Fallback and Black Hole Production in Massive Stars

The compact remnants of core collapse supernovae - neutron stars and blackholes - have properties that reflect both the structure of their stellarprogenitors and the physics of the explosion. In particular, the masses ofthese remnants are sensitive to the density structure of the presupernova starand to the explosion energy. To a considerable extent, the final mass isdetermined by the ``fallback'', during the explosion, of matter that initiallymoves outwards, yet ultimately fails to escape. We consider here the simulatedexplosion of a large number of massive stars (10 to 100 \Msun) of Population I(solar metallicity) and III (zero metallicity), and find systematic differencesin the remnant mass distributions. As pointed out by Chevalier(1989),supernovae in more compact progenitor stars have stronger reverse shocks andexperience more fallback. For Population III stars above about 25 \Msun andexplosion energies less than $1.5 \times 10^{51}$ erg, black holes are a commonoutcome, with masses that increase monotonically with increasing main sequencemass up to a maximum hole mass of about 35 \Msun. If such stars produce primarynitrogen, however, their black holes are systematically smaller. For modernsupernovae with nearly solar metallicity, black hole production is much lessfrequent and the typical masses, which depend sensitively on explosion energy,are smaller. We explore the neutron star initial mass function for bothpopulations and, for reasonable assumptions about the initial mass cut of theexplosion, find good agreement with the average of observed masses of neutronstars in binaries. We also find evidence for a bimodal distribution of neutronstar masses with a spike around 1.2 \Msun (gravitational mass) and a broaderdistribution peaked around 1.4 \Msun.Comment: Accepted for publication in Ap