Diverse Supernova Sources for the r-Process

We present a simplified analysis using equations for the charge flow, which include ν_e capture, for the production of r-process nuclei in the context of the recent supernova hot bubble model. The role of ν_e 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.