THE THORIUM CHRONOMETER IN CS-22892-052 - ESTIMATES OF THE AGE OF THEGALAXY

We analyze the recent thorium detection in the metal-poor halo star CS 22892-052. Sneden et al. have demonstrated that all of the stable ele ments with Z greater than or equal to 56, including those near the tho rium nuclear region, are consistent with the solar r-process abundance s. This result strongly suggests that thorium (formed in the r-process ) was also produced in solar proportions in the progenitor of CS 22892 -052. Theoretical calculations, presented here, that reproduce the obs erved stable solar system r-process abundances predict a thorium/europ ium (Th/Eu) ratio in close agreement with the extrapolated, corrected r-process-only ratio (0.463) at the time of the formation of the solar system, Sneden et al, found that Th/Eu = 0.219 in CS 22892-052, subst antially below the current solar ratio, indicating a much greater age for this star. Ignoring additional production of thorium over time, an d thus any chemical evolution effects, comparison between the observed and the solar system corrected Th/Eu ratios gives a simple radioactiv e-decay age for CS 22892-052 of 15.2 +/- 3.7 Gyr. Additional age estim ates, based upon theoretically determined Th/Eu ratios, also suggest a range of 11.5 less than or similar to t(CS 22892-052) less than or si milar to 18.8 Gyr and are thus consistent with that simple radioactive -decay age determination for this star. Since additional Galactic prod uction of thorium leads to an increase in this decay time, this is a l ower limit on the stellar age, and hence the age of the Galaxy. We eva luate the magnitude of this effect on the chronometric age determinati ons by employing several simple chemical evolution models, including a closed-box model and one which allows for Galactic infall. These Gala ctic chemical evolution models suggest an age of 17 +/- 4 Gyr for CS 2 2892-052. We discuss the implications and possible sources of errors a nd uncertainties in these age estimates.