937 research outputs found
A 3D radiative transfer framework: V. Homologous Flows
Observations and theoretical calculations have shown the importance of
non-spherically symmetric structures in supernovae. Thus, the interpretation of
observed supernova spectra requires the ability to solve the transfer equation
in 3-D moving atmospheres. We present an implementation of the solution of the
radiative transfer equation in 3-D homologously expanding atmospheres in
spherical coordinates. The implementation is exact to all orders in v/c. We use
a new affine method that makes use of the fact that photons travel on straight
lines. We compare our results in 3-D for spherically symmetric test problems
with high velocity fields and find excellent agreement. Our well-tested 1-D
results are based on methods where the momentum directions vary along the
characteristic (co-moving momentum directions). Thus, we are able to verify
both the analytic framework and its numerical implementation. Additionally, we
have been able to test the parallelization over characteristics. Using 512^2
momentum angles we ran the code on 16,384 Opteron processors and achieved
excellent scaling. It is now possible to calculate synthetic spectra from
realistic 3D hydro simulations. This should open an era of progress in hydro
modeling, similar to that that occurred in the 1980s when 1-D models were
confronted with synthetic spectra.Comment: 6 pages, 3 figures, Astronomy & Astrophysics, in pres
Non-LTE treatment of molecules in the photospheres of cool stars
We present a technique to treat systems with very many levels, like
molecules, in non-LTE. This method is based on a superlevel formalism coupled
with rate operator splitting. Superlevels consist of many individual levels
that are assumed to be in LTE relative to each other. The usage of superlevels
reduces the dimensionality of the rate equations dramatically and, thereby,
makes the problem computationally more easily treatable. Our superlevel
formalism retains maximum accuracy by using direct opacity sampling (dOS) when
calculating the radiative transitions and the opacities. We developed this
method in order to treat molecules in cool dwarf model calculations in non-LTE.
Cool dwarfs have low electron densities and a radiation field that is far from
a black body radiation field, both properties may invalidate the conditions for
the common LTE approximation. Therefore, the most important opacity sources,
the molecules, need to be treated in non-LTE. As a case study we applied our
method to carbon monoxide. We find that our method gives accurate results since
the conditions for the superlevel method are very well met for molecules. Due
to very high collisional cross sections with hydrogen, and the high densities
of H_2 the population of CO itself shows no significant deviation from LTE.Comment: AASTeX v50, 35 pages including 12 figures, accepted by Ap
Parallel Implementation of the PHOENIX Generalized Stellar Atmosphere Program
We describe the parallel implementation of our generalized stellar atmosphere
and NLTE radiative transfer computer program PHOENIX. We discuss the parallel
algorithms we have developed for radiative transfer, spectral line opacity, and
NLTE opacity and rate calculations. Our implementation uses a MIMD design based
on a relatively small number of MPI library calls. We report the results of
test calculations on a number of different parallel computers and discuss the
results of scalability tests.Comment: To appear in ApJ, 1997, vol 483. LaTeX, 34 pages, 3 Figures, uses
AASTeX macros and styles natbib.sty, and psfig.st
The color signature of the transit of HD 209458: Discrepancies between stellar atmospheric models and observations
Exoplanetary transits produce a double-horned color signature that is
distinct from both binaries and blends and can thus be used to separate
exoplanets from false positives in transit searches. Color photometry with
precision sufficient to detect this signal in transits of HD 209458 is
available in the literature. Analysis of these observations reveals that, while
the signature does exhibit the expected shape, it is significantly stronger
than PHOENIX atmospheric models predict.Comment: 4 pages, 3 figures, accepted to A&
Phase-Dependent Properties of Extrasolar Planet Atmospheres
Recently the Spitzer Space Telescope observed the transiting extrasolar
planets, TrES-1 and HD209458b. These observations have provided the first
estimates of the day side thermal flux from two extrasolar planets orbiting
Sun-like stars. In this paper, synthetic spectra from atmospheric models are
compared to these observations. The day-night temperature difference is
explored and phase-dependent flux densities are predicted for both planets. For
HD209458b and TrES-1, models with significant day-to-night energy
redistribution are required to reproduce the observations. However, the
observational error bars are large and a range of models remains viable.Comment: 8 pages, 7 figures, accepted for publication in the Astrophysical
Journa
The NextGen Model Atmosphere grid: II. Spherically symmetric model atmospheres for giant stars with effective temperatures between 3000 and 6800~K
We present the extension of our NextGen model atmosphere grid to the regime
of giant stars. The input physics of the models presented here is nearly
identical to the NextGen dwarf atmosphere models, however spherical geometry is
used self-consistently in the model calculations (including the radiative
transfer). We re-visit the discussion of the effects of spherical geometry on
the structure of the atmospheres and the emitted spectra and discuss the
results of NLTE calculations for a few selected models.Comment: ApJ, in press (November 1999), 13 pages, also available at
http://dilbert.physast.uga.edu/~yeti/PAPERS and at
ftp://calvin.physast.uga.edu/pub/preprints/NG-giants.ps.g
Solid-Solid Phase Transformations and Their Kinetics in Ti–Al–Nb Alloys
The application of light-weight intermetallic materials to address the growing interest and necessity for reduction of CO2 emissions and environmental concerns has led to intensive research into TiAl-based alloy systems. However, the knowledge about phase relations and transformations is still very incomplete. Therefore, the results presented here from systematic thermal analyses of phase transformations in 12 ternary Ti-Al-Nb alloys and one binary Ti-Al measured with 4–5 different heating rates (0.8 to 10 °C/min) give insights in the kinetics of the second-order type reaction of ordered (βTi)o to disordered (βTi) as well as the three first-order type transformations from Ti3Al to (αTi), ωo (Ti4NbAl3) to (βTi)o, and O (Ti2NbAl) to (βTi)o. The sometimes-strong heating rate dependence of the transformation temperatures is found to vary systematically in dependence on the complexity of the transformations. The dependence on heating rate is nonlinear in all cases and can be well described by a model for solid-solid phase transformations reported in the literature, which allows the determination of the equilibrium transformation temperatures
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