873 research outputs found
Non-LTE spectral analyses of the lately discovered DB-gap white dwarfs from the SDSS
For a long time, no hydrogen-deficient white dwarfs have been known that have
effective temperature between 30 kK and < 45 kK, i.e. exceeding those of DB
white dwarfs and having lower ones than DO white dwarfs. Therefore, this
temperature range was long known as the DB-gap. Only recently, the SDSS
provided spectra of several candidate DB-gap stars. First analyses based on
model spectra calculated under the assumption of local thermodynamic
equilibrium (LTE) confirmed that these stars had 30 kK < Teff < 45 kK
(Eisenstein et al. 2006). It has been shown for DO white dwarfs that the
relaxation of LTE is necessary to account for non local effects in the
atmosphere caused by the intense radiation field. Therefore, we calculated a
non-LTE model grid and re-analysed the aforementioned set of SDSS spectra. Our
results confirm the existence of DB-gap white dwarfs.Comment: 4 pages, 2 figures, to appear in: Proceedings of the 16th European
Workshop on White Dwarf
The Classical Stellar Atmosphere Problem
We introduce the classical stellar atmosphere problem and describe in detail
its numerical solution. The problem consists of the solution of the radiation
transfer equations under the constraints of hydrostatic, radiative and
statistical equilibrium (non-LTE). We outline the basic idea of the Accelerated
Lambda Iteration (ALI) technique and statistical methods which finally allow
the construction of non-LTE model atmospheres considering the influence of
millions of metal absorption lines. Some applications of the new models are
presented.Comment: accepted for publication in The Journal of Computational and Applied
Mathematics, Computational Astrophysics, eds. H. Riffert, K. Werne
Long-term EXOTIME photometry and follow-up spectroscopy of the sdB pulsator HS 0702+6043
Pulsating subdwarf B (sdB) stars oscillate in short-period p-modes or
long-period g-modes. HS0702+6043 (DW Lyn) is one of a few objects to show
characteristics of both types and is hence classified as hybrid pulsator. It is
one of our targets in the EXOTIME program to search for planetary companions
around extreme horizontal branch objects. In addition to the standard exercise
in asteroseismology to probe the instantaneous inner structure of a star,
measured changes in the pulsation frequencies as derived from an O-C diagram
can be compared to theoretical evolutionary timescales. Based on the
photometric data available so far, we are able to derive a high-resolution
frequency spectrum and to report on our efforts to construct a multi-season O-C
diagram. Additionally, we have gathered time-resolved spectroscopic data in
order to constrain stellar parameters and to derive mode parameters as well as
radial and rotational velocities.Comment: 2 pages, JENAM 2008 proceedings, to be published in 'Communications
in Asteroseismology', 15
Relativistic model for nuclear matter and atomic nuclei with momentum-dependent self-energies
The Lagrangian density of standard relativistic mean-field (RMF) models with
density-dependent meson-nucleon coupling vertices is modified by introducing
couplings of the meson fields to derivative nucleon densities. As a
consequence, the nucleon self energies, that describe the effective in-medium
interaction, become momentum dependent. In this approach it is possible to
increase the effective (Landau) mass of the nucleons, that is related to the
density of states at the Fermi energy, as compared to conventional relativistic
models. At the same time the relativistic effective (Dirac) mass is kept small
in order to obtain a realistic strength of the spin-orbit interaction.
Additionally, the empirical Schroedinger-equivalent central optical potential
from Dirac phenomenology is reasonably well described. A parametrization of the
model is obtained by a fit to properties of doubly magic atomic nuclei. Results
for symmetric nuclear matter, neutron matter and finite nuclei are discussed.Comment: 14 pages, 7 figures, 5 tables, extended introduction and conclusions,
additional references, minor corrections, accepted for publication in Phys.
Rev.
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