Evolution and nucleosynthesis of asymptotic giant branch stars and accreting white dwarfs

Abstract

Type Ia supernovae (SNIa) are luminous stellar explosions which mark the fatal disruption of white dwarfs in a binary system. They are the major producers of Iron group elements in the solar system and also give relevant contribution to the alpha-elements Silicon, Solfur, Calcium and Titanium. Within specific conditions SNIa may also produce about 30 proton-rich isotopes heavier than iron. It is controversial what is the relevance of this p-process component for the abundance of these isotopes in the Galaxy and in our solar system. Its efficiency depends on the products of neutron capture processes active during the accretion phase to reach the Chandrasekhar mass. The aim of this thesis is to provide for the first time comprehensive stellar simulations for investigating the possibility of producing this seeds distribution for p-process nucleosynthesis, calculating it modelling the accretion phase onto a white-dwarfs increasing mass toward the Chandrasekhar limit. The main stellar model properties during the accretion phase are not so different from the asymptotic giant branch phase, before the star becomes a WD and the accretion phase starts. We have used the same stellar code MESA (revision 4219) to produce AGB stellar models, implementing the best known physics and producing eleven one-dimensional AGB stellar models with initial mass M = 2 and 3 solar masses, and with initial metal content Z=0.01 and Z=0.02. The convective boundary-mixing below Thermal Pulses and the Third-Dredge Up is included directly in stellar calculations to take into account Kelvin-Helmholtz instability and gravity waves. Rotation and magnetic field are not included. The same parameterization adopted for AGB models was consistently used for the accretion models, calculating 4 WD models with initial mass 0.856, 1.025, 1.259 and 1.376 solar masses accreting Z=0.01 metal content material. Post-processing calculations are finally done with the Mppnp NuGrid code.