Thorium and Uranium Chronometers Applied to CS 31082‐001

Abstract

We use the classical r -process model to explore the implications of the recently reported first observation of U in the extremely metal-poor, r -process element-enriched halo star CS 31082-001 for U and Th cosmochronometry. Using updated nuclear physics input and performing a new, conservative, analysis of the remaining uncertainties in the classical r -process model, we confirm that U (together with Th) abundance observations in metal-poor stars are a promising tool for dating r -process events in the early Galaxy, independent of assumptions on Galactic chemical evolution. We show that nuclear physics uncertainties limit the present accuracy of estimated U/Th ages to about 2 Gyr. Critical nuclear data that are required to lower this uncertainty include β-delayed fission branchings and reliable predictions of the onset of deformation in the vicinity of the N = 184 shell closure around 244 Tl, as both directly affect predicted U/Th ratios in r -process models. In this paper we apply, for the first time, the new HFBCS-1 mass model within the framework of the classical r -process model. We find that the predicted U and Th abundances are incompatible with the solar U and Th abundances and trace this back to a different prediction of the onset of deformation around 244 Tl. In the case of CS 31082-001, we find it likely that the zero-age U and Th abundances were enhanced by about a factor of 2.5 compared to both (1) a theoretical extrapolation from the observed stable elements using the classical r -process model and (2) the zero-age abundances of Th and U in other r -process-enhanced, metal-poor halo stars. Although presently ad hoc, this "actinide boost" assumption solves the apparent problem of the relative age difference compared with other metal-poor halo stars and, at the same time, the problem of the inconsistency of ages based on U/(stable nucleus), Th/(stable nucleus) and U/Th ratios. There clearly exist differences, among some r -process-enhanced, metal-poor stars, in the level of the elemental abundances of actinides beyond the third r -process peak. Whether CS 31082-001 is a relatively rare case or commonplace awaits the identification of larger numbers of r -process-enhanced, metal-poor stars in which both U and Th can be measured. Using the U/Th ratio, we obtain a best age estimate for the r -process elements in CS 31082-001 of 15.5 ± 3.2 Gyr. Future observations of Pb and Bi and a better determination of the r -process contribution to solar Pb are needed to put the age estimates for this and other stars on a more solid basis. For our most likely scenario, we provide predictions of the expected upper and lower limits on the abundances of the elements Pb and Bi in CS 31082-001.