The Astrophysical Origin of the s-Elements

In this thesis we shall treat the astrophysical origin of the s-process. The subject, that can at first sight appear as very specific, is actually very wide, so_ that we shall concentrate on one aspect, that is we shall consider mainly the s-process which occurs in massive stars (MS), leading to the so-called weak component. The aim is to reach a consistent picture that can explain the solar distribution of the s-isotopes belonging to the weak component, as well as interpret the observation of chemical abundances in the atmospheres of stars of different metallicity. The comparison with such empirical data is a way to test the validity of the theoretical predictions. Indeed, the solar abundances are in most cases well known, with typical uncertainties of about 10 3, while the latest improvements of the observational techniques using high resolution spectroscopy can provide us with abundance determinations within 0.1-0.2 dex