Accretion on compact stars: hypercritical accretion in the Induced Gravitational Collapse and the post-merger evolution of white dwarfs mergers.

The Induced Gravitational Collapse (IGC) paradigm points to a binary origin for the long-duration gamma-ray burst (GRBs) associated with supernovae (SN). In this one, a carbon-oxygen core (COcore) explodes in a Type Ib/c SN in presence of a close neutron star (NS) companion. The SN triggers an hypercritical accretion into the NS and depending on the initial binary parameters, two outcomes are possible. In a first scenario, also referred as binary-driven hypernova (BdHNe), the binary is enough tight, so the accretion rate onto NS can be enough high to lead the NS reaches its critical mass, and collapses to a black hole (BH), emitting a GRB. A second scenario can happen for binary systems with larger binary separations, then the hypercritical accretion onto the NS is not sufficient to induce its gravitational collapse, giving place to a x-ray flash (XRF). The first part of this thesis focus on the hypercritical accretion process in the IGC paradigm. We constructed an analytical framework based on the Bondi-Hoyle accretion formalism, in order to identify the separatrix of systems in which a BH is formed and the ones where there is no BH formation and characterize the observational signatures of the BdHNe and the XRF systems. We show that the material entering into the Bondi-Hoyle region possesses sufficient angular momentum to circularize around the NS, forming a disk-like structure. We estimate the maximum orbital period, as a function of the NS initial mass, up to which the NS companion can reach by hypercritical accretion the critical mass for gravitational collapse leading to BH formation. We also studied the hydrodynamics within the accretion flow. We find that the temperature developed near the NS surface is around 1-10 MeV, hence electron–positron annihilation into neutrinos becomes the main cooling channel. Additionally, 3D numerical simulations of the IGC paradigm are performed with the smoothed particle hydrodynamics (SPH) technique. The fate of the binary system is explored for a wide parameter space including different COcore masses, orbital periods and SN explosion geometry and energies. We determine whether the star gravitational collapse is possible and assess if the binary holds gravitationally bound or it becomes unbound by the SN explosion. The second part of this thesis is about the evolution of postmergers remnants of white dwarfs binary systems. The simulations of coalescence between white dwarfs have shown that the final result consists of a central remnant made of the undisturbed primary star. The secondary star is totally disrupted and about half of the material is accreted by the primary, forming a hot corona surrounding it, and the rest of the material forms a rapidly rotating Keplerian disk, since little mass is ejected from the system during the coalescence process. In this thesis the evolution of metastable, magnetized super-Chandrasekhar white dwarfs formed in the aftermath of the merger of close binary systems has been modeled taking into account the magnetic torques acting on the star, accretion from the Keplerian disk, the threading of the magnetic field lines through the disk, as well as the thermal evolution of the white dwarf core. We explore the binary parameters that lead the white dwarf central remnant to evolve toward the gravitational collapse forming a neutron star or toward explosive carbon ignition leading to a type Ia supernova.