The s-process efficiency in massive stars

The s-process is studied in the He burning core of a M_α = 8 M_⊙ He star using temperature and density profiles obtained with a standard stellar evolution model and an updated nuclear network. We discuss in detail the s-process efficiency for metallicities Z/Z_⊙ ranging between 1 and 10^-3 in the light of recent nuclear data. We show that ¹⁶O acts as a powerful neutron poison at low Z although its effect on the s-process yields is still subject to large uncertainties due to our imperfect knowledge of some nuclear reaction rates and of the galactic evolution of nuclei which determine the neutron economy of the s-process.