4,502 research outputs found
Effect of Radiative Levitation on Calculations of Accretion Rates in White Dwarfs
Elements heavier than hydrogen or helium that are present in the atmospheres
of white dwarfs with effective temperatures lower than 25,000 K, are believed
to be the result of accretion. By measuring the abundances of these elements
and by assuming a steady-state accretion, we can derive the composition of the
accreted matter and infer its source. The presence of radiative levitation,
however, may affect the determination of the accretion rate. We present
time-dependent diffusion calculations that take into account radiative
levitation and accretion. The calculations are performed on C, N, O, Ne, Na,
Mg, Al, Si, S, Ar, and Ca in hydrogen-rich white dwarf models with effective
temperatures lower than 25,000 K and a gravity of log g = 8.0. We show that in
the presence of accretion, the abundance of an element supported by the
radiative levitation is given by the equilibrium between the radiative and
gravitational accelerations, unless the abundance predicted by the steady-state
accretion is much greater than the abundance supported by the radiative
acceleration.Comment: 6 pages, to be published in the proceedings of the 17th European
White Dwarf Workshop that was held in Tubingen, Germany, on August 16-20,
201
Martin boundary of a reflected random walk on a half-space
The complete representation of the Martin compactification for reflected
random walks on a half-space is obtained. It is shown that the
full Martin compactification is in general not homeomorphic to the ``radial''
compactification obtained by Ney and Spitzer for the homogeneous random walks
in : convergence of a sequence of points to a
point of on the Martin boundary does not imply convergence of the sequence
on the unit sphere . Our approach relies on the large
deviation properties of the scaled processes and uses Pascal's method combined
with the ratio limit theorem. The existence of non-radial limits is related to
non-linear optimal large deviation trajectories.Comment: 42 pages, preprint, CNRS UMR 808
Description de Mollusques nouveaux recueilis au cours du voyage de S.A.R le prince Léopold de Belgique aux Indes Orientales Néerlandaises (2<sup>e</sup> note)
Parameter-free Stark Broadening of Hydrogen Lines in DA White Dwarfs
We present new calculations for the Stark broadening of the hydrogen line
profiles in the dense atmospheres of white dwarf stars. Our improved model is
based on the unified theory of Stark broadening from Vidal, Cooper & Smith, but
it also includes non-ideal gas effects from the Hummer & Mihalas occupation
probability formalism directly inside the line profile calculations. This
approach improves upon previous calculations that relied on the use of an
ad-hoc free parameter to describe the dissolution of the line wing opacity in
the presence of high electric microfields in the plasma. We present here the
first grid of model spectra for hot Teff >~ 12,000 K DA white dwarfs that has
no free parameters. The atmospheric parameters obtained from optical and UV
spectroscopic observations using these improved models are shown to differ
substantially from those published in previous studies.Comment: 8 pages, 8 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
Quantum Hall effect anomaly and collective modes in the magnetic-field-induced spin-density-wave phases of quasi-one-dimensional conductors
We study the collective modes in the magnetic-field-induced spin-density-wave
(FISDW) phases experimentally observed in organic conductors of the Bechgaard
salts family. In phases that exhibit a sign reversal of the quantum Hall effect
(Ribault anomaly), the coexistence of two spin-density waves gives rise to
additional collective modes besides the Goldstone modes due to spontaneous
translation and rotation symmetry breaking. These modes strongly affect the
charge and spin response functions. We discuss some experimental consequences
for the Bechgaard salts.Comment: Final version (LaTex, 8 pages, no figure), to be published in
Europhys. Let
Generalized Entropies
We study an entropy measure for quantum systems that generalizes the von
Neumann entropy as well as its classical counterpart, the Gibbs or Shannon
entropy. The entropy measure is based on hypothesis testing and has an elegant
formulation as a semidefinite program, a type of convex optimization. After
establishing a few basic properties, we prove upper and lower bounds in terms
of the smooth entropies, a family of entropy measures that is used to
characterize a wide range of operational quantities. From the formulation as a
semidefinite program, we also prove a result on decomposition of hypothesis
tests, which leads to a chain rule for the entropy.Comment: 21 page
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