THREE-DIMENSIONAL SIMULATIONS OF MIXING INSTABILITIES IN SUPERNOVA EXPLOSIONS

We present the first three-dimensional (3D) simulations of the large-scale mixing that takes place in the shockheated stellar layers ejected in the explosion of a 15.5 M-circle dot blue supergiant star. The blast is initiated and powered by neutrino-energy deposition behind the stalled shock by means of choosing sufficiently high neutrino luminosities from the contracting, nascent neutron star, whose high-density core is excised and replaced by a retreating inner grid boundary. The outgoing supernova shock is followed beyond its breakout from the stellar surface more than 2 hr after the core collapse. Violent convective overturn in the post-shock layer causes the explosion to start with significant large-scale asphericity, which acts as a trigger of the growth of Rayleigh-Taylor instabilities at the composition interfaces of the exploding star. Despite the absence of a strong Richtmyer-Meshkov instability at the H/He interface, which only a largely deformed shock could instigate, deep inward mixing of hydrogen is found as well as fast-moving, metal-rich clumps penetrating with high velocities far into the hydrogen envelope of the star as observed, for example, in the case of Supernova 1987A. Also individual clumps containing a sizeable fraction of the ejected iron-group elements (up to several 10(-3) M-circle dot) are obtained in some models. The metal core of the progenitor is partially turned over with nickel-dominated fingers overtaking oxygen-rich bullets and both nickel and oxygen moving well ahead of the material from the carbon layer. Comparing with corresponding two-dimensional (axially symmetric; 2D) calculations, we determine the growth of the Rayleigh-Taylor fingers to be faster, the deceleration of the dense metal-carrying clumps in the helium and hydrogen layers to be reduced, the asymptotic clump velocities in the hydrogen shell to be higher (up to similar to 4500 km s(-1) for the considered progenitor and an explosion energy of 10(51) erg, instead of less than or similar to 2000 km s(-1) in 2D), and the outward radial mixing of heavy elements and inward mixing of hydrogen to be more efficient in 3D than in 2D. We present a simple argument that explains these results as a consequence of the different action of drag forces on moving objects in the two geometries.