Neutron star accretion and the neutrino fireball

Abstract

We suggest a mechanism for driving supernova explosions through neutrino energy deposition beyond the formation and the cooling of the neutron star. The mechanism depends upon convection next to the neutron star and terminates when the entropy of buoyant bubbles increases sufficiently due to neutrino heating to shut off the down flow of low entropy matter. This convection is initially caused by the heat deposited from neutrinos emitted by the cooling neutron star in a region gravitationally dominated by the neutron star. This convection by providing an efficient heat transport mechanism avoids the overheating and excessive neutrino energy losses in the regions close to the neutron stars and has been recently modeled by Herant, Benz and Colgate. In this model the explosion shock, initially driven by the bounce followed by neutron star derived neutrino heating is further driven by the up flow of the bouyant, high entropy bubbles. The down flow between the bubbles of low entropy matter originating from behind the bounce and later the explosion shock will build up a modest entropy atmosphere in pressure equilibrium with the surface of the cooling neutron star. For modest entropies, S rad ≈ 10 or S total ≈ 18 and condensed neutron stars, R/ M ≈ 10 km per solar mass, the total mass of such an atmosphere is small, 1.7 × 10 −3 M ⊙, but the temperature at the base of such an atmosphere is extremely high, T ≈ 10 MeV. The paradoxical result of neutrino emission is to further condense the atmosphere and increase the temperature until the compression heating is exceeded by the neutrino emission. The absolute limit of compression heating is free fall collapse, which may be approached but not exceeded. Such an accretion event approaching this limit may reach a temperature high enough to create a neutrino “fireball”, a region hot enough, ≈ 11 MeV, so as to be partially opaque to its own (neutrino) radiation. The further heating of the already high entropy bubbles by the neutrino deposition resulting from less extreme, but a continuing high temperature atmosphere, will augment the explosion shock. This convection-driven accretion should continue until the low entropy matter fllowing downwards onto the neutron star is mixed to a high enough entropy by entrainment with the rising high entropy bubbles to prevent further accretion. This process, by providing for a feedback mechanism, may result in a robust supernova explosion in the sense that prior processes resulting in a failed or weak explosion will be augmented with further accretion energy until an explosion of the appropriate energy occurs.