Neutrinos

Core-Collapse Supernovae (CCSNe) are some of the most powerful events in the universe liberating an astonishing 3×1053 ergs of the gravitational binding energy released by the collapse of the stellar core to a nascent neutron star (PNS) that is formed in these events. The visible display is capable of outshining the entire galaxy where it inhabits. Most of this energy, ~ 99%, is carried away by neutrinos of all flavors, however.
According to the favored theory of CCSNe, the production and transport of neutrinos from the dense core through the less dense mantle is believed to deposit energy in the mantle and thereby initiate the supernova explosion. Numerically modeling these events realistically to validate the model therefore requires an accurate neutrino transport algorithm coupled to sophisticated neutrino microphysics to compute the emission, transport, and energy deposition of neutrinos.
The CHIMERA code is a radiation-hydrodynamics code that has been developed to numerically model CCSNe in multiple spatial dimensions. The neutrino transport algorithm currently incorporated in CHIMERA is based upon the Multigroup Flux-Limited Diffusion (MGFLD) method. This current method basically uses only the zeroth angular moment of the Boltzmann equation and closes the system with terms dropped from the first angular moment to produce a diffusion-like equation. A flux-limiter is added to interpolate between the diffusive and free-streaming regimes, and to prevent the algorithm from computing acausal, i.e., faster than light transport, in regions where the neutrino mean free paths are large.

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.

We examine the effects of general relativity on neutrino transport during the critical shock reheating phase of core collapse supernovae. We derive the equations for general relativistic, static, multi-group, flux-limited diffusion (MGFLD), and use these to modify a Newtonian MGFLD code. The steady-state neutrino distributions calculated by the revised code for several post-collapse stellar cores are compared to those computed with the original Newtonian MGFLD code. The general relativistic transport calculations display the expected reductions in neutrino rms energy and luminosity arising from redshift and curvature effects. Although the effects of general relativity on neutrino transport work against the shock revival, firm conclusions cannot be drawn at this time because of other important general relativistic effects on the structure of the core. Nevertheless, it is clear that modeling with Newtonian transport has little connection to reality.