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Session 13 - Stellar Spectra.
Display session, Wednesday, January 07
Exhibit Hall,

[13.01] Thorium Ages for Extremely Metal-Poor Stars

J. A. Johnson (UCSC), M. Bolte (UCO/Lick)

By measuring the abundance of the radioactive isotope thorium-232 (Z=90), Sneden, et al. (1996) found a lower limit of 15 Gyrs to the age of CS 22892-052, a metal-poor giant. Their estimate of the initial thorium abundance was based on a detailed study of many neutron-capture elements, up to Z=76, which matched a scaled solar system r-process pattern. We have obtained Keck HIRES data for a larger sample of field giants and show the results for two stars here: HD 115444 ([Fe/H] \sim -3.0) and HD 108577 ([Fe/H] \sim -2.3). We present abundances for heavy elements in these two stars to check whether the solar-system r-process pattern is indeed invariant, to test the extrapolation from Z \sim 70 to Z=90, and to determine the ages of two more metal-poor stars.

Program listing for Wednesday