Th Ages for Metal-poor Stars

With a sample of 22 metal-poor stars, we demonstrate that the heavy-element abundance pattern (Z>=56) is the same as the r-process contributions to the solar nebula. This bolsters the results of previous studies that there is a universal r-process production pattern. We use the abundance of thorium in five metal-poor stars, along with an estimate of the initial Th abundance based on the abundances of stable r-process elements, to measure their ages. We have four field red giants with errors of 4.2 Gyr in their ages and one M92 giant with an error of 5.6 Gyr, based on considering the sources of observational error only. We obtain an average age of 11.4 Gyr, which depends critically on the assumption of an initial Th/Eu production ratio of 0.496. If the universe is 15 Gyr old, then the (Th/Eu)₀ should be 0.590, in agreement with some theoretical models of the r-process.