185 research outputs found
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&
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
The third dredge-up and the carbon star luminosity functions in the Magellanic Clouds
We investigate the formation of carbon stars as a function of the stellar
mass and parent metallicity. Theoretical modelling is based on an improved
scheme for treating the third dredge-up in synthetic calculations of thermally
pulsing asymptotic giant branch (TP-AGB) stars. In this approach, the usual
criterion (based on a constant minimum core mass for the occurrence of
dredge-up, M_c^min) is replaced by one on the minimum temperature at the base
of the convective envelope, T_b^dred, at the stage of the post-flash luminosity
maximum. Envelope integrations then allow determination of M_c^min as a
function of stellar mass, metallicity, and pulse strength (see Wood 1981), thus
inferring if and when dredge-up first occurs. Moreover, the final possible shut
down of the process is predicted. Extensive grids of TP-AGB models were
computed using this scheme. We present and discuss the calibration of the two
dredge-up parameters (lambda and T_b^dred) aimed at reproducing the carbon star
luminosity function (CSLF) in the LMC. It turns out that the faint tail is
almost insensitive to the history of star formation rate (SFR) in the parent
galaxy (it is essentially determined by T_b^dred), in contrast to the bright
wing which may be more affected by the details of the recent SFR. Once the
faint end is reproduced, the peak location is a stringent calibrator of lambda.
The best fit to the observed CSLF in the LMC is obtained with Z=0.008,
lambda=0.50, log(T_b^dred)=6.4, and a constant SFR up to 5x10^8 yr ago. A good
fit to the CSLF in the SMC is then easily derived from the Z=0.004 models, with
a single choice of parameters, and a constant SFR over the entire significant
age interval. The results are consistent with the theoretical expectation that
the third dredge-up is more efficient at lower Zs.Comment: 22 pages with 15 figures, to appear in A&
Chemical yields from low- and intermediate-mass stars: model predictions and basic observational constraints
In this work we analyse the role of low- and intermediate-mass stars in
contributing to the chemical enrichment of the interstellar medium. First we
present new sets of stellar yields basing on the results of updated
evolutionary calculations, which extend from the ZAMS up to the end of the AGB
phase (Girardi et al. 2000; Marigo et al. 1999a). These new yields, that
present a significant dependence on metallicity, are then compared to those of
other available sets (Renzini & Voli 1981; van de Hoek & Groenewegen 1997). The
resulting differences are explained in terms of different model assumptions --
i.e. treatment of convective boundaries, mass loss, dredge-up, hot-bottom
burning --, and further discussed on the basis of important empirical
constraints which should be reproduced by theory -- i.e. the initial-final mass
relation, white dwarf mass distribution, carbon star luminosity function, and
chemical abundances of planetary nebulae. We show that present models are able
to reproduce such constraints in a satisfactory way.Comment: 24 pages, 12 figures, 16 tables, to appear in A&A. Data Tables are
available in electronic format at http://pleiadi.pd.astro.i
Can the third dredge-up extinguish hot-bottom burning in massive AGB stars?
Marigo (2002) has highlighted the crucial importance of molecular opacities
in modelling the evolution of AGB stars at varying surface C/O ratio. In
particular, it has been shown the large inadequacy of solar-scaled opacities
when applied to models of carbon stars, and hence the need for correctly
coupling the molecular opacities to the current surface chemical composition of
AGB stars. The aim of the present follow-up study is to investigate the effects
of variable molecular opacities on the evolutionary properties of luminous AGB
stars with massive envelopes, i.e. with initial masses from ~3.5 Msun up to 5-8
Msun, which are predicted to experience both the third dredge-up and hot-bottom
burning. It is found that if the dredge-up of carbon is efficient enough to
lead to an early transition from C/O1, then hot-bottom burning may be
weakened, extinguished, or even prevented. The physical conditions for this
occurrence are analysed and a few theoretical and observational implications
are discussed. Importantly, it is found that the inclusion of variable
molecular opacities could significantly change the current predictions for the
chemical yields contributed by intermediate-mass AGB stars, with M~3.5 - 4.0
Msun that make as much as ~ 30-50 % of all stars expected to undergo hot-bottom
burning.Comment: 10 pages, 5 figures, accepted for publication in A&
Calibrating the role of TP-AGB stars in the cosmic matter cycle
In the last ten years three main facts about the thermally pulsing asymptotic
giant branch (TP-AGB) have become evident: 1) the modelling of the TP-AGB phase
is critical for the derivation of basic galaxy properties (e.g. mass and age)
up to high redshift, with consequent cosmological implications; 2) current
TP-AGB calibrations based on Magellanic Cloud (MC) clusters come out not to
work properly for other external galaxies, yielding a likely TP-AGB
overestimation; 3) the significance of the TP-AGB contribution in galaxies,
hence their derived properties, are strongly debated, with conflicting claims
in favour of either a heavy or a light TP-AGB. The only way out of this
condition of persisting uncertainty is to perform a reliable calibration of the
TP-AGB phase as a function of the star's initial mass (hence age) over a wide
range of metallicity, from very low to super-solar values. In this context, I
will review recent advancements and ongoing efforts towards a physically-sound
TP-AGB calibration that, moving beyond the classical use of the MC clusters,
combines increasingly refined TP-AGB stellar models with exceptionally
high-quality data for resolved TP-AGB stars in nearby galaxies. Preliminary
results indicate that a sort of "TP-AGB island" emerges in the age-metallicity
plane, where the contribution of these stars is especially developed, embracing
preferentially solar- and MC-like metallicities, and intermediate ages (~ few
Gyr).Comment: Invited review at the conference "Why Galaxies Care About AGB Stars
III", July 28 - August 1, 2014, Vienna, (eds. F. Kerschbaum, J. Hron, and B.
Wing), to appear in the ASP Conference Series; 10 pages, 10 postscript
figure
The Core Mass Growth and Stellar Lifetime of Thermally Pulsing Asymptotic Giant Branch Stars
We establish new constraints on the intermediate-mass range of the
initial-final mass relation by studying white dwarfs in four young star
clusters, and apply the results to study the evolution of stars on the
thermally pulsing asymptotic giant branch (TP-AGB). We show that the stellar
core mass on the AGB grows rapidly from 10% to 30% for stars with = 1.6 to 2.0 . At larger masses, the core-mass growth
decreases steadily to 10% at = 3.4 . These
observations are in excellent agreement with predictions from the latest TP-AGB
evolutionary models in Marigo et al. (2013). We also compare to models with
varying efficiencies of the third dredge-up and mass loss, and demonstrate that
the process governing the growth of the core is largely the stellar wind, while
the third dredge-up plays a secondary, but non-negligible role. Based on the
new white dwarf measurements, we perform an exploratory calibration of the most
popular mass-loss prescriptions in the literature. Finally, we estimate the
lifetime and the integrated luminosity of stars on the TP-AGB to peak at
3 Myr and = 1.2 10 yr for 2 ( 2 Myr for luminosities brighter than
the RGB tip at 3.4), decreasing to = 0.4 Myr and
= 6.1 10 yr for stars with
3.5 . The implications of these results are discussed with
respect to general population synthesis studies that require correct modeling
of the TP-AGB phase of stellar evolution.Comment: 14 pages, 7 figures, 4 tables. Accepted for publication in Ap
Evolutionary models of zero metallicity stars
We present new evolutionary models for zero-metallicity stars, covering a
large range of initial masses (from 0.8 to 100 M_sun). Models are computed with
overshooting from stellar cores and convective envelopes, and assuming
mass-loss from the most massive stars. We discuss the main evolutionary
features of these stars, and provide estimates of the amount of
newly-synthesized elements dredged-up to the stellar surface, and possibly lost
by stellar winds from the most massive stars. Full details of these models will
be given in Marigo et al. (2000, in preparation).Comment: 2 pages, to appear in the proceedings of the MPA/ESO Workshop "The
first stars
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