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J. J. Cowan (U. Oklahoma), B. Pfeiffer, K. L. Kratz (U. Mainz), F. K. Thielemann (U. Basel), C. Sneden (U. Texas), S. Burles (U. Chicago), D. Tytler (UCSD), T. C. Beers (Michigan State U.)
We have made rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing internally consistent and physically realistic nuclear physics input (QRPA \beta-decay rates and the ETFSI-Q nuclear mass model). These theoretical computations assume the classical waiting-point approximation of (n,\gamma)-(\gamma,n) equilibrium and reproduce in detail both the isotopic and the elemental r-process solar system abundance distributions. These same elemental abundance predictions are compared with ground-based and HST observations of neutron-capture elements in the metal-poor halo stars: CS 22892--052, HD 115444, HD 122563 and HD 126238. The elemental abundances in all four metal-poor stars are reproduced by the theoretical calculations and are consistent with the solar r-process elemental distribution for the elements Z \ge 56. These results strongly suggest, at least for those elements, that the relative elemental r-process abundances have not changed over the history of the Galaxy. This further suggests that there is one r-process site in the Galaxy, at least for the elements Z \ge 56.
Employing the observed stellar abundances of stable elements, in conjunction with the solar r-process abundances to constrain the calculations, predictions for the zero decay-age abundances of the radioactive elements Th and U are made. We compare these predictions with newly derived observational values in three very metal-poor halo stars: HD 115444, CS 22892--052 and HD 122563. Within the observational errors the observed ratio of [Th/Eu] is the same in both CS 22892--052 and HD 115444. Comparing with the theoretical ratio suggests an average age of approximately 13.0 ± ~ 5 Gyr for these two very metal-poor stars, consistent with earlier radioactive age estimates and recent globular and cosmological age estimates.
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