Production of the Light $p$-Process Nuclei in Neutrino Driven Winds

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Session 33 -- Supernovae
Oral presentation, Monday, 9, 1995, 2:00pm - 3:30pm

[33.09] Production of the Light $p$-Process Nuclei in Neutrino Driven Winds

R. D. Hoffman, S. E. Woosley (UCO/Lick), G. M. Fuller (UCSD), B. S. Meyer (Clemson)

Recent studies of the nucleosynthesis that occurs in the neutrino-driven wind following the delayed explosion of a Type II or Ib supernova have provided a promising site for the synthesis of the $r$-process isotopes. One worrisome aspect of such models has been a large overproduction of N=50 (closed neutron shell) nuclei. The conditions in the wind that favor production of these nuclei, $Y_e \approx 0.45 - 0.46, S/N_Ak \approx 50 - 100$, arise in about 0.01 M\sun \ of the ejecta at early times ($\sim 2$ s). The overproductions are so great that, if these results were typical of all such supernovae, essentially no heavy element production (other than these rare nuclei) could occur. However, a successful $r$-process, which gives an appropriate total $r$-abundance ejecta per event, and reproduces the solar isotopic $r$-abundance distribution, does emerge from the same model at later times ($\sim$ 10 - 20 s) when the entropy is higher ($\sim 400$) and $Y_e$ is lower ($\sim 0.40$).

We have re-explored the nucleosynthesis in the same supernova model and included $\nu_e$ and $\bar \nu_e$ capture reactions on free nucleons and heavy nuclei during the freeze out from nuclear statistical equilibrium. For only slightly larger ($\sim 5$\%) values of $Y_e$ (i.e., $Y_e \approx 0.485$ instead of 0.46), the problematic nucleosynthesis disappears and is replaced by the moderate production of some long sought $p-process$ nuclei, $^{74}$Se, $^{78}$Kr, $^{84}$Sr, $^{92}$Mo, and $^{96}$Ru, thus turning the principal failure of this model into a success. The necessary minor adjustments to $Y_e$ can probably be accounted for by small corrections to the electron and anti-electron neutrino fluences and temperatures in the model at early times without adversely altering the $r$-process that occurs at a later time. It is interesting that both $r$- and $p$-process nuclei can be produced in such close proximity. They would probably be mixed in the explosion.

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