Th/Nd cosmochronology and a constant r/s ratio

The production history of both the r-process and the s-process nuclei plays a crucial role in the determination of the galactic age from cosmochronological models. An outstanding puzzle of galactic nucleosynthesis theory has been the constancy of the r/s ratio observed in stars with [Fe/H] > -2.0. It has recently been suggested that the effect of metallicity on the resultant neutron exposure may help explain this surprising observation. A series of numerical calculations are carried out in order to investigate this possibility. We find that although the neutron exposure, and the resultant s-process production, are very sensitive to initial stellar metallicity, this effect alone cannot stimulate a primary s-process production. In fact, we find that low metallicity by itself results in too much s-process production. In order to reconcile theory with observation we propose that an additional effect, namely the increased production of 14N associated with increased proton mixing, should be included in any s-process calculation. We identify a relationship between mixing and metallicity which results in a primary s-process production. Finally, we construct a new cosmochronological model which assumes early spikes in both the r-and s-process production histories, followed by a period of nucleosynthesis characterized by a constant r/s ratio. From comparison with recent stellar observations of the Th/Nd and the Eu/Ba ratios, we determine a relatively young age of ≤ 12 Gyr for the galaxy. (author)