Proton-rich production of lanthanides: the $\hat{I}½i$ process

Abstract

The astrophysical origin of the lanthanides is an open question in nuclear astrophysics. Besides the widely studied $s$, $i$, and $r$ processes in moderately-to-strongly neutron-rich environments, an intriguing alternative site for lanthanide production could in fact be robustly $\text{proton-rich}$ matter outflows from core-collapse supernovae under specific conditions -- in particular, high-entropy winds with enhanced neutrino luminosity and fast dynamical timescales. In this environment, excess protons present after charged particle reactions have ceased can continue to be converted to neutrons by (anti-)neutrino interactions, producing a neutron capture reaction flow up to A~200. This scenario, christened the $\hat{I}½i$ process in a recent paper, has previously been discussed as a possibility. Here, we examine the prospects for $\hat{I}½i$ process through the lens of stellar abundance patterns, bolometric lightcurves, and galactic chemical evolution models, with a particular focus on hypernovae as candidate sites. We identify specific lanthanide signatures for which the $\hat{I}½i$ process can provide a credible alternative to $r$/$i$ processes.

15 pages, 5 figures, v3 matches version to appear in ApJL