The Weak s-Component and Nucleosynthesis in Massive Stars

The difficulties of the phenomenological analysis of the weak s-component and the need for a comparison with the astrophysical picture are discussed. New results of s-process calculations in massive stars during both core He and shell C burning are presented. They are obtained after adopting an updated nuclear network, which includes the deviation from the 1/υ trends of neutron capture cross sections, as well as an extrapolation of beta decay rates to the high temperatures characterizing carbon burning. For a complete discussion of the branching at <SUP>85</SUP>Kr, we additionally present a new measurement of the <SUP>87</SUP>Rb neutron capture cross section. The reproduction of the weak component by successive stellar generations is investigated through a global check which takes into account the interference of the main component in the atomic mass range from Fe to Sr. Consequently, the results of s-processing in low-mass stars during the thermally pulsing asymptotic giant branch phase are also considered, starting from the same set of revised nuclear parameters. A satisfactory match of the solar s-abundances is reached, where the problem of the <SUP>86</SUP>Kr overproduction by the main component is overcome by varying the <σ<SUB>n,γ</SUB>> of <SUP>84,85,86</SUP>Kr inside their present uncertainties, and by considering an additional contribution to <σ<SUB>n,γ</SUB>> (<SUP>86</SUP>Kr) at low temperature by direct captures or undetected resonances. A detailed analysis of the neutron flow at branchings and at bottleneck points is performed, stressing the analogies and differences between the phenomenological and astrophysical picture.