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Session 97 - Supernovae.
Oral session, Wednesday, January 17
Salon del Rey Central, Hilton

[97.05] Model Independent r -Process Nucleosynthesis - Constraints on the Key Parameters

R. D. Hoffman (UCSC), Y. Z. Qian (CalTech), S. E. Woosley (UCSC)

During the first ten seconds following the launch of the shock that powers a successful Type II supernova, a variety of interesting nucleosynthesis occurs in the material heated by neutrinos in the vicinity of the neutron star. This region has been previously identified as a likely site for producing the r-process, and detailed nucleosynthesis studies have been published in the context of two specific supernova models (Woosley et al. 1994; Takahashi, Witti, amp; Janka, 1994). While both works exhibited a common failure (overproduction of N=50 closed neutron shell nuclei) as well as some overall success in reproducing the solar r-process abundance distribution, such aspects were in large part attributable to specifics of the models themselves. Additionally, both groups determined that the region of interest (the ``hot bubble'') had to attain very high entropies (S/N_Ak \gtaprx 400) in order to produce a neutron to heavy seed ratio sufficient to make the heaviest r-process peak elements (A\sim 195), an attribute which has to date only been realized in the former study. This ``high-entropy'' solution, although attractive, may not be a ``necessary'' requirement for the production of a successful r-process.

We present a model independent survey which determines, for a reasonable range of dynamic time scales characterizing the cooling of material from (9-2.5) \times 10^9 K, the minimum value of entropy for a given electron mole number (Y_e) that could produce a successful r-process. These results can be used to gauge the suitability of other delayed explosion models (or other astrophysical environments) as potential r-process sites provided the synthesis follows a freeze-out from nuclear statistical equilibrium. The requisite values of Y_e range from 0.20 for low values of entropy (S/N_Ak \ltaprx 100) such that all \alpha-particles recombine into heavies, to values approaching Y_e\sim 0.5 if the entropy per baryon is \gtaprx 400.

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