Metallicity dependence of the s-process in massive stars: theoretical predictions .

At solar−like metallicity the s−process (slow neutron capture process) in massive stars (M ≥ 10M ) is efficient during convective core He−burning and convective shell C−burning. In both the two phases the 22Ne(α,n)25Mg reaction is the main neutron source. However, moving to metallicities lower than solar, the C shell contribution to the total neutron exposure τ becomes marginal with respect to the s−process contribution in the He core. For this reason, at low metallicity the s−process signature beyond iron is mainly given by the He core. In these conditions, the definition of the 22Ne abundance in the last phases of the He core is crucial for the weak s−process behaviour. Without considering nuclear and models development uncertainties, the initial isotopic distribution of light isotopes severely affects the final s−process distribution. In particular, this point is related to the spectroscopic observations discrepancy of oxygen, that is the most abundant element within the initial CNO nuclei which are converted in the main neutron source 22Ne.