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J. A. Johnson (UCSC), M. Bolte (UCO/Lick/UCSC)
The best local measurements of the age of the Universe have long come from the luminosity of the main-sequence turnoff stars in globular clusters. With the discovery of and abundance analysis for CS 22892-052 ([Fe/H]=-3.1), the dating of stars using the abundances of radioactive elements became a viable method for determining the agesof the halo stars (Sneden et al. 1996). Those authors found that the heavy-element abundance ratios matched those for the r-process derived from the Sun with the exception of radioactive thorium, which was underabundant. They then assumed that a lower limit to the original thorium abundance in CS 22892-052 could be predicted from the solar abundances. The lower limit to the age of CS 22892-052 from the depletion of thorium is 15 ± 3 Gyr -- this once again brings local age estimates in conflict with standard cosmological models.
Our goal is to increase the sample of stars to improve the accuracy of the age limit and to test the crucial assumption that the r-process always produces the same abundance ratios. Using HIRES on the Keck telescope, we have obtained echelle spectra of five more metal-poor stars that are rich in neutron-capture elements. This allows us to measure the abundance of a wide range of heavy elements. We will present our age estimates for HD108577, HD115444, HD186478, BD +8 2548, and M92 VII-18.