S-process nucleosynthesis in massive stars and the weak component. I - Evolution and neutron captures in a 25 solar mass star

Evolutionary calculations of a typical massive star are run in order to analyze the n-capture process occurring during the hydrostatic core He-burning phase, studying the effects of variations in the C-12(alpha, gamma)O-16 and Ne-22(alpha, n)Mg-25 rates within their present uncertainties and in other nuclear inputs. The nucleosynthetic mechanism is characterized by a low mean neutron density, which never exceeds 10 to the 6th n/cu cm. A good production of the s-isotopes in the atomic mass range 70-90 is found, the actual neutron exposure depending on some critical parameters. Important contributions are also found for several light n-rich species and for the heavy rare isotopes Gd-152 and Ta-180. How s-processing in a typical massive star compares with predictions by classical analysis is addressed.