5,170 research outputs found
Evolution and chemical yields of AGB stars: effects of low-temperature opacities
The studies focused on the Thermally-Pulsing Asymptotic Giant Branch phase
experienced by low- and intermediate-mass stars are extremely important in many
astrophysical contexts. In particular, a detailed computation of their chemical
yields is essential for several issues, ranging from the chemical evolution of
galaxies, to the mechanisms behind the formation of globular clusters. Among
all the uncertainties affecting the theoretical modelling of this phase, and
described in the literature, it remains to be fully clarified which results are
severely affected by the use of inadequate low-temperature opacities, that are
in most cases calculated on the basis of the original chemical composition of
the stars, and do not consider the changes in the surface chemistry due to the
occurrence of the third dredge-up and hot-bottom burning. Our investigation is
aimed at investigating this point. By means of full evolutionary models
including new set of molecular opacities computed specifically with the AESOPUS
tool, we highlight which stellar models, among those present in the literature,
need a substantial revision, mainly in relation to the predicted chemical
yields. The interplay among convection, hot bottom burning and the
low-temperature opacity treatment is also discussedComment: 6 pages, 2 figure
Chemical yields from low- and intermediate-mass stars
We present new sets of chemical yields from low- and intermediate-mass stars
with 0.8 Msun <= M <= Mup ~ 5 Msun, and three choices of the metallicity,
Z=0.02, Z=0.008, and Z=0.004 (Marigo 2000, in preparation). These are then
compared with the yields calculated by other authors on the basis of different
model prescriptions, and basic observational constraints which should be
reproduced.Comment: 6 pages, 4 postscript figures, to appear in "The chemical evolution
of the Milky Way: stars versus clusters", Proceedings of the Roma-Trieste
Workshop, (Vulcano, September 1999), eds. F. Matteucci and F. Giovannell
Envelope burning over-luminosity: a challenge to synthetic TP-AGB models
Until recently synthetic AGB models had not taken into account the break-down
of the core mass-luminosity (Mc-L) relation due to the occurrence of envelope
burning in the most massive (M > 3.5 Msun) and luminous (Mbol > -6) stars.
Marigo et al. (1998) made the first attempt to consistently include the related
over-luminosity effect (i.e. above the Mc-L relation) in synthetic TP-AGB
calculations. In this paper the reliability of the solution scheme is tested by
comparison with the results of complete evolutionary calculations for a 7 Msun
AGB star undergoing envelope burning (e.g. Bloecker & Schoenberner 1991).
Indeed, the method proves to be valid as it is able to reproduce with
remarkable accuracy several evolutionary features of the 7 Msun star. We
present extensive synthetic TP-AGB calculations for stars with initial masses
of 3.5, 4.0, 4.5, and 5.0 Msun, and three choices of the initial metallicity,
i.e. Z=0.019, Z=0.008, and Z=0.004. Three values of the mixing-length parameter
are used, i.e. alpha=1.68, 2.0, 2.5. We investigate the dependence of envelope
burning on such stellar parameters (M, Z, and alpha). The comparison between
different cases gives hints on the interplay between envelope burning
over-luminosity and mass loss, and related effects on TP-AGB lifetimes.Comment: 19 pages, 9 postscript figures, 2 tables, submitted to A&
Evolution of AGB stars at varying surface C/O ratio: The crucial effect of molecular opacities
This study calls attention to the importance of properly coupling the
molecular opacities to the actual surface abundances of TP-AGB stars that
experience the third dredge-up and/or hot-bottom burning, i.e. with surface
abundances of carbon and oxygen varying with time. New TP-AGB calculations with
variable opacities -- replacing the usually adopted solar-scaled opacity tables
-- have proven to reproduce, for the first time, basic observables of carbon
stars, like their effective temperatures, C/O ratios, and near-infrared
colours. Moreover, it turns out that the effect of envelope cooling -- due to
the increase in molecular opacities -- may cause other important effects,
namely: i) shortening of the C-star phase; ii) possible reduction or shut-down
of the third dredge-up in low-mass carbon stars; and iii) weakening or even
extinction of hot-bottom burning in intermediate-mass stars.Comment: 6 pages, 5 postscript figures, proceedings of contributed talk at the
St. Luc conference ``CNO in the Universe'', eds. C. Charbonnel, D. Schaerer,
& G. Meynet, to be published in the ASP Conference Serie
Population synthesis models including AGB stars and their ingredients
I will briefly review the state of the art of evolutionary population
synthesis (EPS) models that include the contribution from AGB stars.Comment: 10 pages, 6 figures, review talk to the workshop "Why galaxies care
about AGB stars", Vienna, August 7-11, 200
Towards simulating the photometry, chemistry, mass loss and pulsational properties of AGB star populations in resolved galaxies
Extended and updated grids of TP-AGB tracks have been implemented in the
TRILEGAL population synthesis code, which generates mock stellar catalogues for
a galaxy given its mass, distance, star formation history and age-metallicity
relation, including also the Milky Way foreground population. Among the stellar
parameters that are simulated, we now include the surface chemistry, mass-loss
rates, pulsation modes and periods of LPVs. This allows us to perform a series
of consistency checks between AGB model predictions and observations, that we
are just starting to explore. We present a few examples of model--data
comparisons, mostly regarding the near-infrared and variability data for AGB
stars in the Magellanic Clouds.Comment: 5 pages, 3 figures, contributed talk to the workshop "Why galaxies
care about AGB stars", Vienna, August 7-11, 200
Constraining the third dredge-up via carbon stars in the Magellanic Clouds
We use the available data for Magellanic Cloud carbon stars to constrain the
efficiency of the third dredge-up process in TP-AGB models. We show that star
counts in LMC clusters provide quite stringent limits to the lifetime of the
C-star phase, with a duration between 2 and 3 Myr for stars in the mass range
from 1.5 to 2.8 Msun. Together with the luminosity functions of field C stars,
this information allows us to re-calibrate the third dredge-up parameters log
T_b_dred and lambda in TP-AGB models that include variable molecular opacities
(Marigo 2002). Preliminary results are presented here.Comment: in proceedings of the St. Luc conference ``CNO in the Universe'',
eds. C. Charbonnel, D. Schaerer, & G. Meynet, ASP Conference Series, in pres
Modelling stellar populations at high redshift
Stellar populations carry information about the formation of galaxies and
their evolution up to the present epoch. A wealth of observational data are
available nowadays, which are analysed with stellar population models in order
to obtain key properties such as ages, star formation histories, stellar
masses. Differences in the models and/or in the assumptions regarding the star
formation history affect the derived properties as much as differences in the
data. I shall review the interpretation of high-redshift galaxy data from a
model perspective. While data quality dominates galaxy analysis at the highest
possible redshifts (z>5), population modelling effects play the major part at
lower redshifts. In particular, I discuss the cases of both star-forming
galaxies at the peak of the cosmic star formation history as well as passive
galaxies at redshift below 1 that are often used as cosmological probes.
Remarks on the bridge between low and high-z massive galaxies conclude the
contribution.Comment: 7 pages, 7 figures, invited review at the IAU Symposium 277 "Tracing
the Ancestry of Galaxies (on the land of our ancestors)", Ouagadougou
(Burkina Faso), December 2010, Editors: Claude Carignan, Francoise Combes,
Ken Freema
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