Diverse Supernova Sources for the r-Process

We present a simplified analysis using equations for the charge flow, which include νe capture, for the production of r-process nuclei in the context of the recent supernova hot bubble model. The role of νe capture in speeding up the charge flow, particularly at the closed neutron shells, is studied together with the β flow at freezeout and the effect of neutrino-induced neutron emission on the abundance pattern after freezeout. It is shown that a semiquantitative agreement with the gross solar r-process abundance pattern from the peak at mass number A ~ 130 through the peak at A ~ 195 and up to the region of the actinides can be obtained by a superposition of two distinctive kinds of r-process events. These correspond to a low frequency case L and a high frequency case H, which take into account the low abundance of 129I and the high abundance of 182Hf in the early solar nebula. The lifetime of 182Hf (τ182 ≈ 1.3 × 107 yr) associates the events in case H with the most common Type II supernovae. These events would be mainly responsible for the r-process nuclei near and above A ~ 195. They would also make a significant amount of the nuclei between A ~ 130 and 195, including 182Hf, but would make very little 129I. In order to match the solar r-process abundance pattern and to satisfy the 129I and 182Hf constraints, the events in case L, which would make the r-process nuclei near A ~ 130 and the bulk of those between A ~ 130 and 195, must occur ~10 times less frequently but eject ~10-20 times more r-process material in each event. Assuming that all of the supernovae producing r-process nuclei represent a similar overall process, we speculate that the usual neutron star remnants, and hence prolonged ejection of r-process material, are associated with the events in case L. We further speculate that the more frequently occurring events in case H have ejection of other r-process material terminated by black hole formation during the neutrino cooling phase of the protoneutron star. This suggests that there is now an inventory of ~5 × 108 black holes with masses ~1 M☉ and ~5 × 107 neutron stars resulting from supernovae in the Galaxy. This r-process model would have little effect on the estimates of the supernova contributions to the non-r-process nuclei.