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 (QRPA β-decay properties and the recent 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 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 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, predictions for the zero decay-age abundances of the radioactive elements Th and U are presented. We compare these predictions (obtained with the mass model ETFSIQ, 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