Supernova shock .8.

The supernova shock breaks out (i.e., increases its speed beyond 10(9) cm s(-1)) at about 300 km simply because the velocity of the infalling material decreases greatly with increasing shock radius. The decrease is not due to a decreasing rate of accretion. Recombination of nucleons into alpha-particles plays only a minor part. The energy of the shock is directly related to the neutrino luminosity at early times, and agrees well with the computation by Wilson et al. and with the observation on SN 1987A. For the calculation of nucleosynthesis it is essential to consider the shock as a sharp discontinuity. The calculated amount of Ni-56 in SN 1987A is then 3 times the observed amount, indicating substantial fallback. Convection between entropy maximum and shock stops at a shock radius of about 1000 km; this keeps the proton fraction Y-e closer to that of the infalling material.