INSIDE THE SUPERNOVA - A POWERFUL CONVECTIVE ENGINE

We present an extensive study of the inception of supernova explosions by following the evolution of the cores of two massive stars (15 and 25 M.) in multidimension. Our calculations begin at the onset of core collapse and stop several hundred milliseconds after the bounce, at wh ich time successful explosions of the appropriate magnitude have been obtained. Similar to the classical delayed explosion mechanism of Wils on, the explosion is powered by the heating of the envelope due to neu trinos emitted by the protoneutron star as it radiates the gravitation al energy liberated by the collapse. However, as was shown by Herant, Bent, and Colgate, this heating generates strong convection outside th e neutrinosphere, which we demonstrate to be critical to the explosion . By breaking a purely stratified hydrostatic equilibrium, convection moves the nascent supernova away from a delicate radiative equilibrium between neutrino emission and absorption. Thus, unlike what has been observed in one-dimensional calculations, explosions are rendered quit e insensitive to the details of the physical input parameters such as neutrino cross sections or nuclear equation of state parameters. As a confirmation, our comparative one-dimensional calculations with identi cal microphysics, but in which convection cannot occur, lead to dramat ic failures. Guided by our numerical results, we have developed a para digm for the supernova explosion mechanism. We view a supernova as an open cycle thermodynamic engine in which a reservoir of low-entropy ma tter (the envelope) is thermally coupled and physically connected to a hot bath (the protoneutron star) by a neutrino flux, and by hydrodyna mic instabilities. Neutrino heating raises the entropy of matter in th e vicinity of the protoneutron star until buoyancy carries it to low-d ensity, low-temperature regions at larger radii. This matter is replac ed by low-entropy downflows with negative buoyancy. In essence, a Carn ot cycle is established in which convection allows out-of-equilibrium heat transfer mediated by neutrinos to drive low-entropy matter to hig her entropy and therefore extracts mechanical energy from the heat gen erated by gravitational collapse. We argue that supernova explosions a re nearly guaranteed and self-regulated by the high efficiency of the thermodynamical engine. The mechanical efficiency is high because mixi ng during the heat exchange is limited by the rapid rise and shape-pre serving expansion of the bubbles in a rho proportional to r(-3) atmosp here. In addition, the ideal Carnot efficiency is high due to the larg e temperature contrast between the surface of the protoneutron star an d the material being convected down from large radii (this contrast re mains large in spite of compression and shock heating which is relativ ely small). By direct P dV integration over the convective cycle, we e stimate the energy deposition to be similar to 4 foes per M. involved. Further, convection, by keeping the temperature low in rising neutrin o-heated high-entropy bubbles, allows the storage of internal energy w hile minimizing the losses due to neutrino emission. Thus convection c ontinues to accumulate energy exterior to the neutron star until a suc cessful explosion has occurred. At this time, the envelope is expelled and therefore uncoupled from the heat source (the neutron star) and t he energy input ceases. This paradigm does not invoke new or modified physics over previous treatments, but relies on compellingly straightf orward thermodynamic arguments. It provides a robust and self-regulate d explosion mechanism to power supernovae that is effective under a wi de range of physical parameters.