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

We demonstrate that nucleosynthesis in rapid, high-entropy expansions ofproton-rich matter from high temperature and density can result in a widervariety of abundance patterns than heretofore appreciated. In particular, suchexpansions can produce iron-group nuclides, p-process nuclei, or even heavy,neutron-rich isotopes. Such diversity arises because the nucleosynthesis entersa little explored regime in which the free nucleons are not in equilibrium withthe abundant alpha particles. This allows nuclei significantly heavier thaniron to form in t he presence of abundant free nucleons early in the expansion.As the temperature drops, nucleons increasingly assemble into alpha particlesand heavier nuclei. If the assembly is efficient, the resulting depletion offree neutrons allows disintegrat ion flows to drive nuclei back down to ironand nickel. If this assembly is inefficient, then the large abundance of freenucleons prevents the disintegration flows and leaves a distribution of heavynuclei after reaction freezeout. For cases in between, an intermediateabundance distribution, enriched in p-process isotopes, is frozen out. Theselast expansions may contribute to the solar system's supply of the p-processnuclides if mildly proton-rich, high-entropy matter is ejected fromproto-neutron stars winds or other astrophysical sites. Also sign ificant isthe fact that, because the nucleosynthesis is primary, the signature of thisnucleosyn thesis may be evident in metal poor stars.Comment: 11 pages, 2 tables, 1 figure. Submitted to ApJ Letter