Nucleosynthesis in Early Supernova Winds. II. The Role of Neutrinos

One of the outstanding unsolved riddles of nuclear astrophysics is the origin of the so-called p-process nuclei from A=92 to 126. Both the lighter and heavier p-process nuclei are adequately produced in the neon and oxygen shells of ordinary Type II supernovae, but the origin of these intermediate isotopes, especially ^92,94Mo and ^96,98Ru, has long been mysterious. Here we explore the production of these nuclei in the neutrino-driven wind from a young neutron star. We consider such early times that the wind still contains a proton excess because the rates for ν_e and positron captures on neutrons are faster than those for the inverse captures on protons. Following a suggestion by Fröhlich and coworkers, we also include the possibility that-in addition to the protons, α-particles, and heavy seed-a small flux of neutrons is maintained by the reaction p(ν¯_e, e⁺)n. This flux of neutrons is critical in bridging the long waiting points along the path of the rp-process by (n, p) reactions. Using the unmodified ejecta histories from a recent two-dimensional supernova model by Janka and coworkers, we find synthesis of p-rich nuclei up to ¹⁰²Pd, although our calculations do not show efficient production of ⁹²Mo. If the entropy of these ejecta is increased by a factor of 2, the synthesis extends to ¹²⁰Te. Still larger increases in entropy, which might reflect the role of magnetic fields or vibrational energy input neglected in the hydrodynamical model, result in the production of nuclei up to A~170. Elements synthesized in these more extreme outflows include numerous s- and p-process nuclei, and even some r-process nuclei can be synthesized in these proton-rich conditions.