1,045 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
The Extent and Cause of the Pre-White Dwarf Instability Strip
One of the least understood aspects of white dwarf evolution is the process
by which they are formed. We are aided, however, by the fact that many H- and
He-deficient pre-white dwarfs (PWDs) are multiperiodic g-mode pulsators.
Pulsations in PWDs provide a unique opportunity to probe their interiors, which
are otherwise inaccesible to direct observation. Until now, however, the nature
of the pulsation mechanism, the precise boundaries of the instability strip,
and the mass distribution of the PWDs were complete mysteries. These problems
must be addressed before we can apply knowledge of pulsating PWDs to improve
understanding of white dwarf formation. This paper lays the groundwork for
future theoretical investigations of these stars. In recent years, Whole Earth
Telescope observations led to determination of mass and luminosity for the
majority of the (non-central star) PWD pulsators. With these observations, we
identify the common properties and trends PWDs exhibit as a class. We find that
pulsators of low mass have higher luminosity, suggesting the range of
instability is highly mass-dependent. The observed trend of decreasing periods
with decreasing luminosity matches a decrease in the maximum (standing-wave)
g-mode period across the instability strip. We show that the red edge can be
caused by the lengthening of the driving timescale beyond the maximum
sustainable period. This result is general for ionization-based driving
mechanisms, and it explains the mass-dependence of the red edge. The observed
form of the mass-dependence provides a vital starting point for future
theoretical investigations of the driving mechanism. We also show that the blue
edge probably remains undetected because of selection effects arising from
rapid evolution.Comment: 40 pages, 6 figures, accepted by ApJ Oct 27, 199
Investigation of transit-selected exoplanet candidates from the MACHO survey
Context: Planets outside our solar system transiting their host star, i. e.
those with an orbital inclination near 90 degree, are of special interest to
derive physical properties of extrasolar planets. With the knowledge of the
host star's physical parameters, the planetary radius can be determined.
Combined with spectroscopic observations the mass and therefore the density can
be derived from Doppler-measurements. Depending on the brightness of the host
star, additional information, e. g. about the spin-orbit alignment between the
host star and planetary orbit, can be obtained.
Aims: The last few years have witnessed a growing success of transit surveys.
Among other surveys, the MACHO project provided nine potential transiting
planets, several of them with relatively bright parent stars. The photometric
signature of a transit event is, however, insufficient to confirm the planetary
nature of the faint companion. The aim of this paper therefore is a
determination of the spectroscopic parameters of the host stars as well as a
dynamical mass determination through Doppler-measurements.
Methods: We have obtained follow-up high-resolution spectra for five stars
selected from the MACHO sample, which are consistent with transits of
low-luminosity objects. Radial velocities have been determined by means of
cross-correlation with model spectra. The MACHO light curves have been compared
to simulations based on the physical parameters of the system derived from the
radial velocities and spectral analyses.
Aims: We show that all transit light curves of the exoplanet candidates
analysed in this work can be explained by eclipses of stellar objects, hence
none of the five transiting objects is a planet.Comment: 6 pages, 3 figures, 1 table, accepted for publication in A&
Helium-rich EHB Stars in Globular Clusters
Recent UV observations of the most massive Galactic globular clusters show a
significant population of hot stars below the zero-age HB (``blue hook''
stars), which cannot be explained by canonical stellar evolution. Stars which
suffer unusually large mass loss on the red giant branch and thus experience
the helium-core flash while descending the white dwarf cooling curve could
populate this region. They should show higher temperatures than the hottest
canonical HB stars and their atmospheres should be helium-rich and probably
C/N-rich. We have obtained spectra of blue hook stars in omega Cen and NGC 2808
to test this possibility. Our analysis shows that the blue hook stars in these
clusters reach effective temperatures well beyond the hot end of the canonical
EHB and have higher helium abundances than canonical EHB stars. These results
support the hypothesis that the blue hook stars arise from stars which ignite
helium on the white dwarf cooling curve.Comment: LaTeX, 8 pages, 3 figures, uses Kluwer style files (included), to
appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics
and Space Science, Kluwer Academic Publishers, proceedings of the meeting
held in Keele, UK, June 16-20, 200
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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