Ascertaining the core collapse supernova mechanism: An emerging picture?

The mechanism for core collapse supernova explosions remains undefined in detail and perhaps even in broad brush. Past multidimensional simulations point to the important role neutrino transport, fluid instabilities, rotation, and magnetic fields play, or may play, in generating core collapse supernova explosions, but the fundamental question as to whether or not these events are powered by neutrinos with the aid of some or all of these other phenomena or by magnetic fields or by a combination of both is unanswered. Here we present the results from two sets of simulations, in two and three spatial dimensions. In two dimensions, the simulations include multifrequency flux-limited diffusion neutrino transport in the "ray-by-ray-plus" approximation, two-dimensional self gravity in the Newtonian limit, and nuclear burning through a 14-isotope alpha network. The three-dimensional simulations are model simulations constructed to reflect the post stellar core bounce conditions during neutrino shock reheating at the onset of explosion. They are hydrodynamics-only models that focus on critical aspects of the shock stability and dynamics and their impact on the supernova mechanism and explosion. The two-dimensional simulations demonstrate the important role nuclear burning may play despite the relatively small total energy deposition behind the shock. The three-dimensional simulations demonstrate the need for three-dimensional multi-physics core collapse supernova models. In two dimensions, with the inclusion of nuclear burning, we obtain explosions (although in one case weak) for two progenitors (11 and 15 M _⊙ models). Moreover, in both cases the explosion is initiated when the inner edge of the oxygen layer accretes through the shock. Thus, the shock is not revived while in the iron core, as previously discussed in the literature. The three-dimensional studies of the development of the stationary accretion shock instability (SASI) demonstrate the fundamentally new dynamics allowed when simulations are performed in three spatial dimensions. The predominant l = 1 SASI mode gives way to a stable m = 1 mode, which in turn has significant ramifications for the distribution of angular momentum in the region between the shock and proto-neutron star and, ultimately, for the spin of the remnant neutron star. Moreover, the three-dimensional simulations make clear, given the increased number of degrees of freedom, that two-dimensional models are severely limited by artificially imposed symmetries.