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.