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

A model for the Globular Cluster extreme anomalies

In spite of the efforts made in the latest years, still there is no comprehensive explanation for the chemical anomalies of globular cluster stars. Among these, the most striking is oxygen depletion, which reaches values down to [O/Fe]~-0.4 in most clusters, but in M13 it goes down to less than [O/Fe]~ - 1. In this work we suggest that the anomalies are due to the super position of two different events: 1) PRIMORDIAL SELF-ENRICHMENT: this is asked to explain the oxygen depletion down to a minimum value [O/Fe]~ -0.4; 2) EXTRA MIXING IN A FRACTION OF THE STARS ALREADY BORN WITH ANOMALOUS COMPOSITION: these objects, starting with already low [O/Fe], will reduce the oxygen abundance down to the most extreme values. Contrary to other models that invoke extra mixing to explain the chemical anomalies, we suggest that it is active only if there is a fraction of the stars in which the primordial composition is not only oxygen depleted, but also extremely helium rich (Y~ 0.4), as found in a few GCs from their main sequence multiplicity. We propose that the rotational evolution (and an associated extra mixing) of extremely helium rich stars may be affected by the fact that they develop a very small or non existent molecular weight barrier during the evolution. We show that extra mixing in these stars, having initial chemistry that has already been CNO processed, affects mainly the oxygen abundance, and to a much smaller extent if affects the sodium abundance. The model also predicts a large fluorine depletion concomitant with the oxygen depletion, and a further enhancement of the surface helium abundance, which reaches values close to Y=0.5 in the computed models. We stress that, in this tentative explanation, those stars that are primordially O--depleted, but ARE NOT extremely helium rich do not suffer deep extra mixing.Comment: 12 pages, 8 figures and 5 tables, accepted for publication in MNRA

Modelling the closest double degenerate system RXJ0806.3+1527 and its decreasing period

In the hypothesis that the 5.4m binary RXJ0806.3+1527 consists of a low mass helium white dwarf (donor) transferring mass towards its more massive white dwarf companion (primary), we consider as possible donors white dwarfs which are the result of common envelope evolution occurring when the helium core mass of the progenitor giant was still very small (~ 0.2Msun), so that they are surrounded by a quite massive hydrogen envelope (~1/100Msun or larger), and live for a very long time supported by proton--proton burning. Mass transfer from such low mass white dwarfs very probably starts during the hydrogen burning stage, and the donor structure will remain dominated by the burning shell until it loses all the hydrogen envelope and begins transferring helium. We model mass transfer from these low mass white dwarfs, and show that the radius of the donor decreases while they shed the hydrogen envelope. This radius behavior, which is due to the fact that the white dwarf is not fully degenerate, has two important consequences on the evolution of the binary: 1) the orbital period decreases, with a timescale consistent with the period decrease of the binary RXJ0806.3+1527; 2) the mass transfer rate is a factor of about 10 smaller than from a fully degenerate white dwarf, easing the problem connected with the small X-ray luminosity of this object. The possibility that such evolution describes the system RXJ0806.3+1527 is also consistent with the possible presence of hydrogen in the optical spectrum of the star, whose confirmation would become a test of the model.Comment: 13 pages, 4 figures, accepted for publication on ApJ, main journa

The Lithium Depletion Boundary and the Age of the Young Open Cluster IC~2391

We have obtained new photometry and intermediate resolution ($\Delta \lambda = 2.7$ \AA\ ) spectra of 19 of these objects (14.9 $\le$ $I_c$ $\le$ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on their $VRI$ photometry, spectral types, radial velocity, and H$\alpha$ emission strengths. Higher S/N spectra were obtained for 8 of these probable cluster members in order to measure the strength of the lithium 6708 \AA\ doublet and thus obtain an estimate of the cluster's age. One of these 8 stars has a definite lithium detection and two other (fainter) stars have possible lithium detections. A color-magnitude diagram for our program objects shows that the lithium depletion boundary in IC~2391 is at $I_c$=16.2. Using recent theoretical model predictions, we derive an age for IC~2391 of 53$\pm$5 Myr. While this is considerably older than the age most commonly attributed for this cluster ($\sim$35 Myr) this result for IC~2391 is comparable those recently derived for the Pleiades and Alpha Persei clusters and can be explained by new models for high mass stars that incorporate a modest amount of convective core overshooting.Comment: ApJ Letters, acccepte