SUPERNOVAE - WHERE AND WHY DO THEY BREAK OFF

The precise mechanism whereby gravitational collapse leads to a type II supernova event is one of the most controversial points in the understanding of the final stages of stellar evolution. The 'bounce-shock' mechanism and the proposed 'long-term neutrino mediated' processes compete to explain the ejection of the outer envelopes of the star. Despite their differences, both descriptions consider the shock as mainly responsible for the ejection of the outer mantle in a supernova burst. In this note we discuss results from collapse calculations in which the shock is considered as an interface separating the quasi-static stiff core from the outer mantle. In the models considered the shock fades out and becomes a new boundary surface delimiting the compact homogeneous remnant. The resulting pictures become intelligible in the light of a recently proposed generalization of the concept of adiabatic index for systems where radiation flux is present.