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Open Access Publications from the University of California

Signatures of the Late Time Core-Collapse Supernova Environment

  • Author(s): Roberts, Luke Forrest
  • Advisor(s): Woosley, Stan E
  • et al.

The hot and dense proto-neutron star (PNS) born subsequent to core-collapse in a type II supernova explosion is an intense source of neutrinos of all flavors. It emits the 3-5 x 10^53 ergs of gravitational binding energy gained during collapse as neutrino radiation on a time scale of tens of seconds as it contracts, becomes increasingly neutron-rich and cools. While the supernova explosion mechanism and associated accretion of material is expected to influence the neutrino emission at early time (i.e. t <~ 1 s post bounce) the late time neutrino signal is shaped by the properties of the PNS, such as the nuclear equation of state (EoS), neutrino opacities in dense matter, and other microphysical properties that affect the cooling timescale by influencing either neutrino diffusion or convection. Detection of significant numbers of late time supernova neutrinos will provide a direct window into the properties of nuclear matter and neutron stars, if the neutrino signal can be modeled accurately. The average emitted neutrino energies also strongly affect nucleosynthesis in the neutrino driven wind, neutrino induced nucleosynthesis further out in the star, and the patterns of neutrino oscillations outside of the PNS.

This thesis examines a number of aspects of this environment. First, the equations of spherically symmetric general relativistic radiation hydrodynamics are discussed, a new code for calculating neutrino transport in PNSs is described, and first results from this code are presented. It is found that the NDW is neutron rich for at least a few seconds, in contrast to other recent work. This change in the expected wind electron fraction is traced to the correct treatment of the nucleon dispersion relations in an interacting medium and turns out to be influenced by the sub-nuclear density symmetry energy. Late time convection in PNSs is also studied. It is found that the density dependence of the symmetry energy may affect the duration of convective activity, which is imprinted in the neutrino luminosity evolution.

The second part of the thesis focuses on the neutrino driven wind (NDW) which is blown from the surface of the PNS. Time-dependent hydrodynamic calculations of the NDW are presented, which include accurate weak interaction physics coupled to a full nuclear reaction network. Using two published models of PNS neutrino luminosities,

predictions of the contribution of the NDW to the integrated nucleosynthetic yield of the entire supernova are made. For the neutrino luminosity histories considered, it is found no r-process occurs in the most basic wind scenario because the NDW entropy is too low, the dynamical timescale is too long, and the wind electron fraction is too high. It is possible that the wind produces the N = 50 closed shell isotopes, but this depends on the neutrino luminosities employed. The effect of a secondary heating source on the wind is then considered. The general characteristics of a secondary heating source required to produce r-process nucleosynthesis are discussed. Then gravitoacoustic power excited either by convection or g-mode oscillations of the PNS is considered as a possible source of this heating. It is found that this a viable mechanism for increasing the wind entropy and decreasing the the dynamical timescale to values that are favorable for the r-process, when the neutrino spectra found in the first part of the thesis are assumed.

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