AAS 196th Meeting, June 2000
Session 5. Be Star and Other Atmosphere Studies
Display, Monday, June 5, 2000, 9:20am-6:30pm, Empire Hall South

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[5.13] The Helium r-Process and Neutron-Capture Element Abundances in Low Metallicity Stars

J.W. Truran (Chicago), J.J. Cowan (Oklahoma)

There is increasingly strong observational evidence that the r-process isotopes identified in solar system matter are in fact the products of two distinct classes of r-process nucleosynthesis events. Spectroscopic observations of extremely metal deficient halo field stars and globular cluster stars provide confirmation of the occurrence of a robust r-process mechanism for the production of the main r-process component, at mass numbers A \gtrsim 130-140. This main component is variously argued to have its origin in regions outside the neutronized core, in magnetic jets from the collapsing core, or in neutron star-neutron star mergers. The stellar abundance data available to date suggests, however, that the bulk of the r-process nuclei in the mass region A \lesssim 130 are not formed in this environment. Further evidence for such a distinct weak component is provided by the finding that the abundances of the short lived r-process nuclei 129I and 182Hf in the early solar system cannot be explained by a single type of r-process event (Wasserburg, Busso, & Gallino 1996). We report here exploratory calculations of the consequences of r-process synthesis in the shock-processed helium shells of Type II supernovae. The conditions of temperature, density, and composition are those predicted by the models of Woosley and Weaver (1995). Our results establish that these conditions are quite consistent with the production of an r-process pattern of abundances for the A \lesssim 130 mass region, although the sensitivity to detailed post shock conditions insures that this process is not as robust as that responsible for the main r-process component A \gtrsim 130-140. We discuss the implications of our numerical results for the interpretation both of the abundances in metal poor stars and of the anomalous r-process-like isotopic abundances observed in meteoritic silicon carbide grains (Pellin et al. 2000).

This research was funded in part by NSF grant AST-9618332 to JJC and by the ASCI/Alliances Center for Astrophysical Thermonuclear Flashes at the University of Chicago under DOE contract B341495 (JWT).

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