Two-dimensional hydrodynamics of pre-core collapse: Oxygen shell burning

By direct hydrodynamic simulation, using the piecewise parabolic method code PROMETHEUS, we study the properties of a convective oxygen-burning shell in a SN 1987A progenitor star (20 M_⊙) prior to collapse. The convection is too heterogeneous and dynamic to be well approximated by one-dimensional diffusion-like algorithms that have previously been used for this epoch. Qualitatively new phenomena are seen. The simulations are two-dimensional, with good resolution in radius and angle, and used a large (90°) slice centered at the equator. The microphysics and the initial model were carefully treated. Many of the qualitative features of previous multidimensional simulations of convection are seen, including large kinetic and acoustic energy fluxes, which are not accounted for by mixing length theory. Small but significant amounts of ¹²C are mixed nonuniformly into the oxygen-burning convection zone, resulting in hot spots of nuclear energy production that are more than an order of magnitude more energetic than the oxygen flame itself. Density perturbations (up to 8%) occur at the "edges" of the convective zone and are the result of gravity waves generated by interaction of penetrating flows into the stable region. Perturbations of temperature and Y_e (or neutron excess η) at the base of the convective zone are of sufficient magnitude to create angular inhomogeneities in explosive nucleosynthesis products and need to be included in quantitative estimates of yields. Combined with the plumelike velocity structure arising from convection, the perturbations will contribute to the mixing of ⁵⁶Ni throughout supernovae envelopes. Runs of different resolution and angular extent were performed to test the robustness of these simulations.