On the validity of the local approximation for the s-process in the Os region, and implications for the 187Re-187Os cosmochronology

The s-process number abundance ratio of 186Os and 187Os, which is of key importance for the 187Re-187Os cosmochronology, is calculated in two schematic s-process models. Special emphasis is put on possible s-process path branchings in the W-Os region, as well as on the role of 187Os electron captures. It is shown that the ''local approximation'' commonly called for in the evaluation of the abovementioned ratio may break down in certain seemingly realistic s-process conditions. This result and the possibly important consequences for the Re-Os chronometry are discussed in combination with the uncertain contribution of the first nuclear excited state of 187Os to its stellar neutron capture cross section. It is concluded that the relative s-process yields of 186Os and 187Os are still uncertain enough for preventing the 187Re-187Os pair from being accepted as a reliable cosmochronometer. In particular, it is stressed that some of those predicted yields are incompatible with the results of a study of that pair in the framework of a chemical evolution model of the Galaxy which satisfies various observational constraints in the solar neighborhood, and which predicts an age of the Galaxy in the 11< or approx.Tsub(G)< or approx.15 Gy range (Yokoi et al., 1983). (orig.)