In my PhD work I carried out a detailed investigation on the final fates and chemical ejecta produced by intermediate-mass and massive stars.
The first part of the thesis is focused on massive and very massive stars. We derive the ejecta for a large number of elemental species (H, He, C, N, O, F, Ne, Na, Mg, Al, Si, S Ar, K, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn) during the pre-supernova evolution and after the explosion or collapse event. We use a set of stellar tracks computed with PAdova and TRieste Stellar Evolution Code (PARSEC), with initial masses in the range between 8 M to 350 M , for thirteen different initial metallicities from Z = 0.0001 to Z = 0.02. Adopting suitable explodability criteria available in the recent literature, for each stellar model we derive the final fate and remnant mass, which critically depend on the initial mass and metallicity. Three main classes of explosion events are considered.
Massive stars with initial masses from 8 Msun to 100 Msun , build a degenerate iron core which eventually collapses either generating a successful explosion and a neutron star, or experiencing an inexorable infall with consequent black hole formation (failed supernovae). Very massive objects (VMOs), with initial mass ∼ 100 M , can end their life either as pulsation pair instability supernovae (PPISN), pair instability supernovae (PISN), or directly collapsing to black hole (DBH). For these objects, the fate is mainly determined by the mass of helium-core.
From our analysis we derive a general scenario on the fate of massive and very massive stars emerges. It is evident that both the pre-SN evolution and the subsequent SN channel are significantly affected by the initial metallicity, as a consequence of its impact on the efficiency of mass loss and the growth of the stellar core. In particular, we find that suitable conditions for the occurrence of PPISN and PISN events are not limited to extremely low metallicities, as invoked in early studies. Rather, such energetic events may take place already at Z > Zsun /3, hence in the local Universe, in agreement with recent findings in the literature.
Once final fates and remnant masses are known, we compute the elemental ejecta for all stars in the grid, accounting for both wind and explosion contributions. The wind ejecta are directly derived from PARSEC stellar evolution models, for all isotopes from 1 H to 28 Si and heavier elements up to Zn. The explosion ejecta are obtained from supernova nucleosynthesis calculations available in the literature, for the three classes here considered(CCSN, PISN or PPISN). Suitable parameters (masses of the CO and He cores) are adopted to link the explosion models to our PARSEC tracks. We also calculate the integrated yields ejected by a simple stellar population with a specified initial mass function in view of comparing the chemical contributions of both winds and explosions from the three classes of stars (CCSNe, PISNe and PPISNe).
As a final result of this work, we aim at releasing a large database of chemical ejecta and compact remnants produced by massive and very massive stars over a large range of initial masses and metallicites. These will be a key relevance in the framework of the galaxy chemical evolution studies.
In the second part of the thesis we investigate the chemical ejecta of intermediate-mass stars, with particular focus on the thermally-pulsing asymptotic giant branch (TP-AGB) stars that experience both the third dredge-up and hot-bottom burning. This study was performed in the context of the LUNA (Laboratory Underground Nuclear Astrophysics) collaboration. Nucleosynthesis calculations were carried out adopting the new rate for the key reaction 22 Ne(p, γ) 23 Na, which plays a major role in determining the abundance of sodium. To this aim we used the PARSEC and COLIBRI codes to compute the complete evolution, from the pre-main sequence up to the termination of the TP-AGB phase, of a set of stellar models with initial masses in the range 3.0 Msun 6.0 Msun , and metallicities Z=0.0005, Z=0.006, and Z = 0.014. We find that the new LUNA measurements have much reduced the nuclear uncertainties of the tors of 22Ne and 23Na AGB ejecta, which drop from fac-10 to only a factor of few for the lowest metallicity models. Relying on the most recent estimations for the destruction rate of 23Na, the uncertainties that still affect the 22Ne and 23Na AGB ejecta are mainly dominated by evolutionary aspects (efficiency of mass-loss, third dredge- up, convection). Finally, we discuss how the LUNA results impact on the hypothesis that invokes massive AGB stars as the main agents of the observed O-Na anti-correlation in Galactic globular clusters. We derive quantitative indications on the efficiencies of key physical processes (mass loss, third dredge-up, sodium destruction) in order to simultaneously reproduce both the Na-rich, O-poor extreme of the anti-correlation, and the observational constraints on the CNO abundance.
While best-fitting AGB models can be singled out, the AGB hypothesis still needs to be validated, as various issues still remain
