r-process abundances and chronometers in metal-poor stars

Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing 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 (ETFSI-Q) nuclear mass model), have been performed. These theoretical computations assume the classical waiting-point approximation of (n, gamma) reversible arrow (gamma, n) equilibrium. The calculations reproduce the solar isotopic r-abundances in detail, including the heaviest stable Pb and Bi isotopes. 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 abundances 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. 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 indicates also that it is unlikely that the solar r-process abundances resulted from a random superposition of varying abundance patterns from different r-process nucleosynthesis sites. This further suggests that there is one r-process site in the Galaxy, at least for elements Z greater than or equal to 56. Employing the observed stellar abundances of stable elements, in conjunction with the solar r-process abundances to constrain the calculations, we present predictions for the zero decay-age abundances of the radioactive elements Th and U. We compare these predictions (obtained with the mass model ETFSI-Q, which reproduces solar r-abundances best) 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 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 these two very metal-poor stars to be 15.6 +/- 4.6 Gyr, consistent with earlier radioactive age estimates and recent globular and cosmological age estimates. Our upper limit on the uranium abundance in HD 115444 also implies a similar age. Such radioactive age determinations of very low metallicity stars avoid uncertainties in Galactic chemical evolution models. They still include uncertainties due to the involved nuclear physics far from beta-stability. However, we give an extensive overview of the possible variations expected and come to the conclusion that this aspect alone should not exceed limits of 3 Gyr. Therefore this method shows promise as an independent dating technique for the Galaxy.