Spherically Symmetric Simulation with Boltzmann Neutrino Transport of Core Collapse and Post-Bounce Evolution of a 15 Solar Mass Star

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Abstract
We present results of a spherically symmetric, Newtonian simulation of the core-collapse and post-bounce evolution of a 15 Msun star with a 1.28 Msun iron core. The time-, energy-, and angle-dependent transport of electron neutrinos and antineutrinos was treated with a new code which iteratively solves the Boltzmann equation and the equations for neutrino number, energy and momentum, including the effects of the motion of the stellar medium to order O(v/c) in the fluid velocity v. The supernova shock expands to a maximum radius of 350 km instead of only ~240 km as in a comparable calculation with multi-group flux-limited diffusion (MGFLD) by Bruenn, Mezzacappa, & Dineva (1995). We explain this by stronger neutrino heating due to the more accurate treatment of the transport in our model. Nevertheless, shock expansion breaks down at 180 ms after bounce, and the shock finally recedes to a radius around 250 km (compared to ~170 km in the MGFLD run). The effect of an accurate neutrino transport is helpful, as expected, but not large enough to cause an explosion of the considered 15 Msun star in spherical symmetry. Therefore postshock convection and/or an enhancement of the core neutrino luminosity by convection or reduced neutrino opacities in the neutron star seem necessary for neutrino-driven delayed explosions of such stars. We find that the electron fraction in the neutrino-heating region is larger than 0.5, which suggests that the overproduction problem of neutron-rich nuclei with mass numbers around A = 90 in the neutrino-heated ejecta is absent when electron neutrino and antineutrino spectra and luminosities are calculated with a Boltzmann solver.

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