As an explosion develops in the collapsed core of a massive star, neutrino emission drives convection in a hot bubble of radiation, nucleons, and pairs just outside a proto-neutron star. Shortly thereafter, neutrinos drive a windlike outflow from the neutron star. In both the convective bubble and the early wind, weak interactions temporarily cause a proton excess (Ye 0.50) to develop in the ejected matter. This situation lasts for at least the first second, and the approximately 0.05-0.1 M☉ that is ejected has an unusual composition that may be important for nucleosynthesis. Using tracer particles to follow the conditions in a two-dimensional model of a supernova explosion calculated by Janka, Buras, & Rampp, we determine the composition of this material. Most of it is helium and 56Ni. The rest is relatively rare species produced by the decay of proton-rich isotopes unstable to positron emission. In the absence of pronounced charged-current neutrino capture, nuclear flow will be held up by long-lived waiting-point nuclei in the vicinity of 64Ge. The resulting abundance pattern can be modestly rich in a few interesting rare isotopes like 45Sc, 49Ti, and 64Zn. The present calculations imply yields that, when compared with the production of major species in the rest of the supernova, are about those needed to account for the solar abundance of 45Sc and 49Ti. Since the synthesis will be nearly the same in stars of high and low metallicity, the primary production of these species may have discernible signatures in the abundances of low-metallicity stars. We also discuss uncertainties in the nuclear physics and early supernova evolution to which abundances of interesting nuclei are sensitive.
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