5,511 research outputs found
Numerical Tests of Rotational Mixing in Massive Stars with the new Population Synthesis Code BONNFIRES
We use our new population synthesis code BONNFIRES to test how surface
abundances predicted by rotating stellar models depend on the numerical
treatment of rotational mixing, such as spatial resolution, temporal resolution
and computation of mean molecular weight gradients. We find that even with
identical numerical prescriptions for calculating the rotational mixing
coefficients in the diffusion equation, different timesteps lead to a deviation
of the coefficients and hence surface abundances. We find the surface
abundances vary by 10-100% between the model sequences with short timestep of
0.001Myr to model sequences with longer timesteps. Model sequences with
stronger surface nitrogen enrichment also have longer main-sequence lifetimes
because more hydrogen is mixed to the burning cores. The deviations in
main-sequence lifetimes can be as large as 20%. Mathematically speaking, no
numerical scheme can give a perfect solution unless infinitesimally small
timesteps are used. However, we find that the surface abundances eventually
converge within 10% between modelling sequences with sufficiently small
timesteps below 0.1Myr. The efficiency of rotational mixing depends on the
implemented numerical scheme and critically on the computation of the mean
molecular weight gradient. A smoothing function for the mean molecular weight
gradient results in stronger rotational mixing. If the discretization scheme or
the computational recipe for calculating the mean molecular weight gradient is
altered, re-calibration of mixing parameters may be required to fit
observations. If we are to properly understand the fundamental physics of
rotation in stars, it is crucial that we minimize the uncertainty introduced
into stellar evolution models when numerically approximating rotational mixing
processes.Comment: 8 pages, 6 figures, accepted by A&
Dielectric function of the semiconductor hole liquid: Full frequency and wave vector dependence
We study the dielectric function of the homogeneous semiconductor hole liquid
of p-doped bulk III-V zinc-blende semiconductors within random phase
approximation. The single-particle physics of the hole system is modeled by
Luttinger's four-band Hamiltonian in its spherical approximation. Regarding the
Coulomb-interacting hole liquid, the full dependence of the zero-temperature
dielectric function on wave vector and frequency is explored. The imaginary
part of the dielectric function is analytically obtained in terms of
complicated but fully elementary expressions, while in the result for the real
part nonelementary one-dimensional integrations remain to be performed. The
correctness of these two independent calculations is checked via Kramers-Kronig
relations.
The mass difference between heavy and light holes, along with variations in
the background dielectric constant, leads to dramatic alternations in the
plasmon excitation pattern, and generically, two plasmon branches can be
identified. These findings are the result of the evaluation of the full
dielectric function and are not accessible via a high-frequency expansion. In
the static limit a beating of Friedel oscillations between the Fermi wave
numbers of heavy and light holes occurs.Comment: 16 pages, 11 figures included. Update: Minor additions and
adjustments, published versio
Theory of thermal spin-charge coupling in electronic systems
The interplay between spin transport and thermoelectricity offers several
novel ways of generating, manipulating, and detecting nonequilibrium spin in a
wide range of materials. Here we formulate a phenomenological model in the
spirit of the standard model of electrical spin injection to describe the
electronic mechanism coupling charge, spin, and heat transport and employ the
model to analyze several different geometries containing ferromagnetic (F) and
nonmagnetic (N) regions: F, F/N, and F/N/F junctions which are subject to
thermal gradients. We present analytical formulas for the spin accumulation and
spin current profiles in those junctions that are valid for both tunnel and
transparent (as well as intermediate) contacts. For F/N junctions we calculate
the thermal spin injection efficiency and the spin accumulation induced
nonequilibrium thermopower. We find conditions for countering thermal spin
effects in the N region with electrical spin injection. This compensating
effect should be particularly useful for distinguishing electronic from other
mechanisms of spin injection by thermal gradients. For F/N/F junctions we
analyze the differences in the nonequilibrium thermopower (and chemical
potentials) for parallel and antiparallel orientations of the F magnetizations,
as evidence and a quantitative measure of the spin accumulation in N.
Furthermore, we study the Peltier and spin Peltier effects in F/N and F/N/F
junctions and present analytical formulas for the heat evolution at the
interfaces of isothermal junctions.Comment: to be published in PRB (in press), 19 pages, 19 figure
Relativistic Iron Lines in Galactic Black Holes: Recent Results and Lines in the ASCA Archive
Recent observations with Chandra and XMM-Newton, aided by broad-band spectral
coverage from RXTE, have revealed skewed relativistic iron emission lines in
stellar-mass Galactic black hole systems. Such systems are excellent
laboratories for testing General Relativity, and relativistic iron lines
provide an important tool for making such tests. In this contribution to the
Proceedings of the 10th Annual Marcel Grossmann Meeting on General Relativity,
we briefly review recent developments and present initial results from fits to
archival ASCA observations of Galactic black holes. It stands to reason that
relativistic effects, if real, should be revealed in many systems (rather than
just one or two); the results of our archival work have borne-out this
expectation. The ASCA spectra reveal skewed, relativistic lines in XTE
J1550-564, GRO J1655-40, GRS 1915+105, and Cygnus X-1.Comment: to appear in the proc. of the 10th Annual Marcel Grossmann Meeting on
General Relativity, 5 pages, 1 figure, uses specific .cls and .sty file
Quantum Quench in the Transverse Field Ising Chain
We consider the time evolution of observables in the transverse field Ising
chain (TFIC) after a sudden quench of the magnetic field. We provide exact
analytical results for the asymptotic time and distance dependence of one- and
two-point correlation functions of the order parameter. We employ two
complementary approaches based on asymptotic evaluations of determinants and
form-factor sums. We prove that the stationary value of the two-point
correlation function is not thermal, but can be described by a generalized
Gibbs ensemble (GGE). The approach to the stationary state can also be
understood in terms of a GGE. We present a conjecture on how these results
generalize to particular quenches in other integrable models.Comment: 4 pages, 1 figur
The profile of an emission line from relativistic outflows around a black hole
Recent observations show strong evidence for the presence of Doppler-shifted
emission lines in the spectrum of both black hole candidates and active
galactic nuclei. These lines are likely to originate from relativistic outflows
(or jets) in the vicinity of the central black hole. Consequently, the profile
of such a line should be distorted by strong gravitational effects near the
black hole, as well as special relativistic effects. In this paper, we present
results from a detailed study on how each process affects the observed line
profile. We found that the profile is sensitive to the intrinsic properties of
the jets (Lorentz factor, velocity profile, and emissivity law), as well as to
the spin of the black hole and the viewing angle (with respect to the axis of
the jets). More specifically, in the case of approaching jets, an intrisically
narrow line (blue-shifted) is seen as simply broadened at small viewing angles,
but it shows a doubly peaked profile at large viewing angles for extreme Kerr
black holes (due to the combination of gravitational focusing and Doppler
effects); the profile is always singly peaked for Schwarzschild black holes.
For receding jets, however, the line profile becomes quite complicated owing to
complicated photon trajectories. To facilitate comparison with observations, we
searched a large parameter space to derive representative line profiles. We
show the results and discuss how to use emission lines as a potential tool for
probing the inner region of a black hole jet system.Comment: 16 pages in emulateapj style, 11 figure
Dielectric function of the semiconductor hole gas
We study the dielectric function of the homogeneous hole gas in p-doped
zinc-blende III-V bulk semiconductors within random phase approximation with
the valence band being modeled by Luttinger's Hamiltonian in the spherical
approximation. In the static limit we find a beating of Friedel oscillations
between the two Fermi momenta for heavy and light holes, while at large
frequencies dramatic corrections to the plasmon dispersion occur.Comment: 4 pages, 1 figure included. Version to appear in Europhys. Let
The radial variation of HI velocity dispersions in dwarfs and spirals
Gas velocity dispersions provide important diagnostics of the forces
counteracting gravity to prevent collapse of the gas. We use the 21 cm line of
neutral atomic hydrogen (HI) to study HI velocity dispersion and HI phases as a
function of galaxy morphology in 22 galaxies from The HI Nearby Galaxy Survey
(THINGS). We stack individual HI velocity profiles and decompose them into
broad and narrow Gaussian components. We study the HI velocity dispersion and
the HI surface density, as a function of radius. For spirals, the velocity
dispersions of the narrow and broad components decline with radius and their
radial profiles are well described by an exponential function. For dwarfs,
however, the profiles are much flatter. The single Gaussian dispersion profiles
are, in general, flatter than those of the narrow and broad components. In most
cases, the dispersion profiles in the outer disks do not drop as fast as the
star formation profiles, derived in the literature. This indicates the
importance of other energy sources in driving HI velocity dispersion in the
outer disks. The radial surface density profiles of spirals and dwarfs are
similar. The surface density profiles of the narrow component decline more
steeply than those of the broad component, but not as steep as what was found
previously for the molecular component. As a consequence, the surface density
ratio between the narrow and broad components, an estimate of the mass ratio
between cold HI and warm HI, tends to decrease with radius. On average, this
ratio is lower in dwarfs than in spirals. This lack of a narrow, cold HI
component in dwarfs may explain their low star formation activity.Comment: Accepted for publication in The Astronomical Journal, 13 pages, 10
figures, 4 table
Rapidly variable Fe K line in NGC 4051
We present a detailed analysis on the variability of the Fe K emission line
in NGC 4051 using ASCA data. Through simple Gaussian line fits, we find not
only obvious Fe K line variability with no significant difference in the X-ray
continuum flux between two ASCA observations which were separated by 440
days, but also rapid variability of Fe K line on time scales s
within the second observation. During the second observation, the line is
strong (EW = 733 eV) and broad (
keV) when the source is brightest, and become weaker (EW = 165
eV) and narrower ( keV) whilst the source is weakest. The
equivalent width of Fe K line correlates positively with the continuum flux,
which shows an opposite trend with another Seyfert 1 galaxy MCG --6-30-15.Comment: 12 pages with 5 figures, to appear in ApJ Vol. 516, L6
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