Neutrino trapping during gravitational collapse of stars

Abstract

Continuing the stellar evolutionary sequence for a massive star (M a = 8 M©), the stages from hydrostatic contraction through hydrodynamic instability and neutronization, up to core bounce, are calculated. Neutronization is treated with some care, being described by a reaction network method self-consistently with hydrodynamics and neutrino transport. Weinberg-Salam theory of the weak interaction is used, with sin 2 d w = 0.35. Neutrino transport is treated by a flux-limited, multigroup nonequilibrium diffusion method, with electron-neutrino scattering being allowed to change neutrino energy in the fluid frame by large or small amounts. The physics of the problem is discussed, with emphasis placed on which processes are dominant, which formulations are applicable, and which uncertainties are crucial. As an aid to clarifying the problem, some detailed numerical results are tabulated. With this assumed weak interaction the neutrino opacities are too large to allow much transport by neutrinos. A large degree of neutrino trapping occurs; about half of the original leptons still remain with the fluid at the time the core bounce occurs. The conditions at core bounce are near the boundary between values which give mass ejection by a reflected shock and those which do not. It appears that neutron stars begin as hot, lepton-rich quasi-static objects, evolving by neutrino diffusion: neutrino stars.