Evolutionary Models for AGB Stars in the Magellanic Clouds

AGB stars in the Magellanic Clouds (MCs) are the most robust calibrators for the modelling of this phase, thanks to the large collection of high-quality data (photometry, chemistry, pulsation properties, mass-loss, dust features, etc.) presently available. As part of an ambitious project aimed at substantially improving the performance of AGB synthetic models for the MCs, we revise the stellar models for the MCs. Particular attention is put on the chemical composition, in terms of both the initial mixture at the epoch of star formation, and the subsequent changes in surface abundances during the TP-AGB phase. We derive the initial chemical composition from recent spectroscopic analyses, which indicate that the MCs present non-scaled-solar ratios for key species such as C, N, O, and α-elements. The depression of oxygen ([O/Fe] ˜ -0.2) and of nitrogen ([N/Fe] ˜ -1.0), and the rather low metallicity (Z˜ 0.005 instead of the standard Z˜ 0.008) are among the most striking features. Evolutionary tracks are calculated with a new updated version of the Padova stellar evolution code, from the pre-main sequence phase up to the beginning of the TP-AGB phase, which is then decribed by means of our synthetic model. The changes in the surface abudances (due to the third dredge-up and hot-bottom burning) are consistenly coupled to variations in the low-temperature opacities that are computed, for the first time, on-the-fly with the AESOPUS code (Marigo & Aringer 2009). Here I decribe the new sets of stellar models, with particular emphasis on the evolution of S-type and C-type stars, as well of more massive AGB stars experiencing hot-bottom burning.