Evolution of Very Low Mass Stars and Very Massive Stars in PARSEC

During my Ph.D. study, I have been concentrating on the evolutionary tracks and atmosphere models of very low mass stars (VLMSs; ∼ 0.1 − 0.6 Msun) and very massive stars (VMSs; ∼ 12−350 Msun), based on the PAdova-TRieste Stellar Evolution Code (PARSEC). For the very low mass models, it appeared that the previous models computed with PARSEC were unable to correctly predict some basic observational relations. These relations include the mass–radius relation and the colour-magnitude relations of cool dwarfs. We replace the approximate boundary conditions used in PARSEC with those provided by more realistic atmosphere modelling. We implement the T – tau relations from Phoenix/BT-Settl model atmospheres as the outer boundary conditions in the PARSEC code, finding that this change alone reduces the discrepancy in the mass–radius relation from 8 to 5 per cent. Furthermore, we convert the theoretical quantities to the magnitudes and colors with the stellar spectral libraries from Phoenix/BT-Settl. We compare the models with multi–band photometries of clusters Praesepe, M 67, 47 Tuc and NGC 6397, showing that the use of T – tau relations clearly improves the description of the optical colours and magnitudes. However, using both Kurucz and Phoenix model spectra, the models are still systematically fainter and bluer than the observations. We then apply a shift to the above T – tau relations, increasing from 0 at Teff = 4730 K to ∼14 per cent at Teff = 3160 K, to reproduce the observed mass–radius relation of dwarf stars. Taking this experiment as a calibration of the T – tau relations, we can reproduce the optical and near-infrared CMDs of low mass stars in the old metal–poor globular clusters NGC 6397 and 47 Tuc, and in the intermediate–age and young Solar–metallicity open clusters M 67 and Praesepe. Thus, we extend PARSEC models using this calibration, providing VLMS models that are more suitable for the lower main sequence stars over a wide range of metallicities and wavelengths. For the very massive stars, the Padova models were computed more than 20 years ago and were not distributed to the community because suitable bolometric corrections for these models were not yet implemented. In this project, we complement the PARSEC data base with the stellar evolutionary tracks of massive stars, from the pre-main sequence phase to the central Carbon ignition. We consider a broad range of metallicities, 0.0001≤ Z ≤ 0.04 and initial masses up to Mini =350 Msun. The main difference with respect to our previous models of massive stars is the adoption of a recent formalism accounting for the mass-loss enhancement when the ratio of the stellar luminosity to the Eddington luminosity, Γe , approaches unity. With this new formalism, the models are able to reproduce the Humphreys-Davidson limit observed in the Galactic and Large Magellanic Cloud colour-magnitude diagrams, without an ad hoc mass-loss enhancement. We also follow the predictions of recent wind models indicating that the metallicity dependence of the mass-loss rates becomes shallower when Γ e approaches unity. We thus find that massive stars may suffer from substantial mass-loss even at low metallicity. We also predict that the Humphreys-Davidson limit should become brighter at decreasing metallicity. We supplement the evolutionary tracks with new theoretical bolometric correction tables, useful for comparing tracks and isochrones with the observations. For this purpose, we homogenize existing stellar atmosphere libraries of hot and cool stars (PoWR, ATLAS9 and Phoenix) and add, where needed, new atmosphere models computed with WM-basic. The model grids are fully adequate in mass, age and metallicity for performing detailed investigations of the properties of very young stellar systems in both local and distant galaxies. The new tracks supersede the previous old Padova models of massive stars. Therefore, my work together with the already updated PARSEC models of the other masses could depict the full evolution of stars across the mass range from ∼ 0.08 Msun to 350 Msun and over a wide range of metallicity from super-Solar (Z = 0.04) to extreme metal-poor (Z = 0.0001). Consequently, they would provide paramount information for studies not only on stars or star clusters but also on galaxy formation and evolution