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 $M_{\rm initial}$ = 1.6 to 2.0 $M_\odot$. At larger masses, the core-mass growth decreases steadily to $\sim$10% at $M_{\rm initial}$ = 3.4 $M_\odot$. 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 $t$ $\sim$ 3 Myr and $E$ = 1.2 $\times$ 10$^{10}$ $L_\odot$ yr for $M_{\rm initial}$ $\sim$ 2 $M_\odot$ ($t$ $\sim$ 2 Myr for luminosities brighter than the RGB tip at $\log(L/L_{\odot})$ $>$ 3.4), decreasing to $t$ = 0.4 Myr and $E$ = 6.1 $\times$ 10$^{9}$ $L_\odot$ yr for stars with $M_{\rm initial}$ $\sim$ 3.5 $M_\odot$. 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