715 research outputs found
Photospheric constraints, current uncertainties in models of stellar atmospheres, and spectroscopic surveys
We summarize here the discussions around photospheric constraints, current
uncertainties in models of stellar atmospheres, and reports on ongoing
spectroscopic surveys. Rather than a panorama of the state of the art, we chose
to present a list of open questions that should be investigated in order to
improve future analyses.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
Red supergiant star studies with CO5BOLD and Optim3D
We describe recent work focused towards a better understanding of red
supergiant stars using 3D radiative-hydrodynamics (RHD) simulations with
CO5BOLD. A small number of simulations now exist that span up to seven years of
stellar time, at various numerical resolutions. Our discussion concentrates on
interferometric and spectroscopic observations. We point out a number of
problems, in particular the line depth and line width that are not well
reproduced by simulations. The most recent introduction of a non-grey treatment
of the radiation field dramatically improved the match with observations,
without solving all difficulties. We also review the newly revived effective
temperature scale controversy, and argue that it will only be solved using 3D
RHD models.Comment: To be published on the proceedings of the CO5BOLD workshop 2012 on
Memorie della SAIt Supplementi, Vol. 24, E. Caffau & L. Sbordone eds.
(http://www.lsw.uni-heidelberg.de/co5bold/workshop/
Bringing VY Canis Majoris Down to Size: An Improved Determination of Its Effective Tempeature
The star VY CMa is a late-type M supergiant with many peculiarities, mostly
related to the intense circumstellar environment due to the star's high
mass-loss rate. Claims have been made that would suggest this star is
considerably more luminous (L = 5 x 10^5 Lo) and larger (R=2800 Ro) than other
Galactic red supergiants (RSGs). Indeed, such a location in the H-R diagram
would be well in the "Hayashi forbidden zone" where stars cannot be in
hydrostatic equilibrium. These extraordinary properties, however, rest upon an
assumed effective temperature of 2800-3000 K, far cooler than recent work have
shown RSGs to be. To obtain a better estimate, we fit newly obtained
spectrophotometry in the optical and NIR with the same MARCS models used for
our recent determination of the physical properties of other RSGs; we also use
and from the literature to derive an effective temperatures. We
find that the star likely has a temperature of 3650 K, a luminosity L = 6 x
10^4 Lo, and a radius of 600Ro. These values are consistent with VY CMa being
an ordinary evolved 15 Mo RSG, and agree well with the Geneva evolutionary
tracks. We find that the circumstellar dust region has a temperature of 760 K,
and an effective radius of approximately 130 AU, if spherical geometry is
assumed for the latter. What causes this star to have such a high mass-loss,
and large variations in brightness (but with little change in color), remains a
mystery at present, although we speculate that perhaps this star (and NML Cyg)
are simply normal RSGs caught during an unusually unstable time.Comment: Accepted by the Astrophysical Journa
New ATLAS9 And MARCS Model Atmosphere Grids for the Apache Point Observatory Galactic Evolution Experiment (APOGEE)
We present a new grid of model photospheres for the SDSS-III/APOGEE survey of
stellar populations of the Galaxy, calculated using the ATLAS9 and MARCS codes.
New opacity distribution functions were generated to calculate ATLAS9 model
photospheres. MARCS models were calculated based on opacity sampling
techniques. The metallicity ([M/H]) spans from -5 to 1.5 for ATLAS and -2.5 to
0.5 for MARCS models. There are three main differences with respect to previous
ATLAS9 model grids: a new corrected H2O linelist, a wide range of carbon
([C/M]) and alpha element [alpha/M] variations, and solar reference abundances
from Asplund et al. 2005. The added range of varying carbon and alpha element
abundances also extends the previously calculated MARCS model grids. Altogether
1980 chemical compositions were used for the ATLAS9 grid, and 175 for the MARCS
grid. Over 808 thousand ATLAS9 models were computed spanning temperatures from
3500K to 30000K and log g from 0 to 5, where larger temperatures only have high
gravities. The MARCS models span from 3500K to 5500K, and log g from 0 to 5.
All model atmospheres are publically available online.Comment: 8 pages, 6 figures, 5 tables, accepted for publication in The
Astronomical Journa
Atmospheric dynamics of red supergiant stars and applications to Interferometry
We have written a 3D radiative transfer code that computes emerging spectra
and intensity maps. We derive from radiative hydrodynamic (RHD) simulations of
RSG stars carried out with CO5BOLD (Freytag et al. 2002) observables expected
for red supergiant stars (RSG) especially for interferometric observations,
with emphasis on small scale structures. We show that the convection-related
surface structures are detectable in the H band with today's interferometers
and that the diameter measurement should not be too dependent on the adopted
model. The simulations are a great improvement over parametric models for the
interpretation of interferometric observations.Comment: 6 pages, SF2A 2007 conferenc
Cool star model atmospheres for Gaia : ATLAS, MARCS, and PHOENIX
International audienceI present the widely used model atmosphere codes ATLAS, MARCS, and PHOENIX, and I compare their output model structures and spectra for cool stars of FGKM-types. While model atmosphere stratifications agree closely with each other in the f-D approximation, this is not the case for spectra. Differences between model spectra from different codes are largest in the blue-UV, but smaller differences appear in all regions, especially in the molecular features of cooler model spectra. I recommend the groups to try to solve these discrepancies together. In the meantime, users must be careful when using these spectra in regimes where they differ. I discuss here only comparisons of spectra at solar metallicity, and this should be extended to other metallicities. Detailed comparisons with carefully calibrated spectrophotometric data, and high resolution spectra for stars with well known parameters are also of prime importance. It appears that we still need better line positions for molecules. Finally we should remember that f-D models are only a step towards a better representation of reality, and we should keep developing, and carefully test 3-D, NLTE models
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