Nucleosynthesis in Fast Expansions of High-Entropy, Proton-rich Matter

Abstract

We demonstrate that nucleosynthesis in rapid, high-entropy expansions of proton-rich matter from high temperature and density can result in a wider variety of abundance patterns than heretofore appreciated. In particular, such expansions can produce iron-group nuclides, p-process nuclei, or even heavy, neutron-rich isotopes. Such diversity arises because the nucleosynthesis enters a little explored regime in which the free nucleons are not in equilibrium with the abundant 4He. This allows nuclei significantly heavier than iron to form in the presence of abundant free nucleons early in the expansion. As the temperature drops, nucleons increasingly assemble into 4He and heavier nuclei. If the assembly is efficient, the resulting depletion of free neutrons allows disintegration flows to drive nuclei back down to iron and nickel. If this assembly is inefficient, then the large abundance of free nucleons prevents the disintegration flows and leaves a distribution of heavy nuclei after reaction freezeout. For cases in between, an intermediate abundance distribution, enriched in p-process isotopes, is frozen out. These last expansions may contribute to the solar system's supply of the p-process nuclides if mildly proton-rich, high-entropy matter is ejected from proto-neutron stars winds or other astrophysical sites. Also significant is the fact that, because the nucleosynthesis is primary, the signature of this nucleosynthesis may be evident in metal-poor stars.