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

One of the outstanding unsolved riddles of nuclear astrophysics is the originof the so called ``p-process'' nuclei from A = 92 to 126. Both the lighter andheavier p-process nuclei are adequately produced in the neon and oxygen shellsof ordinary Type II supernovae, but the origin of these intermediate isotopes,especially 92,94Mo and 96,98Ru, has long been mysterious. Here we explore theproduction of these nuclei in the neutrino-driven wind from a young neutronstar. We consider such early times that the wind still contains a proton excessbecause the rates for electron neutrino and positron captures on neutrons arefaster than those for the inverse captures on protons. Following a suggestionby Frohlich et al. 2005, we also include the possibility that, in addition tothe protons, alpha-particles, and heavy seed, a small flux of neutrons ismaintained by the reaction p(bar(nu_e),e+)n. This flux of neutrons is criticalin bridging the long waiting points along the path of the rp-process by (n,p)and (n,gamma) reactions. Using the unmodified ejecta histories from a recenttwo-dimensional supernova model by Janka et al. 2003, we find synthesis ofp-rich nuclei up to 102Pd. However, if the entropy of these ejecta is increasedby a factor of two, the synthesis extends to 120Te. Still larger increases inentropy, that might reflect the role of magnetic fields or vibrational energyinput neglected in the hydrodynamical model, result in the production ofnumerous r-, s-, and p-process nuclei up to A approximately 170, even in windsthat are proton-rich