6.2 Thorium Ages

A new measurement of the age of a very metal poor star in the halo of our Galaxy has recently been made by Cowan et al. (1997), following a technique introduced by Butcher (1987). These authors make use of very high-resolution echelle spectra of CS22892-052, a star with a metallicity of only [Fe/H] = -3.1. They find that the observed abundances for stable elements in this star match the observed r-process elemental abundances observed in the Sun. However, for the radioactive element thorium, the abundance is down by a factor of 40 relative to solar. Allowing for the radioactive decay of thorium relative to (stable) europium yields a minimum age for this star of 15.2 ± 3.7 Gyr (1-sigma). If instead of europium alone, an average abundance for all r-process elements from Eu-Er is used, an age of 13.8 ± 3.7 Gyr results. This lower limit to the age is independent of any model of Galactic evolution (which only serve to increase the total age estimates for the Universe). It depends on both the decay rate and the initial abundance of thorium. Although the current sample is small (1 star!) and the uncertainties are correspondingly large, there is excellent promise for the future once the sample is enlarged. Methods like this one are particularly important because of the opportunity of having high-quality ages completely independent of the globular cluster age scale.