Accretional Heating of Asymmetric Supernova Cores

Abstract

The role of accretion in heating a stalled bounce shock ina core-collapse supernova is investigated. We show that effectiveaccretional heating causes an asymmetric expansion of the shock,sufficient to impart a net impulse of ~300-400 km s -1 to theneutron core. To simplify the analysis, we consider a failed accretionshock. Below such a shock, inward advection is faster than neutrinoheating and the usual gain criterion does not suffice to determine asuccessful explosion. A mechanism that enhances buoyancy and inhibitsmixing between hot and cold postshock fluid elements is requiredto revive the shock. We focus on the response of a magnetic fieldto the accretion flow. Ram heating and shearing of a low-density,magnetized fluid phase (``M-fluid'') is shown to be faster than neutrinocooling. The long duration of the accretion flow compared with thedynamical time allows for a large amplification of the magnetic energy.We calculate the stability of a spherical shock in the presence ofa low-density hydrostatic atmosphere below it and show that belowa critical atmospheric density the shock is unstable to a globalRayleigh-Taylor mode. We then calculate the equilibrium structure ofthis Rayleigh-Taylor plume as it accumulates energy and the criticalsize beyond which quasi-static expansion is no longer possible andits outer boundary converts to a running shock. Accretion continueswhile the shock expands, and an energy of ~10 51 ergs is adirect consequence of the efficiency of ram heating close to theneutron core. The linear momentum imparted to the core is directlyrelated to the mass profile of the precollapse core and explainsthe proper motions of (most) radio pulsars. We also estimate thenet circulation imparted to the last 0.1-0.2 M solar ofcollapsing material, which appears sufficient to torque the core downto a spin period of 1-100 ms. The effect of photodissociation onthe shock jump conditions is calculated, and the implications fornucleosynthesis of iron peak elements are considered. Finally, the residualmagnetic field advected out into the eventual supernova remnant iscompared with the field generated by a rapidly spinning neutron star.