365 research outputs found
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/
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
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
Optical and near-infrared spectrophotometric properties of Long Period Variables and other luminous red stars
Based on a new and large sample of optical and near-infrared spectra obtained
at the Mount Stromlo and Siding Spring Observatories (Lancon & Wood 1998;
Lancon & Wood, in preparation), spectrophotometric properties of cool oxygen-
and carbon-rich Long Period Variables and supergiants are presented.
Temperatures of oxygen-rich stars are assigned by comparison with synthetic
spectra computed from up-to-date oxygen-rich model atmosphere grids. The
existence of reliable optical and near-infrared temperature indicators is
investigated. A narrow relation between the bolometric correction BC(I) and the
broad-band colour I-J is obtained for oxygen-rich cool stars. The ability of
specific near-infrared indices to separate luminosity classes, atmospheric
chemistry or variability subtypes is discussed. Some comments are also given on
extinction effects, water band strengths in Long Period Variables and the
evaluation of 12CO/13CO ratio in red giants.Comment: 14 pages, 21 figures, Latex, accepted for publication in Astronomy
and Astrophysics main journal. Also available at
http://www-astro.ulb.ac.be/~ralvarez
Circumstellar effects on the Rb abundances in O-rich AGB stars
For the first time we explore the circumstellar effects on the Rb (and Zr)
abundance determination in O-rich asymptotic giant branch (AGB) stars by
considering the presence of a gaseous circumstellar envelope with a radial
wind. A modified version of the spectral synthesis code Turbospectrum was used
to deal with extended atmosphere models and velocity fields. The Rb and Zr
abundances were determined from the resonant 7800A Rb I line and the 6474A ZrO
bandhead, respectively, in five representative O-rich AGB stars with different
expansion velocity and metallicity. By using our new dynamical models, the Rb I
line profile (photospheric and circumstellar components) is very well
reproduced. Interestingly, the derived Rb abundances are much lower (by 1-2
dex) in those O-rich AGB stars showing the higher circumstellar expansion
velocities. The Zr abundances, however, remain close to the solar values. The
Rb abundances and Rb/Zr ratios derived here significantly alleviate the problem
of the present mismatch between the observations of intermediate-mass (4-8
solar masses) Rb-rich AGB stars and the AGB nucleosynthesis theoretical
predictions.Comment: Accepted for publication in Astronomy & Astrophysics Letters (7
pages, 5 figures, and 2 tables); final version (language corrected
Envelope tomography of long-period variable stars: I. The Schwarzschild mechanism and the Balmer emission lines
This paper is the first one in a series devoted to the study of the dynamics
of the atmospheres of long-period variable stars. Results from a two-month-long
monitoring of the Mira variables RT Cyg and X Oph around maximum light with the
ELODIE spectrograph at the Haute-Provence Observatory are presented. The
monitoring covers phases 0.80 to 1.16 for RT Cyg and phases 0.83 to 1.04 for X
Oph. The cross-correlation profile of the spectrum of RT Cyg with a K0 III mask
confirms that the absorption lines of RT Cyg in the optical domain appear
double around maximum light. No line doubling was found in the optical spectrum
of X Oph around maximum light, indicating that this feature is not common to
all long-period variables. This paper also presents the application to RT Cyg
of a new tomographic technique deriving the velocity field across the
atmosphere by cross-correlating the optical spectrum with numerical masks
constructed from synthetic spectra and probing layers of increasing depths.
This technique reveals that both the temporal evolution of the line doubling,
and its variation with depth in the atmosphere of RT Cyg, are consistent with
the ``Schwarzschild scenario''. This scenario relates the temporal evolution of
the red and blue peaks of the double absorption lines to the progression of a
shock wave in the atmosphere. The temporal evolution of the Balmer Halpha, H
beta, Hgamma and Hdelta emission lines around maximum light is also presented
for RT Cyg and X Oph. The velocity variations of Halpha and of the absorption
lines are discussed in the framework of two competing models for the formation
of Balmer emission lines in long-period variable stars.Comment: 11 pages, 8 figures, Latex, accepted for publication in Astronomy and
Astrophysics main journal. Also available at
http://www-astro.ulb.ac.be/Html/ps.htm
Potassium: a new actor on the globular cluster chemical evolution stage. The case of NGC 2808
We derive [K/Fe] abundance ratios for 119 stars in the globular cluster NGC
2808, all of them having O, Na, Mg and Al abundances homogeneously measured in
previous works. We detect an intrinsic star-to-star spread in the Potassium
abundance. Moreover [K/Fe] abundance ratios display statistically significant
correlations with [Na/Fe] and [Al/Fe], and anti-correlations with [O/Fe] and
[Mg/Fe]. All the four Mg deficient stars ([Mg/Fe]<0.0) discovered so far in NGC
2808 are enriched in K by ~0.3 dex with respect to those with normal [Mg/Fe].
NGC 2808 is the second globular cluster, after NGC 2419, where a clear Mg-K
anti-correlation is detected, albeit of weaker amplitude. The simultaneous
correlation/anti-correlation of [K/Fe] with all the light elements usually
involved in the chemical anomalies observed in globular cluster stars, strongly
support the idea that these abundance patterns are due to the same
self-enrichment mechanism that produces Na-O and Mg-Al anti-correlations. This
finding suggests that detectable spreads in K abundances may be typical in the
massive globular clusters where the self-enrichment processes are observed to
produce their most extreme manifestations.Comment: Accepted for publication by ApJ, 5 pages, 3 figure
Late-Type Red Supergiants: Too Cool for the Magellanic Clouds?
We have identified seven red supergiants (RSGs) in the Large Magellanic Cloud
(LMC) and four RSGs in the Small Magellanic Cloud (SMC), all of which have
spectral types that are considerably later than the average type observed in
their parent galaxy. Using moderate-resolution optical spectrophotometry and
the MARCS stellar atmosphere models, we determine their physical properties and
place them on the H-R diagram for comparison with the predictions of current
stellar evolutionary tracks. The radial velocities of these stars suggest that
they are likely all members of the Clouds rather than foreground dwarfs or halo
giants. Their locations in the H-R diagram also show us that those stars are
cooler than the current evolutionary tracks allow, appearing to the right of
the Hayashi limit, a region in which stars are no longer in hydrodynamic
equilibrium. These stars exhibit considerable variability in their V
magnitudes, and three of these stars also show changes in their effective
temperatures (and spectral types) on the time-scales of months. One of these
stars, [M2002] SMC 055188, was caught in an M4.5 I state, as late as that seen
in HV 11423 at its recent extreme: considerable later, and cooler, than any
other supergiant in the SMC. In addition, we find evidence of variable
extinction due to circumstellar dust and changes in the stars' luminosities,
also consistent with our recent findings for HV 11423 - when these stars are
hotter they are also dustier and more luminous. We suggest that these stars
have unusual properties because they are in an unstable (and short-lived)
evolutionary phase.Comment: 26 pages, 6 figures; submitted to the Astrophysical Journa
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