The explosion of massive stars and their compact remnant

Massive stars (M ≥ 8Ms) end their life exploding as Core Collapse Supernovae (CCSN). As a result of such a kind of explosions, a very dense and compact remnant, either a Neutron Star (NS) or a Black Hole (BH), is left. The understanding of how the remnant is formed and how its mass is linked with the properties of both the progenitor star (initial mass and initial metallicity) and the explosion (explosion energy, light curve and spectra) is fundamental in several astrophysical areas: for example, the formation of high massive remnants (1) limits the ejection of the heavy elements produced during either the hydrostatic and explosive nucleosynthesis and therefore this may have a significant impact on the chemical evolution of the galaxies as well as on the behavior of the light curve and spectra; (2) contributes significantly to the population of high mass compact objects, and (3) constitutes potential sources of gravitational waves (GWs) through BH-BH or NS-BH mergers. Unfortunately, at present, there is no self consistent hydrodynamic model for CCSN in which the explosion is obtained naturally and systematically. Even in those few cases where the explosion is successful, the results are not fully compatible with the observations (e.g., the energy of the explosion in these cases is a factor of 3 to 10 lower than that usually observed). In addition, these sophisticated 3D hydro simulations cannot predict with certainty of precision the mass of the remnant. The reason is that the fallback occurs on timescales (few hours) much longer than the typical timescales followed by the hydro calculations (few seconds). For all these reasons, at present, the systematic simulations of CCSNe are are still based on artificially induced explosions. In these calculations an arbitrary amount of energy is injected in the presupernova model (typically close to the edge of the iron core) and the shock wave that is generated in this way is followed during its propagation within the exploding mantle. In this context we substantially improved the 1D hydrodynamic code (HYPERION), extensively used for the explosive nucleosynthesis calculations, mainly with the inclusion of the radiative transport in the flux limited diffusion approximation and with a better treatment of the inner boundary conditions. By means of this new version of the code we computed the explosions, and the associated bolometric light curves, remnant masses and explosive nucleosynthesis, of a subset of red super giant presupernova models extracted from the database published by Limongi and Chieffi in 2018. In total 203 explosions have been computed, for different values of the explosion energy. In this way we were able to study the dependence of the light curve behavior (the maximum luminosity, the luminosity at 50 days, the plateau duration, the radioactive tail) and the mass of the remnant on the properties of the progenitor star (mass, metallicity) and on the explosion energy. Such a theoretical predictions constitute a fundamental reference framework for the interpretation of a number of astrophysical topical subjects among which the gravitational waves and their sources.