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 beta-decay rates and the ETFSI-Q nuclear mass model), have been made.These calculations are compared with ground-based and HST observations ofneutron-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 starsare consistent with the solar r-process elemental distribution for the elementsZ >/= 56. These results strongly suggest, at least for those elements, that therelative elemental r-process abundances have not changed over the history ofthe Galaxy. This further suggests that there is one r-process site in theGalaxy, at least for elements Z >/= 56. Employing the observed stellar abundances of stable elements, in conjunctionwith the solar r-process abundances to constrain the calculations, predictionsfor the zero decay-age abundances of the radioactive elements Th and U aremade. We compare these (least-squares-fit ETFSI-Q) predictions with newlyderived observational values in three very metal-poor halo stars: HD 115444, CS22892--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 thetheoretical ratio suggests an average age of these two very metal poor stars tobe approximately 15.6 +/- 4.6 Gyr, consistent with earlier radioactive ageestimates and recent globular and cosmological age estimates.Comment: Astrophysical Journal, in pres