The Role of Neutrinos in Explosive Nucleosynthesis in Core Collapse Supernova Models with Neutrino Transport

Abstract

The problem of core collapse supernova explosions is long standing and attempts to understand the mechanism have been ongoing. On one hand, a full understanding of the underlying mechanism is still pending. On the other hand, there is a need to provide correct nucleosynthesis abundances for the progressing fields of galactic evolution and observations of low-metallicity stars. Traditionally, nucleosynthesis predictions rely on artificially induced explosions which is justifiable for the outer stellar layers but does not account for the effects in the innermost ejecta directly related to the explosion mechanism. The composition of the innermost ejecta is directly linked to the electron fraction Ye = Z/A . This dissertation contains the first investigation of explosive core collapse nucleosynthesis which consistently includes all weak interactions responsible for changes in Ye (neutrino/antineutrino captures on free nucleons and on nuclei, electron/positron captures, and β − /β + -decays). A second novelty of the nucleosynthesis calculations in this thesis is that they are based on core collapse models where the mass cut emerges consistently from the simulation. This is of importance for predicting the amount of Fe-group elements ejected (this is a free parameter in explosions induced by means of a thermal bomb or piston and has to be constrained from observations). Two different approaches are used to achieve explosions (in otherwise non-explosive models): We apply parametrized variations to the neutrino absorption cross sections in order to mimic in one dimension the possible increase of neutrino luminosities caused by uncertainties in proto-neutron star convection in a multi-D scenario. Alternatively, we apply parametrized variations to the neutrino absorption cross section on nucleons in the gain region to mimic the increased neutrino energy deposition which convective turnover of matter in the gain region is expected to provide. We find that both measures lead to explosions and that Ye > 0.5 in the innermost ejected layers (i.e. a proton-rich environment). The nucleosynthesis calculations show that • The proton-rich environment results in enhanced abundances of 45 Sc, 49 Ti, and by chemical evolution studies and observations of low-metallicity stars. • Antineutrino absorption reactions in the proton-rich environment produce neutrons which are immediately captured by neutron-deficient nuclei. • A new nucleosynthesis process (νp-process) takes places in supernovae (and possibly gamma-ray bursts) allowing for appreciable synthesis of elements with mass numbers A > 64. • The νp-process is a candidate for explaining the large Sr abundance seen in a hyper-metal poor star, for the suggested lighter element primary process, and possibly for the origin of the solar abundances of the light p-nuclei.