THRESHOLDS ON STAR-FORMATION AND THE CHEMICAL EVOLUTION OF GALACTIC DISCS - COSMOCHRONOLOGY AND THE AGE OF THE GALAXY

In this paper we analyse different chronometers based on the models of chemical evolution developed by Chamcham, Pitts & Tayler (CPT) and Ch amcham & Tayler (CT). In those papers we discussed the ability of our models to reproduce the observed G-dwarf distribution in the solar nei ghbourhood, the age-metallicity relation and radial chemical abundance gradients. We now examine their response to the predictions of cosmoc hronology. We use the recent production ratios of the actinoid pairs U -235/U-238 and Th-232/U-238 provided by Cowan, Thielemann & Truran and the observed abundance ratios from Anders & Grevesse to determine the duration of nucleosynthesis in the solar neighbourhood, and thus to d etermine maximum likelihood estimates and confidence intervals for the infall parameter, beta, which controls the growth rate of the disc in our models. We compare our predictions for the age of the disc with t he age of the Galaxy estimated from models of white dwarf cooling and from the age of globular clusters. From our statistical analysis we fi nd that these three methods of age prediction appear to be consistent for a range of maximum likelihood values of beta which is in good agre ement with the values considered by CPT and CT, which were found to gi ve a good fit to the observational data considered in those papers, We also briefly consider the consistency of our results with the age of the Universe predicted in different cosmological models - a topic whic h we will investigate more fully in future work.