The r-Process without Excess Neutrons

Abstract

The r-process of nucleosynthesis requires a large neutron-to-seed nucleus ratio. This does not, however, that there be an excess of neutrons over protons. If the expansion of the material is sufficiently rapid and the entropy per nucleon is sufficiently high, the nucleosynthesis enters a heavy-element synthesis regime heretofore unexplored. In this extreme regime, characterized by a persistent disequilibrium between free nucleons and the abundant alpha particles, heavy r-process nuclei can form even in matter with more protons than neutrons. This observation bears on the issue of the site of the r-process, on the variability of abundance yields from r-process events, and on cnstraints on neutrino physics derived from nucleosynthesis. It also clarifies the difference between nucleosynthesis in the early universe and that in less extreme stellar explosive environments.