Stellar winds

This thesis considers the neutrino-driven wind that arises from a proto-neutron star following a supernova explosion as a possible site for the synthesis of the heavy elements by the r-process. We first review the conditions necessary to obtain an r-process, the constraints on the r-process yields for each event, and show the neutrino-driven winds from proto-neutron stars are suitable for an r-process. We next discuss some of the modifications of the supernova code that were necessary in order to numerically simulate the neutrino-driven winds and steps necessary to initiate these conditions. Three important parameters of the wind characterizing the nucleosynthesis are the net electron fraction, the entropy per baryon, and the expansion time-scale. We derive approximate analytic expressions for the neutrino luminosities and mean energies, and the final entropy and net electron fraction of the wind, and compare those against a numerical simulation. We finally present the results of a numerical simulation of the first several seconds of the wind phase, and conclude with an assessment of whether or not an r-process will occur at this time.

This work is a simulation of the Accretion-Induced Collapse of a 1.37 solar mass white dwarf into a neutron star and the subsequent generation of a neutrino-driven wind, with an examination as to whether the event is a candidate for r-process nucleosynthesis. The simulation utilizes a new radiation hydrodynamic code, RadHyd, to model the AIC event. We examine the process of Accretion-Induced Collapse utilizing two sets of neutrino-scattering and absorption rates: The first, and simpler of the two has been in use since they were first introduced in 1985. The second includes a more accurate implementation of neutrino-nucleon scattering and nucleon bremsstrahlung. The improved nue - nue-nucleon scattering rate now permits energy to be exchanged between neutrinos and matter by this process, and is therefore important for the numu's and nutau's, as their only channels for exchanging energy in the standard rates was by the relatively weak NES and pair processes. Neutrino-nucleon bremmsstrahlung is also important for numu's and nutau's as this opens another channel (beside pair process) for their production. Both simulations show a neutrino-driven wind being generated after core bounce and shock propagation. We examine the conditions in these winds to ascertain whether the requisite conditions are attained for an r-process. In neither case are these achieved during the time of the simulations (i.e. 2 seconds). However, these simulations need to be carried out at least an order of magnitude longer before firm conclusions can be drawn about the applicability of this site for the r-process.