Neutrino transport in core collapse supernovae

Core collapse supernovae occupy a special place in the cosmic hierarchy for their role in disseminating and producing most elements in the Universe heavier than hydrogen and helium, without which life as we know it would not be possible. These catastrophic events mark the end of a massive star's life and the birth of a neutron star or a black hole, and are the most energetic explosions in the Cosmos. Core collapse supernovae result when the iron core of a massive star becomes unstable at the end of the star's evolution, collapses on itself, rebounds at ultra-high densities, and produces a shock wave that will ultimately be responsible for disrupting the star. As infalling core material passes through the shock, it is compressed and heated, and the core nuclei are dissociated (broken up) at the expense of thermal, pressure-producing energy behind the shock, thereby weakening it. In addition to this energy loss, energy is carried away from the shocked region by massless particles know as "neutrinos". The shock stalls, and is later thought to be revived by a "neutrino heating" mechanism. At the time the shock stalls, the core consists of an inner "neutrinosphere" radiating neutrinos and "antineutrinos" of three "flavors": "electron", "muon", and "tau" neutrinos and their antineutrinos. This inner core will ultimately radiate away its thermal energy, cool, and go on to form a neutron star or a black hole. Revival of the stalled shock above the neutrinosphere is mediated by the absorption of electron neutrinos and antineutrinos emerging from the radiating proto-neutron star. This heating depends sensitively on the neutrino luminosities, spectra, and distribution of neutrino direction cosines in the region behind the shock. In turn, this depends on the neutrino transport through three regions: the "neutrino-thick", diffusion region deep within the core below the neutrinosphere, the "semitransparent" region encompassing the neutrinosphere, and the "neutrino-thin", streaming region at larger radii. Sufficient accuracy for a definitive simulation of the supernova outcome can be obtained only via a solution of the neutrino Boltzmann transport equations and their coupling to the hydrodynamics equations governing the evolution of the core material. In this article we present a numerical method to solve the neutrino Boltzmann equations coupled to the core hydrodynamics. Spherical symmetry is assumed, but our methods extend to multidimensional Boltzmann transport simulations. (With the assumption of spherical symmetry, radius is the only spatial variable, rendering the simulation, at least as far as the spatial dimensions are concerned, one-dimensional. However, as we will discuss, the Boltzmann equation is a "phase space" equation in radius and neutrino direction cosine and energy, and therefore, inherently multidimensional even when spherical symmetry is assumed.) We also present the results of comparisons of "multigroup flux-limited diffusion" (approximate) neutrino transport and Boltzmann (exact) neutrino transport in post-core bounce supernova environments, with an eye toward the quantities central to the neutrino-heating, shock-revival mechanism. Multigroup flux-limited diffusion is the most sophisticated transport approximation implemented thus far in core collapse supernova simulations, and will be described in some detail in this article. Our results demonstrate that differences significant to shock revival are obtained with the two transport schemes, supporting the claim that accurate neutrino transport is paramount in simulations of these important cosmic events. We discuss the ramifications our results have for our ongoing simulations of core collapse supernovae with exact Boltzmann neutrino transport.