The effect of 12C + 12C rate uncertainties on the weak s-process component

The contribution by massive stars (M > 15M{sub {circle_dot}}) to the weak s-process component of the solar system abundances is primarily due to the {sup 22}Ne neutron source, which is activated near the end of helium-core burning. The residual {sup 22}Ne left over from helium-core burning is then reignited during carbon burning, initiating further s-processing that modifies the isotopic distribution. This modification is sensitive to the stellar structure and the carbon burning reaction rate. Recent work on the {sup 12}C + {sup 12}C reaction suggests that resonances located within the Gamow peak may exist, causing a strong increase in the astrophysical S-factor and consequently the reaction rate. To investigate the effect of such a rate, 25M{sub {circle_dot}} stellar models with different carbon burning rates, at solar metallicity, were generated using the Geneva Stellar Evolution Code (GENEC) with nucleosynthesis post-processing calculated using the NuGrid Multi-zone Post-Processing Network code (MPPNP). A strongly enhanced rate can cause carbon burning to occur in a convective core rather than a radiative one and the convective core mixes the matter synthesized there up into the carbon shell, significantly altering the initial composition of the carbon-shell. In addition, an enhanced rate causes carbon-shell burning episodes to ignitemore » earlier in the evolution of the star, igniting the {sup 22}Ne source at lower temperatures and reducing the neutron density.« less