The weak s process takes place in massive stars and it produces the majority of s-only isotopes in the atomic mass range from 60 to 90. This process is qualitatively well understood. However, there are still large uncertainties remaining on the quantitative side. Rotation has a strong effect on the stellar structure and mixing, but its impact on the s process has not been studied yet. We implemented an extended and flexible reaction network inside the Geneva stellar evolution code (GENEC) to be able to study the influence of rotation on the s process. For a star with a partic-ular initial mass and composition rotation increases the He core size and the central temperature enhancing the s-process efficiency during core helium burning. In turn the C-shell contribution is reduced since more 22Ne has already been burnt during He-burning. Mixing induced by rotation also affects the contribution of the He-burning shell, since it leads to the production of primary 14N and primary 22Ne. 22Ne and 4He can again be transported to regions with higher temperatures below the convective He-shell, where 22Ne(α,n) becomes an efficient neutron source. To inves-tigate the influence of reaction rate uncertainties besides the uncertainties of stellar structure and mixing, we have developed a one-zone post-processing network including Monte Carlo variations of the rates.

The s-process production in massive stars at very low metallicities is expected to be negligible due to the low abundance of the neutron source 22 Ne, to primary neutron poisons and decreasing iron seed abundances. However, recent models of massive stars including the effects of rotation show that a strong production of 22 Ne is possible in the helium core, as a consequence of the primary nitrogen production (observed in halo metal poor stars). Using the PPN post-processing code (developed within the NuGrid collaboration), we study the impact of this primary 22 Ne on the s process. We find a large production of s elements between strontium and barium, starting with the amount of primary 22 Ne predicted by stellar models including the effects of rotation. There are several key reaction rate uncertainties influencing the s-process efficiency. For example, within the nuclear reaction rate uncertainty, the 17 O(α,γ) reaction may either be critically important or negligible. We also report on the development of the new parallel (MPI) post-processing (MPPNP) variant of the PPN code designed to follow the complete nucleosynthesis in stars on highly resolved grids. We present here the first post-processing run from the ZAMS up to the end of helium burning for a 15 M ⊙ model.