A Long, Cold, Early $r$-process? $\nu$-induced Nucleosynthesis in He Shells Revisited

We reexamine a $\nu$-driven $r$-process mechanism in the He shell of a core-collapse supernova with low initial metallicity. We use the hydrodynamic code KEPLER to calculate the nucleosynthesis both before and after the passage of shock, in recent pre-supernova models. We find that for an inverted neutrino mass hierarchy, $A\sim 130$ and 195 abundance peaks can be produced over $\sim 20$--50~s for initial metallicities $\leq 10^{-3}$ the solar value. The mechanism is sensitive to the amount of $^{28}$Si and $^{32}$S present in the He shell in the pre-supernova model, as well as on the $\nu$ emission model and oscillations. We discuss the implications of this early $r$-process which could alter interpretations of abundance data from metal-poor stars.