Neutrino astrophysics

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.

This work is a simulation of a 15 solar mass star from initial core collapse through the formation of the neutrino-driven wind with an eye towards a possible site for the r-process. We use a new radiation hydrodynamics code known as RadHyd, which is a merger of EVH-1 and MGFLD. The EVH-1 code is a piecewise parabolic method, or PPM, Godunov solver with Newtonian gravitation and MGFLD is a sophisticated multi-group, flux-limited diffusion radiation transport code. Core collapse is initiated from a progenitor model S15s7b of Weaver and Woosley. A maximum radius of 2.0 x 10^11 cm is chosen to retain the shock on the grid for the desired duration of the simulation. The stagnated shock is revived after 300 ms by increasing the internal energy between the shock and the gain radius over a period of 50 ms. Boundary conditions provided by the outward moving shock yield a subsonic wind. The progenitor model is evolved with two different explosion energies (1.5 and 2.5 foes) and two sets of neutrino reaction rates. The first set is the standard set from Bruenn [5], whereas the second contains neutrino-nucleon elastic scattering with nucleon recoil and blocking and nucleon bremsstrahlung. In the analysis, we compare the structure differences caused by the different neutrino rates in the time from bounce to 300 ms after bounce, when the energy addition will take place. Finally, we discuss the further evolution of the model and characterize properties of the neutrino-driven wind for the two sets of rates and explosion energies.