The Thorium Chronometer in CS 22892−052: Estimates of the Age of the Galaxy

Abstract

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 elements with Z ≥ 56, including those near the thorium nuclear region, are consistent with the solar r-process abundances. 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 observed stable solar system r-process abundances predict a thorium/europium (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, substantially below the current solar ratio, indicating a much greater age for this star. Ignoring additional production of thorium over time, and thus any chemical evolution effects, comparison between the observed and the solar system corrected Th/Eu ratios gives a simple radioactive-decay age for CS 22892-052 of 15.2 ± 3.7 Gyr. Additional age estimates, based upon theoretically determined Th/Eu ratios, also suggest a range of 11.5 t_{CS 22892-052} 18.8 Gyr and are thus consistent with that simple radioactive-decay age determination for this star. Since additional Galactic production of thorium leads to an increase in this decay time, this is a lower limit on the stellar age, and hence the age of the Galaxy. We evaluate the magnitude of this effect on the chronometric age determinations by employing several simple chemical evolution models, including a closed-box model and one which allows for Galactic infall. These Galactic chemical evolution models suggest an age of 17 ± 4 Gyr for CS 22892-052. We discuss the implications and possible sources of errors and uncertainties in these age estimates.