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

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

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

Evolution of zero-metallicity massive stars

We discuss the evolutionary properties of primordial massive and very massive stars, supposed to have formed from metal-free gas. Stellar models are presented over a large range of initial masses (8 Msun <= Mi <= 1000 Msun), covering the hydrogen- and helium-burning phases up to the onset of carbon burning. In most cases the evolution is followed at constant mass. To estimate the possible effect of mass loss via stellar winds, recent analytic formalisms for the mass-loss rates are applied to the very massive models (Mi >= 120 Msun).Comment: Invited talk at IAU Symp. 212, ``A Massive Star Odyssey, from Main Sequence to Supernova'', K.A. van der Hucht, A. Herrero, C. Esteban (eds.), 7 pages, 5 postscript figure

A unified formalism for the core mass-luminosity relations of shell-burning stars

The luminosity evolution of stars with highly condensed cores surrounded by nuclear-burning shell(s) is analytically investigated with the aid of homology relations. With respect to earlier works using a similar approach (e.g. Refsdal & Weigert 1970; Kippenhahn 1981), the major improvement is that we derive all the basic dependences (i.e. on core mass, core radius, and chemical composition) in a completely generalised fashion, then accounting for a large range of possible physical properties characterising the burning shell(s). Parameterised formulas for the luminosity are given as a function of the (i) relative contribution of the gas to the total pressure (gas plus radiation), (ii) opacity source, and (iii) dominant nuclear reaction rates. In this way, the same formalism can be applied to shell-burning stars of various metallicities and in different evolutionary phases. In particular, we present some applications concerning the luminosity evolution of RGB and AGB stars with different chemical compositions, including the case of initial zero metallicity. It turns out that homology predictions provide a good approximation to the results of stellar model calculations. Therefore, the proposed formalism is useful to understand the possible differences in the luminosity evolution of shell-burning stars within a unified interpretative framework, and can be as well adopted to improve the analytical description of stellar properties in synthetic models.Comment: 11 pages, 7 postscript figures, accepted for publication in A&A, Main Journa

Pre-MS depletion, accretion and primordial 7Li

We reconsider the role of pre-main sequence (pre-MS) Li depletion on the basis of new observational and theoretical evidence: i) new observations of Halpha emissions in young clusters show that mass accretion could be continuing till the first stages of the MS, ii) theoretical implications from helioseismology suggest large overshooting values below the bottom of the convective envelopes. We argue here that a significant pre-MS 7Li destruction, caused by efficient overshoot mixing, could be followed by a matter accretion after 7Li depletion has ceased on MS thus restoring Li almost to the pristine value. As a test case we show that a halo dwarf of 0.85 Msun with an extended overshooting envelope starting with an initial abundance of A(Li) = 2.74 would burn Li completely, but an accretion rate of the type 1e-8xe^{-t/3e6} Msun yr$^{-1}$ would restore Li to end with an A(Li) = 2.31. A self-regulating process is required to produce similar final values in a range of different stellar masses to explain the PopII Spite plateau. However, this framework could explain why open cluster stars have lower Li abundances than the pre-solar nebula, the absence of Li in the most metal poor dwarfs and a number of other features which lack of a satisfactory explanation.Comment: To be published in Memorie della Societ\`a Astronomica Italiana Supplementi Vol. 22, Proceedings of Lithium in the cosmos, Iocco F., Bonifacio P., Vangioni E., ed

An extensive grid of DARWIN models for M-type AGB stars I. Mass-loss rates and other properties of dust-driven winds

The purpose of this work is to present an extensive grid of dynamical atmosphere and wind models for M-type AGB stars, covering a wide range of relevant stellar parameters. We used the DARWIN code, which includes frequency-dependent radiation-hydrodynamics and a time-dependent description of dust condensation and evaporation, to simulate the dynamical atmosphere. The wind-driving mechanism is photon scattering on submicron-sized Mg$_2$SiO$_4$ grains. The grid consists of $\sim4000$ models, with luminosities from $L_\star=890\,{\mathrm{L}}_\odot$ to $L_\star=40000\,{\mathrm{L}}_\odot$ and effective temperatures from 2200K to 3400K. For the first time different current stellar masses are explored with M-type DARWIN models, ranging from 0.75M$_\odot$ to 3M$_\odot$. The modelling results are radial atmospheric structures, dynamical properties such as mass-loss rates and wind velocities, and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of condensed Si). We find that the mass-loss rates of the models correlate strongly with luminosity. They also correlate with the ratio $L_*/M_*$: increasing $L_*/M_*$ by an order of magnitude increases the mass-loss rates by about three orders of magnitude, which may naturally create a superwind regime in evolution models. There is, however, no discernible trend of mass-loss rate with effective temperature, in contrast to what is found for C-type AGB stars. We also find that the mass-loss rates level off at luminosities higher than $\sim14000\,{\mathrm{L}}_\odot$, and consequently at pulsation periods longer than $\sim800$ days. The final grain radii range from 0.25 micron to 0.6 micron. The amount of condensed Si is typically between 10% and 40%, with gas-to-dust mass ratios between 500 and 4000.Comment: Accepted to A&A, 17 pages, 15 figure

Coupling emitted light and chemical yields from stars: a basic constraint to population synthesis models of galaxies

In this paper we emphasize the close connection between the chemical and spectrophotometric evolution of stellar systems: Chemical yields from stars correspond to a precise fraction of their emitted light. We translate this concept quantitatively. Starting from simple stellar populations, we derive useful analytical relations to calculate the stellar fuel consumption (emitted light) as a function of basic quantities predicted by stellar models, i.e. the mass of the core and the chemical composition of the envelope. The final formulas explicate the relation between integrated light contribution (total or limited to particular evolutionary phases), chemical yields and stellar remnants. We test their accuracy in the case of low- and intermediate-mass stars, and indicate the way to extend the analysis to massive stars. This formalism provides an easy tool to check the internal consistency between the different stellar inputs adopted in galaxy models: The fuel computed by means of the analytical formulas (corresponding to a given set of chemical yields) should be compared to the exact values given by the luminosity integration along the stellar evolutionary tracks or isochrones (corresponding to a given set of spectrophotometric models). Only if both estimates of the fuel are similar, the stellar inputs can be considered self-consistent in terms of their energetics. This sets an important requirement to galaxy models, also in consideration of the fact that different sources of input stellar data are frequently used to model their spectro-photometric and chemical evolution.Comment: 17 pages, 7 figures, 3 tables, accepted for publication in A&

Effects of dark matter annihilation on the first stars

We study the evolution of the first stars in the universe (Population III) from the early pre-Main Sequence until the end of helium burning in the presence of WIMP dark matter annihilation inside the stellar structure. The two different mechanisms that can provide this energy source are the contemporary contraction of baryons and dark matter, and the capture of WIMPs by scattering off the gas with subsequent accumulation inside the star. We find that the first mechanism can generate an equilibrium phase, previously known as a "dark star", which is transient and present in the very early stages of pre-MS evolution. The mechanism of scattering and capture acts later, and can support the star virtually forever, depending on environmental characteristic of the dark matter halo and on the specific WIMP model.Comment: Proceedings of the IAU Symposium 255, "Low-Metallicity Star Formation: From the First Stars to Dwarf Galaxies"; L.K. Hunt, S. Madden and R. Schneider ed

Evolution of planetary nebulae II. Population effects on the bright cut-off of the PNLF

We investigate the bright cut-off of the [OIII]l5007 planetary nebula luminosity function (PNLF), that has been suggested as a powerful extragalactic distance indicator. Theoretical PNLFs are constructed via Monte-Carlo simulations of populations of PNe, whose individual properties are described with the aid of recent PN synthetic models (Marigo et al. 2001), coupled to a detailed photoionisation code (CLOUDY). The basic dependences of the cut-off magnitude M* are then discussed. We find that: (i) In galaxies with recent or ongoing star formation, the modelled PNLF present M* values between -4 and -5, very close to the observationally-calibrated value for the LMC. (ii) In these galaxies, the PNLF cut-off is produced by PNe with progenitor masses of about 2.5 Msun, while less massive stars give origin to fainter PNe. As a consequence M* is expected to depend strongly on the age of the last burst of star formation, dimming by as much as 5 mag as we go from young to 10-Gyr old populations. (iii) Rather than on the initial metallicity of a stellar population, M* depends on the actual [O/H] of the observed PNe, a quantity that may differ significantly from the initial value (due to dredge-up episodes), especially in young and intermediate-age PN populations. (iv) Also the transition time from the end of AGB to the PN phase, and the nuclear-burning properties (i.e. H- or He-burning) of the central stars introduce non-negligible effects on M*. The strongest indication derived from the present calculations is a serious difficulty to explain the age-invariance of the cut-off brightness over an extended interval, say from 1 to 13 Gyr, that observations of PNLFs in galaxies of late-to-early type seem to suggest.Comment: 22 pages, to appear in Astronomy & Astrophysic