R‐Process Abundances and Chronometers in Metal‐poor Stars

Abstract

Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing internally consistent and physically realistic nuclear physics input (quasi-particle random-phase approximation [QRPA] β-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,γ) ⇄ (γ,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 ≥ 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 ≥ 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 β-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.