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 92,94 Mo and 96,98 Ru, 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 ( 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 102 Pd, although our calculations do not show efficient production of 92 Mo. If the entropy of these ejecta is increased by a factor of 2, the synthesis extends to 120 Te. 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.