Fallback and Black Hole Production in Massive Stars

The compact remnants of core-collapse supernovae--neutron stars and black holes--have properties that reflect both the structure of their stellar progenitors and the physics of the explosion. In particular, the masses of these remnants are sensitive to the density structure of the presupernova star and to the explosion energy. To a considerable extent, the final mass is determined by the "fallback," during the explosion, of matter that initially moves outward, yet ultimately fails to escape. We consider here the simulated explosion of a large number of massive stars (9-100 M) of Population I (solar metallicity) and III (zero metallicity) and find systematic differences in the remnant mass distributions. As pointed out by Chevalier, supernovae in more compact progenitor stars have stronger reverse shocks and experience more fallback. For Population III stars above about 25 M and explosion energies less than <IMG SRC="eq-00001.gif" ALT="1.5\times 10^{51}"/> 1.5 x 1051 ergs, black holes are a common outcome, with masses that increase with increasing main-sequence mass up to a maximum hole mass, for very low explosion energy, of about 40 M. If such stars produce primary nitrogen, however, their black holes are systematically smaller. For modern supernovae with nearly solar metallicity, black hole production is much less frequent and the typical masses, which depend sensitively on explosion energy, are smaller. The maximum black hole mass is about 15 M. We explore the neutron star initial mass function for both populations and, for reasonable assumptions about the initial mass cut of the explosion, find good agreement with the average of observed masses of neutron stars in binaries. We also find evidence for a bimodal distribution of neutron star masses with a spike around 1.2 M (gravitational mass) and a broader distribution peaked around 1.4 M.