S-process nucleosynthesis in massive stars and the weak component. II - Carbon burning and galactic enrichment

Abstract

In Paper I we calculated the s-processing that takes place in a typical massive star of 25 M 0 during He burning. Here we follow the subsequent shell carbon-burning phase. The reactivation of the 22 Ne(a, n) 25 Mg neutron source makes a small s-process occur, with a neutron exposure of t = 0.06 mbarn-1 and a neutron density starting from a high initial value of ^lO 11 cm-3. The s-abundances are consequently greatly modified with respect to He exhaustion, and this may affect most of the CO core, especially if C burning takes place as a convective nuclear stage. In evaluating the s-processed material ejected by a 25 M 0 supernova, we must also consider that the outer region of the CO core has a variable chemical composition due to the shrinkage of the convective core at the end of He-burning. Furthermore, in the inner core the s-abundances are destroyed by photodissociations that occur from O-burning onward. We then calculate the yields of s-isotopes from a stellar generation of massive stars, relying upon the pre-supernova models by Nomoto and Hashimoto. The normalization is obtained by asking for the solar oxygen to be reproduced. Stars more massive than 50 M 0 are supposed to participate in the chemical enrichment of the Galaxy only through stellar wind, i.e., they are not allowed to explode as supernovae; their cores will presumably collapse into black holes. The massive stars' contributions to the solar s-abundances are then added to the contributions from low-mass stars on the asymptotic giant branch. We find that the s-process in massive stars can completely account for the weak component observed in the solar system, and can put important constraints on the scenario for the chemical evolution of the Galaxy.