2,435 research outputs found
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
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
Planet formation from the ejecta of common envelopes
The close binary system NN Serpentis must have gone through a common envelope
phase before the formation of its white dwarf. During this phase, a substantial
amount of mass was lost from the envelope. The recently detected orbits of
circumbinary planets are likely inconsistent with planet formation before the
mass loss.We explore whether new planets may have formed from the ejecta of the
common envelope and derive the expected planetary mass as a function of
radius.We employed the Kashi & Soker model to estimate the amount of mass that
is retained during the ejection event and inferred the properties of the
resulting disk from the conservation of mass and angular momentum. The
resulting planetary masses were estimated from models with and without
radiative feedback. We show that the observed planetary masses can be
reproduced for appropriate model parameters. Photoheating can stabilize the
disks in the interior, potentially explaining the observed planetary orbits on
scales of a few AU. We compare the expected mass scale of planets for 11
additional systems with observational results and find hints of two
populations, one consistent with planet formation from the ejecta of common
envelopes and the other a separate population that may have formed earlier. The
formation of the observed planets from the ejecta of common envelopes seems
feasible. The model proposed here can be tested through refined observations of
additional post-common envelope systems. While it appears observationally
challenging to distinguish between the accretion on pre-existing planets and
their growth from new fragments, it may be possible to further constrain the
properties of the protoplanetary disk through additional observations of
current planetary candidates and post-common envelope binary systems.Comment: 12 pages, 8 figures, 3 tables. Accepted at A&
The quest for companions to post-common envelope binaries IV: The 2:1 mean-motion resonance of the planets orbiting NN Serpentis
We present 69 new mid-eclipse times of the young post-common envelope binary
(PCEB) NN Ser, which was previously suggested to possess two circumbinary
planets. We have interpreted the observed eclipse-time variations in terms of
the light-travel time effect caused by two planets, exhaustively covering the
multi-dimensional parameter space by fits in the two binary and ten orbital
parameters. We supplemented the fits by stability calculations for all models
with an acceptable chi-square. An island of secularly stable 2:1 resonant
solutions exists, which coincides with the global chi-square minimum. Our
best-fit stable solution yields current orbital periods P_o = 15.47 yr and P_i
= 7.65 yr and eccentricities e_o = 0.14 and e_i = 0.22 for the outer (o) and
inner (i) planets, respectively. The companions qualify as giant planets, with
masses of 7.0 M_Jup and 1.7 M_Jup for the case of orbits coplanar with that of
the binary. The two-planet model that starts from the present system parameters
has a lifetime greater than 10^8 yr, which significantly exceeds the age of NN
Ser of 10^6 yr as a PCEB. The resonance is characterized by libration of the
resonant variable Theta_1 and circulation of omega_i-omega_o, the difference
between the arguments of periapse of the two planets. No stable non-resonant
solutions were found, and the possibility of a 5:2 resonance suggested
previously by us is now excluded at the 99.3% confidence level.Comment: 8 pages, 8 figure
Characterisation of cyclic variability in an optically accessible IC Engine by means of phase-independent POD
Investigation of cyclic variability in engine operation has recently received new impulse with the widespread application of advanced numerical and experimental techniques. The present work attempts to shed some light on the existence and nature of correlations between coherent structures dynamics and cyclic variability in IC engines by means of phase-independent Proper Orthogonal Decomposition applied to highly-resolved PIV measurements obtained in an optically accessible, motored engine. Analysis of the conditional means and variances of the reconstruction coefficients reveal interesting patterns in the break-up of coherent structures which are also confirmed by experimental observation and leave room for speculation on the true nature of the flow field at different crank angles. A first attempt has also been carried out to reconstruct missing information from available measurements, with encouraging results: the development of such interpolation/reconstruction technique could obviously have a great impact on the reduction of the cost normally involved in experimental and computational campaigns
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
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