Astrophysics

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.

The starting point of any general relativistic numerical simulation is a solution
of the Hamiltonian and momentum constraints that (ideally) represents an astrophysically realistic scenario. This dissertation presents a new method to produce initial data sets for binary neutron stars with arbitrary spins and orbital eccentricities. The method only provides approximate solutions to the constraints. However, it was
shown that the corresponding constraint violations subside after a few orbits, becoming
comparable to those found in evolutions of standard conformally flat, helically
symmetric binary initial data. This dissertation presents the first spinning neutron
star binary simulations in circular orbits with a orbital eccentricity less then 0.01. The
initial data sets corresponding to binaries with spins aligned, zero and anti-aligned
with the orbital angular momentum were evolved in time. These simulations show
the orbital “hang-up” effect previously seen in binary black holes. Additionally, they
show orbital eccentricities that can be up to one order of magnitude smaller than
those found in helically symmetric initial sets evolutions.

The propagation of shock and detonation waves through non-uniform dense
stellar material was numerically followed by characteristics. A number
of computations were performed for various initial shock strengths for
cases in which burning of materials was and was not present . For cases
in which burning was included, an analytic expression for the burning of
carbon was used. From such computations it was concluded that: (1)
due to the divergent nature of spherical systems and the subsequent decay
of shocks (for a region sufficiently close to the center of the star)
it may not be possible for the shock to support a steady detonation and
(2) after reaching a minimum strength, the shock may again accelerate,
thus forming a new detonation wave.