NuGrid: S process in massive stars

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 ²²Ne, 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 ²²Ne 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 ²²Ne on the s process. We find a large production of s elements between strontium and barium, starting with the amount of primary ²²Ne 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 ¹⁷O(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.