Dineva, Tamara Simeoneova

We use a semianalytical approach to derive criteria for the presence of doubly diffusive instabilities in postcollapsed stellar matter believed to be essential for producing a supernova explosion. Critical stability equations are obtained from both Boltzmann Equation Moment formalism and Single Particle Eigenvalue approach. Computer experiments are performed to numerically evaluate the key equilibration timescales contained in these equations. Contrary to the widely accepted view, we find that the core, if unstable, is unstable to semiconvection, rather than to neutron fingers. We also find for a given density and entropy there is a critical value for the lepton fraction Y1, below which the stellar core is completely stable to doubly diffusive instabilities of either kind. A considerable fraction of the core proves to lie below the critical Y1, immediately following shock propagation. As the core evolves this fraction quickly encompasses the entire core. We conclude that doubly diffusive instabilities of any kind are unlikely to play a role in the supernova explosion mechanism.

The models of exploding stars--supernovae--do not explode. This dissertation investigates the transfer of energy from the interior to the outer layers in such stars to try to understand what is missing in these models that would solve the supernova problem. Hydrodynamic instabilities and aspects in the microphysics of the neutrino transport in postcollapsed stellar matter are considered. In Chapter II we derive criteria for the presence of doubly diffusive instabilities believed to be essential for producing a supernova explosion. Contrary to the widely accepted view, we find that the core, if unstable, is unstable to semiconvection, rather than to neutron fingers. A critical value for the lepton fraction, Y1, is found for a given density and entropy, below which the stellar core is completely stable to instabilities. A considerable fraction of the stellar core is found to lie below the critical Y1. As the core evolves this fraction quickly encompasses the entire core. Thus doubly diffusive instabilities of any kind are unlikely to play a role in the supernova explosion mechanism. A strong magnetic field may modify the neutrino-nucleon absorption rates which are critical for shock reheating. In Chapter III we derive the cross section of neutrino absorption on neutrons in the presence of a strong magnetic field. We calculate values for the neutrino inverse mean free path and numerically compare them to the values in the non magnetic case. We find that they exhibit an oscillatory behavior, with huge peaks present due to discontinuities in the density of state. We conclude that the presence of a strong magnetic field does not yield a dramatic reduction in the inverse mean free paths which would be necessary to substantially increase the neutrino luminosity and revive the shock. Neutrino-neutrino scattering in the vicinity of the neutrino sphere may modify the neutrino luminosities and therefore affect shock reheating. In the last Chapter we calculate the neutrino-neutrino scattering cross sections, incorporating them into the source term of the Boltzmann equation for subsequent numerical computation. Inclusion of these scattering rates in transport codes will increase the accuracy of neutrino transport calculations.