulations with different resolutions
and box sizes as well as those of Ma \& Bertchinger (1994), we show that for
the CHDM models with $\Omega_\nu$=0.2--0.3 the PS approximation should be used
with Gaussian filter with $\delta_c=1.3-1.4$ if one tries to recover the total
mass of a collapsed halo and to include nonlinear effects, due to waves both
longer and shorter than those within the simulation box.
|
9410022v1
|
1995-12-05
|
Small Scale Structure and High Redshift HI
|
Cosmological simulations with gas dynamics suggest that the Lyman-alpha
forest is produced mainly by "small scale structure" --- filaments and sheets
that are the high redshift analog of today's galaxy superclusters. There is no
sharp distinction between Lyman-alpha clouds and "Gunn-Peterson" absorption
produced by the fluctuating IGM -- the Lyman-alpha forest {\it is} the
Gunn-Peterson effect. Lyman limit and damped Lyman-alpha absorption arises in
the radiatively cooled gas of forming galaxies. At $z~2-3$, most of the gas is
in the photoionized, diffuse medium associated with the Lyman-alpha forest, but
most of the {\it neutral} gas is in damped Lyman-alpha systems. We discuss
generic evolution of cosmic gas in a hierarchical scenario of structure
formation, with particular attention to the prospects for detecting 21cm
emission from high redshift HI. A scaling argument based on the present-day
cluster mass function suggests that objects with $M_{HI} >~ 5e11 h^{-1} \msun$
should be extremely rare at $z~3$, so detections with existing instruments will
be difficult. An instrument like the proposed Square Kilometer Array could
detect individual damped Lyman-alpha systems at high redshift, making it
possible to map structure in the high redshift universe in much the same way
that today's galaxy redshift surveys map the local large scale structure.
|
9512016v1
|
1996-04-03
|
APM z>4 QSO Survey: Spectra and Intervening Absorption Systems
|
The APM multicolor survey for bright z > 4 objects, covering 2500 deg^2 of
sky to m(R)~19, resulted in the discovery of thirty-one quasars with z > 4.
High signal-to-noise optical spectrophotometry at 5A resolution has been
obtained for the twenty-eight quasars easily accessible from the northern
hemisphere. These spectra have been surveyed to create new samples of high
redshift Lyman-limit systems, damped Lyman-alpha absorbers, and metal
absorption systems (e.g. CIV and MgII). In this paper we present the spectra,
together with line lists of the detected absorption systems. The QSOs display a
wide variety of emission and absorption line characteristics, with 5 exhibiting
broad absorption lines and one with extremely strong emission lines
(BR2248-1242). Eleven candidate damped Ly-alpha absorption systems have been
identified covering the redshift range 2.8<z<4.4 (8 with z>3.5). An analysis of
the measured redshifts of the high ionization emission lines with the low
ionization lines shows them to be blueshifted by 430+/-60 km/s. In a previous
paper (Storrie-Lombardi et. al. 1994) we discussed the redshift evolution of
the Lyman limit systems catalogued here. In subsequent papers we will discuss
the properties of the Ly-alpha forest absorbers and the redshift and column
density evolution of the damped Ly-alpha absorbers.
|
9604021v1
|
1996-06-13
|
Damping of Cosmic Magnetic Fields
|
We examine the evolution of magnetic fields in an expanding fluid composed of
matter and radiation with particular interest in the evolution of cosmic
magnetic fields. We derive the propagation velocities and damping rates for
relativistic and non-relativistic fast and slow magnetosonic, and Alfv\'en
waves in the presence of viscous and heat conducting processes. The analysis
covers all MHD modes in the radiation diffusion and the free-streaming regimes.
When our results are applied to the evolution of magnetic fields in the early
universe, we find that cosmic magnetic fields are damped from prior to the
epoch of neutrino decoupling up to recombination. Our findings have multifold
implications for cosmology. The dissipation of magnetic field energy into heat
during the epoch of neutrino decoupling ensures that most magnetic field
configurations generated in the very early universe satisfy big bang
nucleosynthesis constraints. Further dissipation before recombination
constrains models in which primordial magnetic fields give rise to galactic
magnetic fields or density perturbations. Finally, the survival of Alfv\'en and
slow magnetosonic modes on scales well below the Silk mass may be of
significance for the formation of structure on small scales (abridged).
|
9606080v2
|
1997-01-09
|
Molecular Hydrogen Absorption in the z= 1.97 Damped Lyman alpha Absorption system toward QSO 0013-004
|
We present a new ultra-violet spectrum of the QSO 0013-004 with 0.9 \AA
resolution obtained with the MMT Blue spectrograph. The \upsilon = 0 - 0, 1 -
0, 2 - 0 and 3 - 0 Lyman bands of H_2 associated with the z = 1.9731 damped Ly
alpah absorption line system have been detected. The H_2 column density is
N(H_2) = 6.9 (\pm 1.6)\times 10^{19} cm^{-2}, and the Doppler parameter b =
15\pm 2 km/s. The populations of different rotational levels are measured and
used to derive the excitation temperatures. The estimated kinetic temperature
T_K\sim 70 K, and the total particle number density n(H) \sim 300 cm^{-3}. The
UV photoabsorption rate $\beta_0 \sim 6.7\times 10^{-9}$ s^{-1}, about a factor
of few times greater than that in a typical diffuse Milky Way interstellar
cloud. The total hydrogen column density is $N(H) = 6.4(\pm 0.5)\times 10^{20}
cm^{-2}$. The fractional H_2 abundance f = 2N(H_2)/(2N(H_2) + N(H I)) \sim 0.22
\pm 0.05 is the highest among all observed damped Ly\al absorbers. The high
fractional H_2 abundance is consistent with the inferred presence of dust and
strong C I absorption in this absorber.
|
9701041v1
|
1997-02-06
|
Abundance Patterns of Heavy Elements in Damped Lyman-Alpha Galaxies
|
We present a quantitative analysis of the abundances of heavy elements in
damped Ly-alpha galaxies in the sample of Lu et al. (1996). In particular, we
compare the observed gas-phase abundances with those expected when the
intrinsic (i.e., nucleosynthetic) pattern is the same as that in either the Sun
or in Galactic halo stars and when the depletion pattern is the same as that in
the warm Galactic interstellar medium, but with various values of the
dust-to-metals ratio. We find that the observations are equally consistent with
the solar and halo-star intrinsic patterns and that they favor some depletion,
the typical dust-to-metals ratio being 40%-90% of that in the Milky Way today.
However, neither intrinsic pattern matches the observations perfectly. For the
solar pattern, the discrepancy is mainly with [Mn/Fe], while for the halo-star
pattern, the discrepancy is with [Zn/Fe], [Ni/Fe], and possibly [Al/Fe]. Our
analysis does not support the claim by Lu et al. that the damped Ly-alpha
galaxies have halo-star abundance patterns and no dust depletion.
|
9702066v2
|
1997-07-15
|
Linear Response, Dynamical Friction and the Fluctuation-Dissipation Theorem in Stellar Dynamics
|
We apply linear response theory to a general, inhomogeneous, stationary
stellar system, with particular emphasis on dissipative processes analogous to
Landau damping. Assuming only that the response is causal, we show that the
irreversible work done by an external perturber is described by the
anti-Hermitian part of a linear response operator, and damping of collective
modes is described by the anti-Hermitian part of a related polarization
operator. We derive an exact formal expression for the response operator, which
is the classical analog of a well-known result in quantum statistical physics.
When the self-gravity of the response can be ignored, and the ensemble-averaged
gravitational potential is integrable, the expressions for the mode energy,
damping rate, and polarization operator reduce to well-known formulae derived
from perturbation theory in action-angle variables. In this approximation,
dissipation occurs only via resonant interaction with stellar orbits or
collective modes. For stellar systems in thermal equilibrium, the
anti-Hermitian part of the response operator is directly related to the
correlation function of the fluctuations. Thus dissipative properties of the
system are completely determined by the spectrum of density fluctuations---the
fluctuation-dissipation theorem. In particular, we express the coefficient of
dynamical friction for an orbiting test particle in terms of the fluctuation
spectrum; this reduces to the known Chandrasekhar formula in the restrictive
case of an infinite homogeneous system with a Maxwellian velocity distribution.
|
9707161v1
|
1997-08-04
|
Violent Relaxation, Phase Mixing, and Gravitational Landau Damping
|
This paper proposes a geometric interpretation of flows generated by the
collisionless Boltzmann equation (CBE), focusing on the coarse-grained approach
towards equilibrium. The CBE is a noncanonical Hamiltonian system with the
distribution function f the fundamental dynamical variable, the mean field
energy H[f] playing the role of the Hamiltonian and the natural arena of
physics being the infinite-dimensional phase space of distribution functions.
Every time-independent equilibrium f_0 is an energy extremal with respect to
all perturbations that preserve the constraints associated with Liouville's
Theorem, local energy minima corresponding to linearly stable equilibria. If an
initial f(t=0) is sufficiently close to some linearly stable lower energy f_0,
its evolution involves linear phase space oscillations about f_0 which, in many
cases, would be expected to exhibit linear Landau damping. If f(t=0) is far
from any stable extremal, the flow will be more complicated but, in general,
one would anticipate that the evolution involves nonlinear oscillations about
some lower energy f_0. In this picture, the coarse-grained approach towards
equilibrium usually termed violent relaxation is interpreted as nonlinear
Landau damping. The evolution of a generic initial f(t=0) involves a coherent
initial excitation, not necessarily small, being converted into incoherent
motion associated with nonlinear oscillations about some equilibrium f_0 which,
in general, will exhibit destructive interference.
|
9708026v1
|
1998-11-23
|
The Physical Nature of the Lyman Limit Systems
|
We analyze Keck HIRES observations of a Lyman limit system at z=2.652 toward
Q2231-00. These observations afford the most comprehensive study of the
physical properties of a LL system to date. By comparing the ionic column
densities for Fe^+, Fe^{++}, Si^+, and Si^{3+} against calculations derived
from the CLOUDY software package, we have strictly constrained the ionization
state of this system. This has enabled us to calculate accurate abundances of a
Lyman limit system for the first time at z > 2, e.g., [Fe/H] = -0.5 \pm 0.1. We
also derive a total hydrogen column density, log N(H) = 20.73 \pm 0.2, which is
comparable to values observed for the damped lya systems.
The system is special for exhibiting CII* 1335 absorption, allowing an
estimate of the electron density, n_e = 6.5 \pm 1.3 x 10^{-2} cm^{-3}. Coupling
this measurement with our knowledge of the ionization state, we derive the
following physical properties: (1) hydrogen volume density, n_H = 5.9 \pm 1.2 x
10^{-2} cm^{-3}, (2) path length, l = 3 \pm 1.6 kpc, and (3) ionizing
intensity, log J_{912} = -20.22 \pm 0.21. We point out that a number of the
physical properties (e.g. [Fe/H], N(H), n_H) resemble those observed for the
damped lya systems, which suggests this system may be the photoionized analog
of a damped system. The techniques introduced in this Letter should be
applicable to a number of Lyman limit systems and therefore enable a survey of
their chemical abundances and other physical properties.
|
9811357v1
|
1998-12-03
|
Gravity-Modes in ZZ Ceti Stars III. Eigenvalues and Eigenfuctions
|
We report on numerical calculations of nonadiabatic eigenvalues and
eigenfunctions for g-modes in ZZ Ceti variables. The spectrum of overstable
$l=1$ modes delineates the instability strip. Its blue edge occurs where
$\omega \tau_c \approx 1$ for the $n=1$ mode. Here $\omega$ is radian frequency
and $\tau_c$ is about four times the thermal timescale at the bottom of the
surface convection zone. As a ZZ Ceti cools, its convection zone deepens,
longer period modes become overstable, but the critical value of $\omega\tau_c$
separating overstable and damped modes rises. The latter is a consequence of
enhanced radiative damping for modes which propagate immediately below the
convection zone. The critical value of $\omega\tau_c$ is of observational
significance because modes with the smallest value of $\omega\tau_c$ are most
observable photometrically. Maximum periods for overstable modes predicted for
our cooler model envelopes are about a factor two longer than the observational
upper limit of $1,200\s$. We assess a number of plausible resolutions for this
discrepancy among which convective overshoot and nonlinear saturation look
promising. The nonadiabatic eigenfunctions enable us to predict relative
amplitudes and phases of photospheric variations of flux and velocity,
quantities made accessible by recent observations. We also present asymptotic
formula for damping rates of high order modes, a result of consequence for
future investigations of nonlinear saturation of the amplidues of overstable
modes.
|
9812085v1
|
1999-01-19
|
Collisionless Dissipative Nonlinear Alfven Waves: Nonlinear Steepening, Compressible Turbulence, and Particle Trapping
|
The magnetic energy of nonlinear Alfven waves in compressible plasmas may be
ponderomotively coupled only to ion-acoustic quasi-modes which modulate the
wave phase velocity and cause wave-front steepening. In the collisionless
plasma with $\beta\not=0$, the dynamics of nonlinear Alfven wave is also
affected by the resonant particle-wave interactions. Upon relatively rapid
evolution (compared to the particle bounce time), the quasi-stationary wave
structures, identical to the so called (Alfvenic) Rotational Discontinuities,
form, the emergence and dynamics of which has not been previously understood.
Collisionless (Landau) dissipation of nonlinear Alfven waves is also a
plausible and natural mechanism of the solar wind heating. Considering a
strong, compressible, Alfvenic turbulence as an ensemble of randomly
interacting Alfvenic discontinuities and nonlinear waves, it is shown that
there exist two distinct phases of turbulence. What phase realizes depends on
whether this collisionless damping is strong enough to provide adequate energy
sink at all scales and, thus, to support a steady-state cascade of the wave
energy. In long-time asymptotics, however, the particle distribution function
is affected by the wave magnetic fields. In this regime of nonlinear Landau
damping, resonant particles are trapped in the quasi-stationary Alfvenic
discontinuities, giving rise to a formation of a plateau on the distribution
function and quenching collisionless damping. Using the virial theorem for
trapped particles, it is analytically demonstrated that their effect on the
nonlinear dynamics of such discontinuities is non-trivial and forces a
significant departure of the theory from the conventional paradigm.
|
9901257v1
|
2000-06-06
|
A Model of Metallicity Evolution in the Early Universe
|
We apply the phenomenological model used to explain the abundances of Fe and
r-process elements in very metal-poor stars in the Galaxy to [Fe/H] of damped
Ly alpha systems. It is assumed that the first stars formed after the Big Bang
were very massive and promptly enriched the interstellar medium to [Fe/H] ~ -3,
at which metallicity formation of normal stars took over. Subsequent Fe
enrichment was provided by Type II supernovae. The range of [Fe/H] at a given
redshift z for damped Ly alpha systems is explained by the time t* after the
Big Bang at which normal star formation started in an individual protogalactic
system. The average t* is approx 80% the age of the universe for damped Ly
alpha systems at z approx 1.5 to 4.5, indicating a long delay between the Big
Bang and the turn-on of protogalaxies. It is inferred that a substantial
fraction of the total baryonic matter may not have been aggregated into
protogalaxies where normal star formation had occurred down to z ~ 1.5. The
data near z = 2.2 suggest that the rate of turn-on of protogalaxies was
initially very low and slowly reached a maximum at ~ 3 Gyr after the Big Bang.
This may be important in understanding the rate of formation of quasars.
|
0006082v2
|
2000-09-27
|
Lensing properties of 7 damped Lyman-alpha absorbing galaxy-QSO pairs
|
Le Brun et al. (1997) presented the first identifications of the galaxies
giving rise to 7 intermediate redshift damped Ly-alpha (DLA) absorption
systems. Here, we study the gravitational lensing properties of these
foreground galaxies based on their observed optical appearance and on the
absence of any secondary lensed quasar image. We consider the possibility that
any secondary image be hidden due to extinction by dust, but find it unlikely.
We derive upper limits on the amplification factor affecting the luminosity of
the background quasars; in each case, this factor is found to be less than 0.3
mag. We also obtain upper limits on the total mass of the damped Ly-alpha
galaxies, within radii equal to the quasar impact parameters. Mass-to-light
ratios are found to be consistent with existing estimates based on X-ray
emission or on motion of dwarf satellites. Although we show that lensing is not
important in this sample, we note that existing DLA surveys used to determine
the cosmological density of gas at z<1 are based on samples of quasars brighter
than the ones considered here and for which the amplification bias is likely to
be stronger.
|
0009433v1
|
2000-11-14
|
Sharp HI edges at high z: the gas distribution from Damped Lyman-alpha to Lyman-limit absorption systems
|
We derive the distribution of neutral and ionized gas in high redshift clouds
which are optically thick to hydrogen ionizing radiation, using published data
on Lyman-limit and Damped Lyman-alpha absorption systems in the redshift range
1.75 < z < 3.25. We assume that the distribution of the hydrogen total (HI+HII)
column density in the absorbers follows a power law K N_H^{-alpha}, whereas the
observed HI column density distribution deviates from a pure power law as a
result of ionization from a background radiation field. Comparison of the
models and observations give Maximum Likelihood solutions for the exponent
alpha and for X, the value of log(N_H/N_HI) when the Lyman-limit optical depth
is unity: alpha=2.7^{+1.0}_{-0.7} and X=2.75\pm0.35. X is much lower than what
would be obtained for a gaseous distribution in equilibrium under its own
gravity but the ratio of dark matter to gas density is not well constrained
being log(eta_0)=1.1\pm 0.8. An extrapolation of our derived power law
distribution towards systems of lower column density, the Lyman-alpha forest,
favours models with log(eta_0) < 1.1 and alpha=2.7-3.3. With alpha appreciably
larger than 2, Lyman-limit systems contain more gas than Damped Lyman-alpha
systems and Lyman-alpha forest clouds even more. Estimates of the cosmological
gas and dark matter density due to absorbers of different column density around
z=2.5 are also given.
|
0011268v1
|
2000-11-29
|
Implications of 21cm observations for damped Ly-$α$ systems
|
We present Giant Metrewave Radio Telescope HI 21cm absorption observations,
of candidate and confirmed damped Lyman-$\alpha$ systems (DLAS). The derived
spin temperatures (T_s) are in all cases $\sim 1000$ K or higher. We have also
collated from the literature a list of DLAS for which 21cm observations exist,
and discuss their implications for the nature of the absorbers.
A cross-comparison of the 21cm profiles with low ionization metal profiles
shows that the 21cm absorption coincides in velocity with the deepest metal
line feature. This is consistent with models in which the deep metal line
features arise from discrete clouds but not with models where the deepest
features are the result of velocity crowding.
We also find that the typical derived spin temperatures of DLAS are
considerably higher than those in the Galaxy or nearby spirals. The only
exceptions are DLAS which are known to be associated with the disks of spirals;
these do, in fact, show low spin temperatures. In a multi-phase medium,the
derived T_s is a weighted average of the temperatures of the individual phases.
High derived T_s values are hence to be expected from small, low metallicity
objects, since these objects should have a lower fraction of the cold phase in
their ISM as compared to large galaxies. The high T_s in DLAS is hence
consistent with their observed low metallicities as well as with recent
observations that DLAS are also associated with dwarf/LSB galaxies. Finally, we
suggest that the following trend may be identified: at low redshift, damped
absorption arises from a range of systems, including spiral galaxy disks,
while, at high redshift, absorption occurs predominantly in smaller systems.
(Abridged)
|
0011540v1
|
2001-01-18
|
A Comprehensive Statistical Analysis of the Gas Distribution in Lyman-limit and Damped Lyman-alpha Absorption Systems
|
In this paper we show how to use data on Lyman-limit and Damped Lyman-alpha
absorption systems to derive the hydrogen ionization fractions and the
distribution of the face-on total gas column density. We consider axially
symmetric, randomly oriented absorbers, ionized by an external background
radiation field in order to relate the face-on total gas distribution to that
of the neutral hydrogen observed along the line of sight. We devise a
statistical procedure based on the Maximum Likelihood criterion, that is able
to treat simultaneously data coming from different surveys and statistically
recovers the "true" column densities in the presence of large uncertainties:
this is especially important for Lyman-limit systems which leave an
unmeasurable residual flux at wavelengths shorter than the Lyman break. We make
use of simulated data to look for possible observational biases and extensively
test our procedure. For a large statistical sample of real data in the redshift
range [1.75,3.25] (collected from all published surveys) our Maximum Likelihood
procedure gives a power-law slope for the total hydrogen distribution of -2.7.
All together Lyman-limit systems therefore contain more gas than Damped
Lyman-alpha systems. Analysis of data at other redshifts shows that more
observations are needed to reach a compelling evidence for a cosmological
evolution of the slope of the gas distribution.
|
0101335v1
|
2001-04-29
|
The First Detection of Co in a Damped Lyman Alpha System
|
The study of elemental abundances in Damped Lyman Alpha systems (DLAs) at
high redshift represents one of our best opportunities to probe galaxy
formation and chemical evolution at early times. By coupling measurements made
in high z DLAs with our knowledge of abundances determined locally and with
nucleosynthetic models, we can start to piece together the star formation
histories of these galaxies. Here, we discuss the clues to galactic chemical
evolution that may be gleaned from studying the abundance of Co in DLAs. We
present high resolution echelle spectra of two QSOs, Q2206-199 and Q1223+17,
both already known to exhibit intervening damped systems. These observations
have resulted in the first ever detection of Co at high redshift, associated
with the z= 1.92 DLA in the sightline towards Q2206-199. We find that the
abundance of Co is approximately 1/4 solar and that there is a clear
overabundance relative to iron, [Co/Fe] = +0.31 +/- 0.05. From the abundance of
Zn, we determine that this is a relatively metal-rich DLA, with a metallicity
approximately 1/3 solar. Therefore, this first detection of Co is similar to
the marked overabundance relative to Fe seen in Galactic bulge and thick disk
stars.
|
0104466v1
|
2001-06-22
|
Self-Absorption of Ionizing Radiation and Extended Lyman Alpha Emission in High- Redshift QSOs
|
We calculate the neutral hydrogen column density of self-absorption in QSOs
predicted in a model where the QSOs are located in the same halos that contain
the gas in damped Lya absorption systems. The model is parameterized by the
probability P_0 that any halo has an active QSO. We assume that the QSOs ionize
the gas, but do not expel or heat it. The derived HI column densities produce
negligible Lyman limit absorption, even in the lowest luminosity QSOs, with an
optical depth of only ~ 10% for luminosity L=0.01 L_*, when P_0=10^{-2}. We
also compute the HeII Lyman limit self-absorption, which is slightly higher but
still negligible. The self-absorption can be higher if the gas is highly
clumped; only in this case the overall emissivity from QSOs could be
significantly reduced due to absorption by the known damped Lya systems, to
affect the predicted intensity of the ionizing background or the epoch of HeII
reionization. The presence of the gas associated with damped absorption systems
around QSOs could also be detected from the narrow Lya emission line, which
should have and angular extent of 0.1 to 1'' in typical high-redshift QSOs.
|
0106424v2
|
2001-06-22
|
Compressible MHD Turbulence in Interstellar Plasmas
|
Radio-wave scintillation observations reveal a nearly Kolmogorov spectrum of
density fluctuations in the ionized interstellar medium. Although this density
spectrum is suggestive of turbulence, no theory relevant to its interpretation
exists. We calculate the density spectrum in turbulent magnetized plasmas by
extending the theory of incompressible MHD turbulence given by Goldreich &
Sridhar to include the effects of compressibility and particle transport. Our
most important results are as follows. (1) Density fluctuations are due to the
slow mode and the entropy mode. Both modes are passively mixed by the cascade
of shear Alfven waves. Since the shear Alfven waves have a Kolmogorov spectrum,
so do the density fluctuations. (2) Observed density fluctuation amplitudes
imply either that the magnetic and gas pressures are comparable, or that the
outer scale of the turbulence is very small. (3) A high degree of ionization is
required for the cascade to survive damping by neutrals and thereby to extend
to small lengthscales. Regions that are insufficiently ionized produce density
fluctuations only on lengthscales larger than the neutral damping scale. These
regions may account for the excess of power that is found on large scales. (4)
Both the entropy mode and the slow mode are damped on lengthscales below that
at which protons can diffuse across an eddy during the eddy's turnover time.
Consequently, eddies whose extents along the magnetic field are smaller than
the proton collisional mean free path do not contribute to the density
spectrum. However, in MHD turbulence eddies are highly elongated along the
magnetic field. From an observational perspective, the relevant lengthscale is
that transverse to the magnetic field. Thus the cut-off lengthscale for density
fluctuations is significantly smaller than the proton mean free path.
|
0106425v1
|
2001-11-20
|
Collisional Cascades in Planetesimal Disks I. Stellar Flybys
|
We use a new multiannulus planetesimal accretion code to investigate the
evolution of a planetesimal disk following a moderately close encounter with a
passing star. The calculations include fragmentation, gas and
Poynting-Robertson drag, and velocity evolution from dynamical friction and
viscous stirring. We assume that the stellar encounter increases planetesimal
velocities to the shattering velocity, initiating a collisional cascade in the
disk. During the early stages of our calculations, erosive collisions damp
particle velocities and produce substantial amounts of dust. For a wide range
of initial conditions and input parameters, the time evolution of the dust
luminosity follows a simple relation, L_d/L_{\star} = L_0 / [alpha +
(t/t_d)^{beta}]. The maximum dust luminosity L_0 and the damping time t_d
depend on the disk mass, with L_0 proportional to M_d and t_d proportional to
M_d^{-1}. For disks with dust masses of 1% to 100% of the `minimum mass solar
nebula' (1--100 earth masses at 30--150 AU), our calculations yield t_d approx
1--10 Myr, alpha approx 1--2, beta = 1, and dust luminosities similar to the
range observed in known `debris disk' systems, L_0 approx 10^{-3} to 10^{-5}.
Less massive disks produce smaller dust luminosities and damp on longer
timescales. Because encounters with field stars are rare, these results imply
that moderately close stellar flybys cannot explain collisional cascades in
debris disk systems with stellar ages of 100 Myr or longer.
|
0111384v1
|
2002-02-11
|
The Contribution of HI-rich Galaxies to the Damped Lyman-alpha Absorber Population at z=0
|
We present a study of the expected properties of the low redshift damped
Lyman-alpha absorber population determined from a sample of HI-selected
galaxies in the local universe. Because of a tight correlation between the HI
mass and HI cross-section, which we demonstrate spans all galaxy types, we can
use our HI-selected sample to predict the properties of the absorption line
systems. We use measurements of the number density and HI cross-section of
galaxies to show that the total HI cross-section at column densities sufficient
to produce damped Lyman-alpha absorption is consistent with no evolution of the
absorber population. We also find that the dN/dz distribution is dominated by
galaxies with HI masses near 10^9 Msolar. However, because of the large
dispersion in the correlation between HI mass and stellar luminosity, we find
that the distribution of dN/dz as a function of L_J is fairly flat.
Additionally, we examine the line widths of the HI-selected galaxies and show
that there may be evolution in the kinematics of HI-rich galaxies, but it is
not necessary for the higher redshift population to contain a greater
proportion of high mass galaxies than we find locally.
|
0202216v2
|
2002-03-18
|
Monte Carlo Simulation of Lyman Alpha Scattering and Application to Damped Lyman Alpha Systems
|
A Monte Carlo code to solve the transfer of Lyman alpha (Lya) photons is
developed, which can predict the Lya image and two-dimensional Lya spectra of a
hydrogen cloud with any given geometry, Lya emissivity, neutral hydrogen
density distribution, and bulk velocity field. We apply the code to several
simple cases of a uniform cloud to show how the Lya image and emitted line
spectrum are affected by the column density, internal velocity gradients, and
emissivity distribution. We then apply the code to two models for damped Lya
absorption systems: a spherical, static, isothermal cloud, and a flattened,
axially symmetric, rotating cloud. If the emission is due to fluorescence of
the external background radiation, the Lya image should have a core
corresponding to the region where hydrogen is self-shielded. The emission line
profile has the characteristic double peak with a deep central trough. We show
how rotation of the cloud causes the two peaks to shift in wavelength as the
slit is perpendicular to the rotation axis, and how the relative amplitude of
the two peaks is changed. In reality, damped Lya systems are likely to have a
clumpy gas distribution with turbulent velocity fields, which should smooth the
line emission profile, but should still leave the rotation signature of the
wavelength shift across the system.
|
0203287v2
|
2002-04-09
|
The Non-Damped Nature of Twelve Low-Redshift Damped Lyman Alpha Candidate Systems
|
Hubble Space Telescope (HST) UV spectroscopy of 12 candidate low-redshift
damped Lyman alpha (DLA) systems in 11 QSOs (z=0.103 in Q0054+144, z=0.969 and
z=0.987 in Q0302-223, z=0.478 in Q0454-220, z=1.476 in Q1047+550, z=1.070 in
Q1206+459, z=1.228 in Q1247+267, z=0.399 in Q1318+290B, z=0.519 in Q1329+412,
z=0.276 in Q1451-375, z=0.204 in Q2112+059, z=0.263 in Q2251+113) are
presented; the observations demonstrate that they are not DLAs with
$N(HI)\ge2\times10^{20}$ atoms cm$^{-2}$. In all cases except two the systems
either do not exist or are well below the DLA threshold column density; the
exceptions are a z=0.474 system in Q0454-220 which has $N(HI)=3\times10^{19}$
atoms cm$^{-2}$ and a z=1.223 system in Q1247+267 which has
$N(HI)=8\times10^{19}$ atoms cm$^{-2}$. Despite the availability of data in the
HST archives demonstrating that these are not suitable targets, many have
unfortunately been approved for observation with Chandra, Gemini, and/or HST
with the intent of doing followup work on low-redshift DLAs. Furthermore, these
results indicate that the low-redshift DLA statistics derived from IUE spectra
and presented by Lanzetta, Wolfe, & Turnshek (1995) and Wolfe et al. (1995) are
invalid.
|
0204155v1
|
2002-05-17
|
High matter density peaks from UVES observations of QSO pairs: correlation properties and chemical abundances
|
We study the transverse clustering properties of high matter density peaks as
traced by high column density absorption systems (either Lyman limit systems
characterized by N(HI)> 2 x 10^{17} cm^{-2} or CIV systems with W_{r}> 0.5 A)
at redshifts between 2 and 3 with UVES spectra of two QSO pairs (UM680/UM681 at
56 arcsec angular separation and Q2344+1228/Q2343+1232 at 5 arcmin angular
separation) and a QSO triplet (Q2139-4433/Q2139-4434/Q2138-4427 at 1, 7 and 8
arcmin angular separation). We find 3 damped Ly-alpha systems (N(HI)> 2 x
10^{20} cm^{-2}): 2 coinciding with strong metal systems in the nearby line of
sight and 1 matching the emission redshift of the paired QSO; plus 7 Lyman
limit systems: 4 forming two matching couples and 3 without a corresponding
metal system within ~3000 km/s in the coupled line of sight. In summary, we
detect five out of ten matching systems within 1000 km/s, indicating a highly
significant overdensity of strong absorption systems over separation lengths
from ~1 to 8 h^{-1} Mpc. The observed coincidences could arise in gas due to
starburst-driven superwinds associated with a quasar or a galaxy, or gas
belonging to large scale structures like filaments or sheets. We also determine
chemical abundance ratios for three damped Ly-alpha systems. In particular, for
the damped system at z ~ 2.53788 in the spectrum of Q2344+1228, new estimates
of the ratios O/Fe, C/Fe are obtained: [C/Fe]<0.06, [O/Fe]<0.2. They indicate
that O and C are not over-solar in this system.
|
0205299v1
|
2002-08-28
|
Complex rotation with internal dissipation. Applications to cosmic-dust alignment and to wobbling comets and asteroids
|
Neutron stars, asteroids, comets, cosmic-dust granules, spacecraft, as well
as whatever other freely spinning body dissipate energy when they rotate about
any axis different from principal. We discuss the internal-dissipation-caused
relaxation of a freely precessing rotator towards its minimal-energy mode (mode
that corresponds to the spin about the maximal-inertia axis). While the body
nutates at some rate, the internal stresses and strains within the body
oscillate at frequencies both higher and lower than this rate. The internal
dissipation takes place mostly the second and higher harmonics. We discuss the
application of our findings to asteroids. Regarding the comets, estimates show
that the currently available angular resolution of spacecraft-based instruments
makes it possible to observe wobble damping within year- or maybe even
month-long spans of time. We also discuss cosmic-dust astrophysics; in
particular, the role played by precession damping in the dust alignment. We
show that this damping provides coupling of the grain's rotational and
vibrational degrees of freedom; this entails occasional flipping of dust grains
due to thermal fluctuations. During such a flip, grain preserves its angular
momentum, but the direction of torques arising from H2 formation reverses. As a
result, flipping grain will not rotate fast in spite of the action of
uncompensated H2 formation torques. The grains get ``thermally trapped,'' and
their alignment is marginal.
|
0208489v1
|
2003-06-16
|
Damping of Tensor Modes in Cosmology
|
An analytic formula is given for the traceless transverse part of the
anisotropic stress tensor due to free streaming neutrinos, and used to derive
an integro-differential equation for the propagation of cosmological
gravitational waves. The solution shows that anisotropic stress reduces the
squared amplitude by 35.6 % for wavelengths that enter the horizon during the
radiation-dominated phase, independent of any cosmological parameters. This
decreases the tensor temperature and polarization correlation functions for
these wavelengths by the same amount. The effect is less for wavelengths that
enter the horizon at later times. At the longest wavelengths the decrease in
the tensor correlation functions due to neutrino free streaming ranges from
10.7% for $\Omega_Mh^2=0.1$ to 9.0% for $\Omega_Mh^2=0.15$. An Appendix gives a
general proof that tensor as well as scalar modes satisfy a conservation law
for perturbations outside the horizon, even when the anisotropic stress tensor
is not negligible.
|
0306304v2
|
2003-12-30
|
Nonlinear Pulsations in Differentially Rotating Neutron Stars: Mass-Shedding-Induced Damping and Splitting of the Fundamental Mode
|
We study small-amplitude, nonlinear pulsations of uniformly and
differentially rotating neutron stars employing a two-dimensional evolution
code for general-relativistic hydrodynamics. Using Fourier transforms at
several points inside the star, both the eigenfrequencies and two-dimensional
eigenfunctions of pulsations are extracted. The centrifugal forces and the
degree of differential rotation have significant effects on the
mode-eigenfunction. We find that near the mass-shedding limit, the pulsations
are damped due to shocks forming at the surface of the star. This new damping
mechanism may set a small saturation amplitude for modes that are unstable to
the emission of gravitational waves. After correcting for the assumption of the
Cowling approximation (used in our numerical code), we construct empirical
relations that predict the range of gravitational-wave frequencies from
quasi-periodic post-bounce oscillations in the core collapse of massive stars.
We also find that the fundamental quasi-radial mode is split, at least in the
Cowling approximation and mainly in differentially rotating stars, into two
different sequences.
|
0312648v2
|
2004-01-16
|
Simulating stochastically excited oscillations. The mode lifetime of xi Hya
|
The discovery of solar-like oscillations in the giant star xy Hya (G7III) was
reported by Frandsen et al. (2002). Their frequency analysis was very limited
due to alias problems in the data set (caused by single-site observations). The
extent to which the aliasing affected their analysis was unclear due to the
unknown damping time of the stellar oscillation modes. In this paper we
describe a simulator created to generate time series of stochastically excited
oscillations, which takes as input an arbitrary window function and includes
both white and non-white noise. We also outline a new method to compare a large
number of simulated time series with an observed time series to determine the
damping time, amplitude, and limited information on the degree of the
stochastically excited modes. For xi Hya we find the most likely amplitude to
be ~ 2 m/s, in good agreement with theory (Houdek & Gough, 2002), and the most
likely damping time to be ~ 2 days, which is much shorter than the theoretical
value of 15-20 days calculated by Houdek & Gough (2002).
|
0401331v1
|
2004-10-13
|
On the Importance of Local Sources of Radiation in Cosmological Absorption Systems
|
An upper limit to the importance of local sources of radiation compared to
the cosmic background in cosmological absorption systems is derived, as a
simple consequence of the conservation of surface brightness. The limit depends
only on the rate of incidence of the absorbers and the mean free path of the
radiation. It is found that, on average, the ionizing radiation intensity from
local sources in Lyman limit systems at z>2 must be less than half of the
intensity of the cosmic background. In absorbers with column densities much
lower than Lyman limit systems, the local source contribution must be
negligible. The limit on the ratio of local source to background intensities is
then applied to the class of damped Lya absorption systems with detectable
excited CII lines. A cooling rate of the gas in these systems has been measured
by Wolfe et al., who assumed that the balancing heating source is photoelectric
heating on dust by light at ~ 1500 A . The intensity from local star formation
at this wavelength in this class of damped Lya systems is found to be at most ~
3 times the background intensity. If the heating source is indeed photoelectric
heating of dust, the background created by sources associated with damped Lya
systems can then be estimated from the average cooling rates measured in the
absorbers. Current results yield a background intensity higher than previous
estimates based on observed galaxy and quasar luminosity functions, although
with a large uncertainty. The possibility of other sources of heating, such as
shock-heating in a turbulent medium, should be explored.
|
0410315v1
|
2005-04-04
|
Relativistic r-modes and Shear viscosity: regularizing the continuous spectrum
|
Within a fully relativistic framework, we derive and solve numerically the
perturbation equations of relativistic stars, including the stresses produced
by a non-vanishing shear viscosity in the stress-energy tensor. With this
approach, the real and imaginary parts of the frequency of the modes are
consistently obtained. We find that, approaching the inviscid limit from the
finite viscosity case, the continuous spectrum is regularized and we can
calculate the quasi-normal modes for stellar models that do not admit solutions
at first order in perturbation theory when the coupling between the polar and
axial perturbations is neglected. The viscous damping time is found to agree
within factor 2 with the usual estimate obtained by using the eigenfunctions of
the inviscid limit and some approximation for the energy dissipation integrals.
We find that the frequencies and viscous damping times for relativistic
$r-$modes lie between the Newtonian and Cowling results. We compare the results
obtained with homogeneous, polytropic and realistic equations of state and find
that the frequencies depend only on the rotation rate and on the compactness
parameter (M/R), being almost independent of the equation of state. Our
numerical results for realistic neutron stars give viscous damping times with
the same dependence on mass and radius as previously estimated, but
systematically larger of about 60%.
|
0504062v1
|
2005-04-29
|
Evidence for a magnitude-dependent bias in the Hamburg/ESO Survey for Damped Lyman-alpha Systems
|
We present preliminary results from the Hamburg/ESO survey for Damped
Ly-alpha (hereafter, DLA) Systems. This survey is characterized by (i) the good
knowledge of the biases affecting the parent QSO survey, (ii) the brightness
and (iii) the relatively wide magnitude distribution of the background QSOs.
Therefore, it is well-suited to study possible magnitude-dependent biases in
DLA surveys, such as the one expected from dust obscuration. We have
systematically searched for damped Lyman-alpha line candidates in 5 A
resolution spectra of the 188 QSOs that constitute our statistical sample.
These candidates have later been reobserved with UVES at the ESO--Very Large
Telescope (VLT) for confirmation and accurate N(HI) measurements. In the
redshift range covered by the survey, 19 DLA systems have been discovered. Over
the whole survey, we find that the number density, n(z), and cosmological
density of gas, Omega_gas, have comparable values to the ones obtained by
CORALS (Ellison et al. 2001). However, the number densities of DLA systems n(z)
in two sub-samples of equal absorption distance path defined by the magnitude
of the background QSOs differ by a factor of about 5. We estimate that the
probability that n(z) is equal in the two sub-samples is < 0.003. A similar,
only slightly less significant difference is found for Omega_gas.
|
0504657v1
|
2005-10-12
|
Ly-alpha Radiative Transfer in Cosmological Simulations and Application to a z~8 Emitter
|
We develop a Ly-alpha radiative transfer (RT) Monte Carlo code for
cosmological simulations.High resolution,along with appropriately treated
cooling can result in simulated environments with very high optical
depths.Thus,solving the Ly-alpha RT problem in cosmological simulations can
take an unrealistically long time.For this reason,we develop methods to speed
up the Ly-alpha RT.With these accelerating methods,along with the
parallelization of the code,we make the problem of Ly-alpha RT in the complex
environments of cosmological simulations tractable.We test the RT code against
simple Ly-alpha emitter models,and then we apply it to the brightest Ly-alpha
emitter of a gasdynamics+N-body Adaptive Refinement Tree (ART) simulation at
z~8.We find that recombination rather than cooling radiation Ly-alpha photons
is the dominant contribution to the intrinsic Ly-alpha luminosity of the
emitter,which is ~4.8x10e43 ergs/s.The size of the emitter is pretty
small,making it unresolved for currently available instruments.Its spectrum
before adding the Ly-alpha Gunn-Peterson absorption (GP) resembles that of
static media,despite some net inward radial peculiar motion.This is because for
such high optical depths as those in ART simulations,velocities of order some
hundreds km/s are not important.We add the GP in two ways.First we assume no
damping wing,corresponding to the situation where the emitter lies within the
HII region of a very bright quasar,and second we allow for the damping
wing.Including the damping wing leads to a maximum line brightness suppression
by roughly a factor of ~62.The line fluxes,even though quite faint for current
ground-based telescopes,should be within reach for JWST.
|
0510347v2
|
2005-12-05
|
Kinematics and star formation activity in the z=2.03954 damped Lyman-alpha system towards PKS 0458-020
|
We present UVES observations of the log N(HI)= 21.7 damped Lyman-alpha system
at z=2.03954 towards the quasar PKS 0458-020. HI Lyman-alpha emission is
detected in the center of the damped Lyman-alpha absorption trough.
Metallicities are derived for MgII, SiII, PII, CrII, MnII, FeII and ZnII and
are found to be -1.21\pm0.12, -1.28\pm0.20, -1.54\pm0.11, -1.66\pm0.10,
-2.05\pm0.11, -1.87\pm0.11, -1.22\pm0.10, respectively, relative to solar. The
depletion factor is therefore of the order of [Zn/Fe]=0.65. We observe metal
absorption lines to be blueshifted compared to the Lyman-alpha emission up to a
maximum of 100 and 200 km/s for low and high-ionization species respectively.
This can be interpreted either as the consequence of rotation in a large
(~7kpc) disk or as the imprint of a galactic wind. The star formation rate
(SFR) derived from the Lyman-alpha emission, 1.6 solar masses/yr, is compared
with that estimated from the observed CII* absorption. No molecular hydrogen is
detected in our data, yielding a molecular fraction f<-6.52. This absence of H2
can be explained as the consequence of a high ambient UV flux which is one
order of magnitude larger than the radiation field in the ISM of our Galaxy and
originates in the observed emitting region.
|
0512118v1
|
2006-02-14
|
On the magnetic structure and wind parameter profiles of Alfven wave driven winds in late-type supergiant stars
|
Cool stars at giant and supergiant evolutionary phases present low velocity
and high density winds, responsible for the observed high mass-loss rates.
Although presenting high luminosities, radiation pressure on dust particles is
not sufficient to explain the wind acceleration process. Among the possible
solutions to this still unsolved problem, Alfven waves are, probably, the most
interesting for their high efficiency in transfering energy and momentum to the
wind. Typically, models of Alfven wave driven winds result in high velocity
winds if they are not highly damped. In this work we determine
self-consistently the magnetic field geometry and solve the momentum, energy
and mass conservation equations, to demonstrate that even a low damped Alfven
wave flux is able to reproduce the low velocity wind. We show that the magnetic
fluxtubes expand with a super-radial factor S>30 near the stellar surface,
larger than that used in previous semi-empirical models. The rapid expansion
results in a strong spatial dilution of the wave flux. We obtained the wind
parameter profiles for a typical supergiant star of 16 M_sun. The wind is
accelerated in a narrow region, coincident with the region of high divergence
of the magnetic field lines, up to 100 km/s. For the temperature, we obtained a
slight decrease near the surface for low damped waves, because the wave heating
mechanism is less effective than the radiative losses. The peak temperature
occurs at 1.5 r_0 reaching 6000 K. Propagating outwards, the wind cools down
mainly due to adiabatic expansion.
|
0602305v1
|
2006-03-17
|
Electron impact excitation of Helium-like ions up to n=4 levels including radiation damping
|
Helium-like ions provide the most important X-ray spectral diagnostics in
high temperature fusion and astrophysical plasmas. We previously presented
computed collision strengths for O~VII including relativistic fine structure,
levels up to the $n=4$ complex and radiation damping of autoionizing
resonances. We have extended this work to other He-like ions (N, Ne, Mg, Al,
Si, S, Ca). The calculations are carried out using the Breit-Pauli R-matrix
(BPRM) method with a 31-level eigenfunction expansion. Collision strengths for
the principal lines important in X-ray plasma diagnostics, w, x, y and z,
corresponding to the 4 transitions to the ground level 1s^2(^1S_0) <-
1s2p(^1P^o_1), 1s2p(^3P^o_2), 1s2p(^3P^o_1), 1s2s(^3S_1), are explicitly shown.
We find the effect of radiation damping to be significant for the forbidden
transitions in heavier He-like ions, which should affect the diagnostic line
ratios. We extrapolated the collision strengths to their values at infinite
energy using the Burgess-Tully extrapolation technique. This is required to
calculate the Maxwellian average collision strengths at high temperature. We
show that the coupling between dipole allowed and inter-combination transitions
affects increasingly the effective collision strengths for the n ^1S_0 - n'
^3P_1 transition as the charge of the ion increases. This clearly affects the
treatment of the extrapolation toward the infinite energy point of the
collision strength. This work is carried out as part of the Iron Project-RmaX
Network.
|
0603482v1
|
2006-07-25
|
Metal-rich Damped/sub-Damped Lyman-alpha Quasar Absorbers at z<1
|
Damped Lyman-alpha absorbers (DLAs), seen in absorption against a background
quasar, provide the most detailed probes available of element abundances in the
Universe over > 90 % of its age. DLAs can be used to observationally measure
the global mean metallicity in the Universe and its evolution with time.
Paradoxically, these observations are more difficult at lower redshifts, where
the absorber rest-frame UV spectra are cut-off due to the atmospheric
absorption. We present here high-resolution VLT/UVES observations of several
elements contained in three DLAs and one sub-DLA with 0.6<z_abs<0.9. We detect
Mg I, Mg II, Fe II, Zn II, Cr II, Mn II, Ti II and Ca II. Our observations more
than double the high-resolution sample of [Zn/H] at z<1. We also report the
discovery of three metal-rich systems, whereas most previous measurements show
low N(HI)-weighted mean metallicity projecting to about 1/6th solar level at
z=0. We derive [Zn/H]=-0.11+/-0.04 at z_abs=0.725, [Zn/H]=-0.54+/-0.20 at
z_abs=0.740 and [Zn/H]=-0.49+/-0.22 at z_abs=0.652, plus one additional upper
limit ([Zn/H]<-0.36 at z_abs=0.842). These measurements confirm the existence
of quasar absorbers with relatively high metallicities based on abundance
estimates free from the effect of dust depletion. Possible implications of
these results for the metallicity of neutral gas phase in the past ~ 8 Gyr are
presented and compared with models.
|
0607561v1
|
1998-06-21
|
Collective intersubband transitions in quantum wells: a comparative density-functional study
|
We use time-dependent (current) density functional theory to study collective
transitions between the two lowest subbands in GaAs/AlGaAs quantum wells. We
focus on two systems where experimental results are available: a wide single
and a narrow asymmetric double well. The aim is to calculate frequency and
linewidth of collective electronic modes damped via electron-electron
interaction only. Since Landau damping is not effective here, the dominant
damping mechanism involves dynamical exchange-correlation effects such as
multipair production. To capture these effects, one has to go beyond the widely
used adiabatic local density approximation (ALDA) and include retardation. We
perform a comparative study of two approaches which fall in this category: the
dynamical extension of the ALDA by Gross and Kohn, and a more recent method
which treats exchange and correlation beyond the ALDA as viscoelastic stresses
in the electron liquid. We find that the former method is more robust: it
performs similarly for strongly different degrees of collectivity of the
electronic motion. Results for quantum wells compare reasonably to experiment,
with a tendency towards overdamping. By contrast, the viscoelastic approach is
superior for systems where the electron dynamics is predominantly collective,
but breaks down if the local velocity field is too rapidly varying, as in the
case of a single-electron-like behavior such as tunneling through a potential
barrier.
|
9806252v1
|
1998-11-27
|
Renormalization group analysis of the quantum non-linear sigma model with a damping term
|
We investigate the behavior of the zero-temperature quantum non-linear sigma
model in d dimensions in the presence of a damping term of the form f(w)~
|w|^alpha, with 1 \le alpha <2. We find two fixed points: a spin-wave fixed
point FP1 showing a dynamic scaling exponent z=1 and a dissipative fixed point
FP2 with z>1. In the framework of the \epsilon-expansion it is seen that there
is a range of values alpha_*(d) \le alpha \le 2 where the point FP1 is stable
with respect to FP2, so that the system realizes a z=1 quantum critical
behavior even in the presence of a dissipative term. However, reasonable
arguments suggest that in d=2 this range is very narrow. In the broken symmetry
phase we discuss a phenomenological scaling approach, treating damping as a
perturbation of the ordered ground state. The relation of these results with
the pseudogap effect observed in underdoped layered cuprates is discussed.
|
9811392v2
|
1999-12-14
|
Spinwave damping in the two-dimensional ferromagnetic XY model
|
The effect of damping of spinwaves in a two-dimensional classical
ferromagnetic XY model is considered. The damping rate $\Gamma_{q}$ is
calculated using the leading diagrams due to the quartic-order deviations from
the harmonic spin Hamiltonian. The resulting four-dimensional integrals are
evaluated by extending the techniques developed by Gilat and others for
spectral density types of integrals. $\Gamma_{q}$ is included into the memory
function formalism due to Reiter and Solander, and Menezes, to determine the
dynamic structure function $S(q,\omega)$. For the infinite sized system, the
memory function approach is found to give non-divergent spinwave peaks, and a
smooth nonzero background intensity (``plateau'' or distributed intensity) for
the whole range of frequencies below the spinwave peak. The background
amplitude relative to the spinwave peak rises with temperature, and eventually
becomes higher than the spinwave peak, where it appears as a central peak. For
finite-sized systems, there are multiple sequences of weak peaks on both sides
of the spinwave peaks whose number and positions depend on the system size and
wavevector in integer units of $2\pi/L$. These dynamical finite size effects
are explained in the memory function analysis as due to either spinwave
difference processes below the spinwave peak or sum processes above the
spinwave peak. These features are also found in classical Monte Carlo --
Spin-Dynamics simulations.
|
9912241v1
|
2002-01-25
|
Time-dependent current density functional theory for the linear response of weakly disordered systems
|
This paper develops a quantitatively accurate first-principles description
for the frequency and the linewidth of collective electronic excitations in
inhomogeneous weakly disordered systems. A finite linewidth in general has
intrinsic and extrinsic sources. At low temperatures and outside the region
where electron-phonon interaction occurs, the only intrinsic damping mechanism
is provided by electron-electron interaction. This kind of intrinsic damping
can be described within time-dependent density-functional theory (TDFT), but
one needs to go beyond the adiabatic approximation and include retardation
effects. It was shown previously that a density-functional response theory that
is local in space but nonlocal in time has to be constructed in terms of the
currents, rather than the density. This theory will be reviewed in the first
part of this paper. For quantitatively accurate linewidths, extrinsic
dissipation mechanisms, such as impurities or disorder, have to be included. In
the second part of this paper, we discuss how extrinsic dissipation can be
described within the memory function formalism. We first review this formalism
for homogeneous systems, and then present a synthesis of TDFT with the memory
function formalism for inhomogeneous systems, to account simultaneously for
intrinsic and extrinsic damping of collective excitations. As example, we
calculate frequencies and linewidths of intersubband plasmons in a 40 nm wide
GaAs/AlGaAs quantum well.
|
0201483v1
|
2003-06-10
|
Oscillations of Bose-Einstein condensates with vortex lattices. II. Finite temperatures
|
We derive the finite temperature oscillation modes of a harmonically confined
Bose-Einstein condensed gas undergoing rigid body rotation supported by a
vortex lattice in the condensate. The hydrodynamic modes separate into two
classes corresponding to in-phase (center-of-mass) and counter-phase (relative)
oscillations of the thermal cloud and the condensate. The in- and counter-phase
oscillations are independent of each other in the case where the thermal cloud
is inviscid for all modes studied, except the radial pulsations which couple
them because the pressure perturbations of the condensate and the thermal cloud
are governed by different adiabatic indices. If the thermal cloud is viscous,
the two classes of oscillations are coupled, i.e. each type of motion involves
simultaneously mass and entropy currents. The counter-phase oscillations are
damped by the mutual friction between the condensate and the thermal cloud
mediated by the vortex lattice. The damping is large for the values of the
drag-to-lift ratio of the order of unity and becomes increasingly ineffective
in either limit of small or large friction. An experimental measurement of a
subset of these oscillation modes and their damping rates can provide
information on the values of the phenomenological mutual friction coefficients,
and hence the quasiparticle-vortex scattering processes in dilute atomic Bose
gases.
|
0306245v2
|
2004-05-14
|
Thermoelastic relaxation in elastic structures with applications to thin plates
|
A new result enables direct calculation of thermoelastic damping in vibrating
elastic solids. The mechanism for energy loss is thermal diffusion caused by
inhomogeneous deformation, flexure in thin plates. The general result is
combined with the Kirchhoff assumption to obtain a new equation for the
flexural vibration of thin plates incorporating thermoelastic loss as a damping
term. The thermal relaxation loss is inhomogeneous and depends upon the local
state of vibrating flexure, specifically, the principal curvatures at a given
point on the plate. Thermal loss is zero at points where the principal
curvatures are equal and opposite, that is, saddle shaped or pure anticlastic
deformation. Conversely, loss is maximum at points where the curvatures are
equal, that is, synclastic or spherical flexure. The influence of modal
urvature on the thermoelastic damping is described through a modal
pparticipation factor. The effect of transverse thermal diffusion on plane wave
propagation is also examined. It is shown that transverse diffusion effects are
always small provided the plate thickness is far greater than the thermal
phonon mean free path, a requirement for the validity of the classical theory
of heat transport. These results generalize Zener's theory of thermoelastic
loss in beams and are useful in predicting mode widths in MEMS and NEMS
oscillators.
|
0405323v2
|
2005-04-21
|
Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals
|
The femtosecond optical pump-probe technique was used to study dynamics of
photoexcited electrons and coherent optical phonons in transition metals Zn and
Cd as a function of temperature and excitation level. The optical response in
time domain is well fitted by linear combination of a damped harmonic
oscillation because of excitation of coherent $E_{2g}$ phonon and a
subpicosecond transient response due to electron-phonon thermalization. The
electron-phonon thermalization time monotonically increases with temperature,
consistent with the thermomodulation scenario, where at high temperatures the
system can be well explained by the two-temperature model, while below
$\approx$ 50 K the nonthermal electron model needs to be applied. As the
lattice temperature increases, the damping of the coherent $E_{2g}$ phonon
increases, while the amplitudes of both fast electronic response and the
coherent $E_{2g}$ phonon decrease. The temperature dependence of the damping of
the $E_{2g}$ phonon indicates that population decay of the coherent optical
phonon due to anharmonic phonon-phonon coupling dominates the decay process. We
present a model that accounts for the observed temperature dependence of the
amplitude assuming the photoinduced absorption mechanism, where the signal
amplitude is proportional to the photoinduced change in the quasiparticle
density. The result that the amplitude of the $E_{2g}$ phonon follows the
temperature dependence of the amplitude of the fast electronic transient
indicates that under the resonant condition both electronic and phononic
responses are proportional to the change in the dielectric function.
|
0504540v1
|
2005-10-10
|
Quantum master equation descriptions of a nanomechanical resonator coupled to a single-electron transistor
|
We analyse the quantum dynamics of a nanomechanical resonator coupled to a
normal-state single-electron transistor (SET). Starting from a microscopic
description of the system, we derive a master equation for the SET island
charge and resonator which is valid in the limit of weak electro-mechanical
coupling. Using this master equation we show that, apart from brief transients,
the resonator always behaves like a damped harmonic oscillator with a shifted
frequency and relaxes into a thermal-like steady state. Although the behaviour
remains qualitatively the same, we find that the magnitude of the resonator
damping rate and frequency shift depend very sensitively on the relative
magnitudes of the resonator period and the electron tunnelling time. Maximum
damping occurs when the electrical and mechanical time-scales are the same, but
the frequency shift is greatest when the resonator moves much more slowly than
the island charge. We then derive reduced master equations which describe just
the resonator dynamics. By making slightly different approximations, we obtain
two different reduced master equations for the resonator. Apart from minor
differences, the two reduced master equations give rise to a consistent picture
of the resonator dynamics which matches that obtained from the master equation
including the SET island charge.
|
0510236v1
|
2005-10-16
|
Magnetic vortex dynamics in a 2D easy plane ferromagnet
|
In this thesis, we consider the dynamics of vortices in the easy plane
insulating ferromagnet in two dimensions. In addition to the quasiparticle
excitations, here spin waves or magnons, this magnetic system admits a family
of vortex solutions carrying two topological invariants, the winding number or
vorticity, and the polarization. A vortex is approximately described as a
particle moving about the system, endowed with an effective mass and acted upon
by a variety of forces. Classically, the vortex has an inter-vortex potential
energy giving a Coulomb-like force (attractive or repulsive depending on the
relative vortex vorticity), and a gyrotropic force, behaving as a self-induced
Lorentz force, whose direction depends on both topological indices. Expanding
semiclassically about a many-vortex solution, the vortices are quantized by
considering the scattered magnon states, giving a zero point energy correction
and a many-vortex mass tensor. The vortices cannot be described as independent
particles--that is, there are off-diagonal mass terms, such as 1/2 Mij vi vj,
that are non-negligible. This thesis examines the full vortex dynamics in
further detail by evaluating the Feynman-Vernon influence functional, which
describes the evolution of the vortex density matrix after the magnon modes
have been traced out. In addition to the set of forces already known, we find
new damping forces acting both longitudinally and transversely to the vortex
motion. The vortex motion within a collective cannot be entirely separated:
there are damping forces acting on one vortex due to the motion of another. The
effective damping forces have memory effects: they depend not only on the
current motion of the vortex collection but also on the motion history.
|
0510414v1
|
2006-05-19
|
Enhancement of noncontact friction between closely spaced bodies by two-dimensional systems
|
. We consider the effect of an external bias voltage and the spatial
variation of the surface potential, on the damping of cantilever vibrations.
The electrostatic friction is due to energy losses in the sample created by the
electromagnetic field from the oscillating charges induced on the surface of
the tip by the bias voltage and spatial variation of the surface potential. A
similar effect arises when the tip is oscillating in the electrostatic field
created by charged defects in a dielectric substrate. The electrostatic
friction is compared with the van der Waals friction originating from the
fluctuating electromagnetic field due to quantum and thermal fluctuation of the
current density inside the bodies. We show that the electrostatic and van der
Waals friction can be greatly enhanced if on the surfaces of the sample and the
tip there are two-dimension (2D) systems, e.g. a 2D-electron system or
incommensurate layers of adsorbed ions exhibiting acoustic vibrations. We show
that the damping of the cantilever vibrations due to the electrostatic friction
may be of similar magnitude as the damping observed in recent experiments of
Stipe \textit{et al} [B.C.Stipe, H.J.Mamin, T.D.Stowe, T.W.Kenny, and D.Rugar,
Phys.Rev. Lett.% \textbf{87}, 0982001]. We also show that at short separation
the van der Waals friction may be large enough to be measured experimentally.
|
0605480v1
|
2006-09-12
|
Slowing down Josephson vortex lattice in Bi_2Sr_2CaCu_2O_{8+d} with pancake vortices
|
We study theoretically and experimentally influence of pancake vortices on
motion of the Josephson vortex lattice in layered high-temperature
superconductors. Mobility of the Josephson vortices in layered superconductors
is strongly suppressed by small amount of pancake-vortex stacks. Moving
Josephson vortex lattice forces oscillating zigzag deformation of the
pancake-vortex stacks contributing to damping. The salient feature of this
contribution is its nonmonotonic dependence on the lattice velocity and the
corresponding voltage. Maximum pancake effect is realized when the Josephson
frequency matches the relaxation frequency of the stacks. The pancake-vortex
damping is strongly suppressed by thermal fluctuations of the pancake vortices.
This theoretical picture was qualitatively confirmed by experiments on two
mesas prepared out of Bi_2Sr_2CaCu_2O_{8+d} whiskers. We found that the
Josephson-vortex flux-flow voltage is very sensitive to small c-axis magnetic
field. The pancake-vortex contribution to the current indeed nonmonotonically
depends on voltage and decreases with increasing temperature and in-plane
magnetic field. We also found that irradiation with heavy ions has no
noticeable direct influence on motion of the Josephson vortices but
dramatically reduces the pancake-vortex contribution to the damping of the
Josephson vortex lattice at low temperatures.
|
0609262v2
|
2006-02-01
|
Radiation Damping in Einstein-Aether Theory
|
This work concerns the loss of energy of a material system due to
gravitational radiation in Einstein-aether theory-an alternative theory of
gravity in which the metric couples to a dynamical, timelike, unit-norm vector
field. Derived to lowest post-Newtonian order are waveforms for the metric and
vector fields far from a nearly Newtonian system and the rate of energy
radiated by the system. The expressions depend on the quadrupole moment of the
source, as in standard general relativity, but also contain monopolar and
dipolar terms. There exists a one-parameter family of Einstein-aether theories
for which only the quadrupolar contribution is present, and for which the
expression for the damping rate is identical to that of general relativity to
the order worked to here. This family cannot yet be declared observationally
viable, since effects due to the strong internal fields of bodies in the actual
systems used to test the damping rate are not included.
|
0602004v5
|
1998-02-20
|
Real-time Relaxation and Kinetics in Hot Scalar QED: Landau Damping
|
The real time evolution of field condensates with soft length scales
k^{-1}>(eT)^{-1} is solved in hot scalar electrodynamics, with a view towards
understanding relaxational phenomena in the QGP and the electroweak plasma. We
find that transverse gauge invariant non-equilibrium expectation values of
fields relax via {\em power laws} to asymptotic amplitudes that are determined
by the quasiparticle poles. The long time relaxational dynamics and relevant
time scales are determined by the behaviour of the retarded self-energy not at
the small frequencies, but at the Landau damping thresholds. This explains the
presence of power laws and not of exponential decay. Furthermore, we derive the
influence functional, the Langevin equation and the fluctuation-dissipation
theorem for the soft modes, identifying the correlation functions that emerge
in the classical limit. We show that a Markovian approximation fails to
describe the dynamics {\em both} at short and long times. We also introduce a
novel kinetic approach that goes beyond the standard Boltzmann equation and
incorporates off-shell processes and find that the distribution function for
soft quasiparticles relaxes with a power law through Landau damping. We also
find an unusual dressing dynamics of bare particles and anomalous (logarithmic)
relaxation of hard quasiparticles.
|
9802370v2
|
2003-03-04
|
Asymptotic quasinormal modes of Reissner-Nordström and Kerr black holes
|
According to a recent proposal, the so-called Barbero-Immirzi parameter of
Loop Quantum Gravity can be fixed, using Bohr's correspondence principle, from
a knowledge of highly-damped black hole oscillation frequencies. Such
frequencies are rather difficult to compute, even for Schwarzschild black
holes. However, it is now quite likely that they may provide a fundamental link
between classical general relativity and quantum theories of gravity. Here we
carry out the first numerical computation of very highly damped quasinormal
modes (QNM's) for charged and rotating black holes. In the Reissner-Nordstr\"om
case QNM frequencies and damping times show an oscillatory behaviour as a
function of charge. The oscillations become faster as the mode order increases.
At fixed mode order, QNM's describe spirals in the complex plane as the charge
is increased, tending towards a well defined limit as the hole becomes
extremal. Kerr QNM's have a similar oscillatory behaviour when the angular
index $m=0$. For $l=m=2$ the real part of Kerr QNM frequencies tends to
$2\Omega$, $\Omega$ being the angular velocity of the black hole horizon, while
the asymptotic spacing of the imaginary parts is given by $2\pi T_H$.
|
0303029v3
|
2005-01-28
|
Summation of divergent series and Borel summability for strongly dissipative equations with periodic or quasi-periodic forcing terms
|
We consider a class of second order ordinary differential equations
describing one-dimensional systems with a quasi-periodic analytic forcing term
and in the presence of damping. As a physical application one can think of a
resistor-inductor-varactor circuit with a periodic (or quasi-periodic) forcing
function, even if the range of applicability of the theory is much wider. In
the limit of large damping we look for quasi-periodic solutions which have the
same frequency vector of the forcing term, and we study their analyticity
properties in the inverse of the damping coefficient. We find that already the
case of periodic forcing terms is non-trivial, as the solution is not analytic
in a neighbourhood of the origin: it turns out to be Borel-summable. In the
case of quasi-periodic forcing terms we need Renormalization Group techniques
in order to control the small divisors arising in the perturbation series. We
show the existence of a summation criterion of the series in this case also,
but, however, this can not be interpreted as Borel summability.
|
0501500v1
|
1997-11-17
|
Fusion and Binary-Decay Mechanisms in the $^{35}$Cl+$^{24}$Mg System at E/A $\approx$ 8 MeV/Nucleon
|
Compound-nucleus fusion and binary-reaction mechanisms have been investigated
for the $^{35}$Cl+$^{24}$Mg system at an incident beam energy of E$_{Lab}$= 282
MeV. Charge distributions, inclusive energy spectra, and angular distributions
have been obtained for the evaporation residues and the binary fragments.
Angle-integrated cross sections have been determined for evaporation residues
from both the complete and incomplete fusion mechanisms. Energy spectra for
binary fragment channels near to the entrance-channel mass partition are
characterized by an inelastic contribution that is in addition to a fully
energy damped component. The fully damped component which is observed in all
the binary mass channels can be associated with decay times that are comparable
to, or longer than the rotation period. The observed mass-dependent cross
sections for the fully damped component are well reproduced by the fission
transition-state model, suggesting a fusion followed by fission origin. The
present data cannot, however, rule out the possibility that a long-lived
orbiting mechanism accounts for part or all of this yield.
|
9711005v1
|
1996-12-31
|
Fluid Models for Kinetic Effects on Coherent Nonlinear Alfven Waves. II. Numerical Solutions
|
The influence of various kinetic effects (e.g. Landau damping, diffusive and
collisional dissipation, and finite Larmor radius terms) on the nonlinear
evolution of finite amplitude Alfvenic wave trains in a finite-beta environment
is systematically investigated using a novel, kinetic nonlinear Schrodinger
(KNLS) equation. The dynamics of Alfven waves is sensitive to the sense of
polarization as well as the angle of propagation with respect to the ambient
magnetic field. Numerical solution for the case with Landau damping reveals the
formation of dissipative structures, which are quasi-stationary, S-polarized
directional (and rotational) discontinuities which self-organize from parallel
propagating, linearly polarized waves. Parallel propagating circularly
polarized packets evolve to a few circularly polarized Alfven harmonics on
large scales. Stationary arc-polarized rotational discontinuities form from
obliquely propagating waves. Collisional dissipation, even if weak, introduces
enhanced wave damping when beta is very close to unity. Cyclotron motion
effects on resonant particle interactions introduce cyclotron resonance into
the nonlinear Alfven wave dynamics.
|
9612018v1
|
2000-09-06
|
Design, fabrication and measurement of the first rounded damped detuned accelerator structure (RDDS1)
|
As a joint effort in the JLC/NLC research program, we have developed a new
type of damped detuned accelerator structure with optimized round-shaped
cavities (RDDS). This paper discusses some important R&D aspects of the first
structure in this series (RDDS1). The design aspects covered are the cell
design with sub-MHz precision, HOM detuning, coupling and damping technique and
wakefield simulation. The fabrication issues covered are ultra-precision cell
machining with micron accuracy, assembly and diffusion bonding technologies to
satisfactorily meet bookshelf, straightness and cell rotational alignment
requirements. The measurements described are the RF properties of single
cavities and complete accelerator section, as well as wakefields from the ASSET
tests at SLAC. Finally, future improvements are also discussed.
|
0009025v1
|
2001-07-19
|
Dipole Wakefield Suppression In High Phase Advance Detuned Linear Accelerators For The JLC/NLC Designed To Minimise Electrical Breakdown And Cumulative BBU
|
Recent experiments at SLAC [1,2] and CERN [3] have revealed evidence of
significant deformation in the form of "pitting" of the cells of the 1.8m
series of structures DDS/RDDS (Damped Detuned Structure/Rounded Damped Detuned
Structure). This pitting occurs in the high group velocity (vg /c = 0.012) end
of the accelerating structure and little evidence of breakdown has been found
in the lower group velocity end of the structure. Additional, albeit
preliminary experimental evidence, suggests that shorter and lower group
velocity structures have reduced breakdown events with increasing accelerating
field strengths. Two designs are presented here, firstly a 90cm structure
consisting of 83 cells with an initial vg/c = 0.0506 (known as H90VG5) and
secondly, an even shorter structure of length 60cm consisting of 55 cells with
an initial vg /c = 0.03 (known as H60VG3). The feasibility of using these
structures to accelerate a charged beam over 10km is investigated. The
particular issue focussed upon is suppression of the dipole wakefields via
detuning of the cell frequencies and by locally damping individual cells in
order to avoid BBU (Beam Break Up). Results are presented on beam-induced
dipole wakefields and on the beam dynamics encountered on tracking the progress
of the beam through several thousand accelerating structures.
[1] C. Adolphsen, ROAA003, this conf. [2] R.H. Miller et al, FPAH062, this
conf. [3] L. Groening et al, MPPH039, this conf
|
0107049v1
|
2000-12-10
|
Chaotic atomic population oscillations between two coupled Bose-Einstein condensates with time-dependent asymmetric trap potential
|
We have investigated the chaotic atomic population oscillations between two
coupled Bose-Einstein condensates (BEC) with time-dependent asymmetric trap
potential. In the perturbative regime, the population oscillations can be
described by the Duffing equation, and the chaotic oscillations near the
separatrix solution are analyzed. The sufficient-necessary conditions for
stable oscillations depend on the physical parameters and initial conditions
sensitively. The first-order necessary condition indicates that the Melnikov
function is equal to zero, so the stable oscillations are Melnikov chaotic. For
the ordinary parameters and initial conditions, the chaotic dynamics is
simulated with numerical calculation. If the damping is absent, with the
increasing of the trap asymmetry, the regular oscillations become chaotic
gradually, the corresponding stroboscopic Poincare sections (SPS) vary from a
single island to more islands, and then the chaotic sea. For the completely
chaotic oscillations, the long-term localization disappears and the short-term
localization can be changed from one of the BECs to the other through the route
of Rabi oscillation. When there exists damping, the stationary chaos
disappears, the transient chaos is a common phenomenon before regular stable
frequency locked oscillations. And proper damping can keep localization
long-lived.
|
0012043v1
|
2003-01-23
|
Revivals and entanglement from initially entangled mixed states of a damped Jaynes-Cummings model
|
An exact density matrix of a phase-damped Jaynes - Cummings model (JCM) with
entangled Bell-like initial states formed from a model two-state atom and sets
of adjacent photon number states of a single mode radiation field is presented.
The entanglement of the initial states and the subsequent time evolution is
assured by finding a positive lower bound on the concurrence of local 2x2
projections of the full 2xinfinity JCM density matrix. It is found that the
time evolution of the lower bound of the concurrence systematically captures
the corresponding collapse and revival features in atomic inversion, relative
entropies of atomic and radiation, mutual entropy, and quantum deficit. The
atom and radiation subsystems exhibit alternating sets of collapses and
revivals in a complementary fashion due to the initially mixed states of the
atom and radiation employed here. This is in contrast with the result obtained
when the initial state of the dissipationless system is a factored pure state
of atom and radiation, where the atomic and radiation entropies are necessarily
the same. The magnitudes of the entanglement lower bound and the atomic and
radiation revivals become larger as both magnitude and phase of the Bell-like
initial state contribution increases. The time evolution of the entropy
difference of the total system and that of the radiation subsystem exhibits
negative regions called "supercorrelated" states which do not appear in the
atomic subsystem. Entangled initial states are found to enhance this
supercorrelated feature. Finally, the effect of phase damping is to randomize
both the subsystems for asymptotically long times .
|
0301126v1
|
2003-03-18
|
A Method for Modeling Decoherence on a Quantum Information Processor
|
We develop and implement a method for modeling decoherence processes on an
N-dimensional quantum system that requires only an $N^2$-dimensional quantum
environment and random classical fields. This model offers the advantage that
it may be implemented on small quantum information processors in order to
explore the intermediate regime between semiclassical and fully quantum models.
We consider in particular $\sigma_z\sigma_z$ system-environment couplings which
induce coherence (phase) damping, though the model is directly extendable to
other coupling Hamiltonians. Effective, irreversible phase-damping of the
system is obtained by applying an additional stochastic Hamiltonian on the
environment alone, periodically redressing it and thereby irreversibliy
randomizing the system phase information that has leaked into the environment
as a result of the coupling. This model is exactly solvable in the case of
phase-damping, and we use this solution to describe the model's behavior in
some limiting cases. In the limit of small stochastic phase kicks the system's
coherence decays exponentially at a rate which increases linearly with the kick
frequency. In the case of strong kicks we observe an effective decoupling of
the system from the environment. We present a detailed implementation of the
method on an nuclear magnetic resonance quantum information processor.
|
0303115v1
|
2005-11-25
|
The electrodynamic 2-body problem and the origin of quantum mechanics
|
We numerically solve the functional differential equations (FDE's) of
2-particle electrodynamics, using the full electrodynamic force obtained from
the retarded Lienard-Wiechert potentials and the Lorentz force law. In
contrast, the usual formulation uses only the Coulomb force (scalar potential),
reducing the electrodynamic 2-body problem to a system of ordinary differential
equations (ODE's). The ODE formulation is mathematically suspect since FDE's
and ODE's are known to be incompatible; however, the Coulomb approximation to
the full electrodynamic force has been believed to be adequate for physics. We
can now test this long-standing belief by comparing the FDE solution with the
ODE solution, in the historically interesting case of the classical hydrogen
atom. The solutions differ.
A key qualitative difference is that the full force involves a `delay'
torque. Our existing code is inadequate to calculate the detailed interaction
of the delay torque with radiative damping. However, a symbolic calculation
provides conditions under which the delay torque approximately balances (3rd
order) radiative damping. Thus, further investigations are required, and it was
prematurely concluded that radiative damping makes the classical hydrogen atom
unstable. Solutions of FDE's naturally exhibit an_infinite_ spectrum of
_discrete_ frequencies. The conclusion is that (a) the Coulomb force is_not_ a
valid approximation to the full electrodynamic force, so that (b) the n-body
interaction needs to be reformulated in various current contexts such as
molecular dynamics.
|
0511235v1
|
2007-05-11
|
Ground-state cooling of a micromechanical oscillator: generalized framework for cold damping and cavity-assisted cooling schemes
|
We provide a general framework to describe cooling of a micromechanical
oscillator to its quantum ground state by means of radiation-pressure coupling
with a driven optical cavity. We apply it to two experimentally realized
schemes, back-action cooling via a detuned cavity and cold-damping
quantum-feedback cooling, and we determine the ultimate quantum limits of both
schemes for the full parameter range of a stable cavity. While both allow to
reach the oscillator's quantum ground state, we find that back-action cooling
is more efficient in the good cavity limit, i.e. when the cavity bandwidth is
smaller than the mechanical frequency, while cold damping is more suitable for
the bad cavity limit. The results of previous treatments are recovered as
limiting cases of specific parameter regimes.
|
0705.1728v6
|
2007-11-08
|
Solving non-linear equations of longitudinal and transverse electron waves in collisionless Maxwellian plasma
|
We have considered an expansion of solutions of the non-linear equations for
both longitudinal and transverse waves in collisionless Maxwellian plasma in
series of non-damping overtones of the field E(x,t) and electron velocity
distribution function f=f(0) +f(1) where f(0) is background Maxwellian electron
distribution function and f(1) is perturbation. The electrical field and
perturbation f(1) are presented as a series of non-damping harmonics with
increasing frequencies of the order n and the same propagation speed. It is
shown presence of recurrent relations for arising overtones. Convergence of the
series is provided by a power law parameter series convergence. There are
proposed also successive procedures of cutting off the distribution function
f(1) to the condition of positivity f near the singularity points where kinetic
equation becomes inapplicable. In this case, at poles absence the solution
reduces to non-damping Vlasov waves (oscillations). In the case of transverse
waves, dispersion equation has two roots, corresponding to the branches of fast
electromagnetic and slow electron waves. There is noted a possibility of
experimental testing appearing exotic results with detecting frequencies and
amplitudes of n-order overtones.
|
0711.1321v8
|
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