Non-spherical core collapse supernovae

II. The late-time evolution of globally anisotropic neutrino-driven explosions and their implications for SN 1987 A

Abstract
Two-dimensional simulations of strongly anisotropic supernova explosions of a nonrotating blue supergiant progenitor are presented, which follow the hydrodynamic evolution from times shortly after shock formation until hours later. It is shown that explosions which around the time of shock revival are dominated by low-order unstable modes (i.e. by a superposition of the l=2 and l=1 modes, in which the former is strongest), are consistent with all major observational features of SN 1987 A, in contrast to models which show high-order mode perturbations only and were published in earlier work. Among other items, the low-mode models exhibit final iron-group velocities of up to km s^-1, strong mixing at the He/H composition interface, with hydrogen being mixed downward in velocity space to only 500 km s^-1, and a final prolate anisotropy of the inner ejecta with a major to minor axis ratio of about 1.6. The success of low-mode explosions with an energy of about  erg to reproduce these observed features is based on two effects: the (by 40%) larger initial maximum velocities of metal-rich clumps compared to our high-mode models, and the initial global deformation of the shock. The first effect protects the (fastest) clumps from interacting with the strong reverse shock that forms below the He/H composition interface, by keeping their propagation timescale through the He-core shorter than the reverse shock formation time. This ensures that the outward motion of the clumps remains always subsonic, and that thus their energy dissipation is minimal (in contrast to the supersonic case). The second effect is responsible for the strong inward mixing of hydrogen: the aspherical shock deposits large amounts of vorticity into the He/H interface layer at early times (around t = 100 s). This triggers the growth of a strong Richtmyer-Meshkov instability that results in a global anisotropy of the inner ejecta at late times (i.e. around s), although the shock itself has long become spherical by then. The simulations suggest a coherent picture, which explains the observational data of SN 1987 A within the framework of the neutrino-driven explosion mechanism using a minimal set of assumptions. It is therefore argued that other paradigms, which are based on (more) controversial physics, may not be required to explain this event.

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