r-process abundances and chronometers in metal-poor stars

Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, emplo ying internally consistent and physically realistic nuclear physics input ( quasi-particle random-phase approximation [QRPA] beta-decay properties and the recent extended Thomas-Fermi with Strutinsky integral and quenching (ET FSI-Q) nuclear mass model), have been performed. These theoretical computat ions assume the classical waiting-point approximation of (n, gamma) reversi ble arrow (gamma, n) equilibrium. The calculations reproduce the solar isot opic r-abundances in detail, including the heaviest stable Pb and Bi isotop es. These calculations are then compared with ground-based and Hubble Space Telescope observations of neutron-capture elements in the metal-poor halo stars CS 22892-052, HD 115444, HD 122563, and HD 126238. The elemental abun dances in all four metal-poor stars are consistent with the solar r-process elemental distribution for the elements Z greater than or equal to 56. The se results strongly suggest, at least for those elements, that the relative elemental r-process abundances have not changed over the history of the Ga laxy. This indicates also that it is unlikely that the solar r-process abun dances resulted from a random superposition of varying abundance patterns f rom different r-process nucleosynthesis sites. This further suggests that t here is one r-process site in the Galaxy, at least for elements Z greater t han or equal to 56. Employing the observed stellar abundances of stable ele ments, in conjunction with the solar r-process abundances to constrain the calculations, we present predictions for the zero decay-age abundances of t he radioactive elements Th and U. We compare these predictions (obtained wi th the mass model ETFSI-Q, which reproduces solar r-abundances best) with n ewly derived observational values in three very metal-poor halo stars: HD 1 15444, CS 22892-052, and HD 122563. Within the observational errors the rat io of [Th/Eu] is the same in both CS 22892-052 and HD 115444. Comparing wit h the theoretical ratio suggests an average age of these two very metal-poo r stars to be 15.6 +/- 4.6 Gyr, consistent with earlier radioactive age est imates and recent globular and cosmological age estimates. Our upper limit on the uranium abundance in HD 115444 also implies a similar age. Such radi oactive age determinations of very low metallicity stars avoid uncertaintie s in Galactic chemical evolution models. They still include uncertainties d ue to the involved nuclear physics far from beta-stability. However, we giv e an extensive overview of the possible variations expected and come to the conclusion that this aspect alone should not exceed limits of 3 Gyr. There fore this method shows promise as an independent dating technique for the G alaxy.