$s$-Process in Massive Carbon-Enhanced Metal-Poor Stars

Observations suggest that the interstellar medium (ISM) might have been highly enriched in carbon at very early times. We explore nucleosynthesis in massive carbon-enhanced metal-poor (CEMP) stars of $12$–$40{\mathrm{M}}_{\odot }$ formed from such an ISM with $[\mathrm{F}\mathrm{e}]\le -2$. We find substantial production of elements heavier than Fe, mostly up to Sr, by the weak \textsl{s}-process in stars with initial abundances of $[\mathrm{C}\mathrm{N}\mathrm{O}]\gtrsim -1.5$. Even heavier elements, up to Ba, can be produced for $[\mathrm{C}\mathrm{N}\mathrm{O}]\gtrsim -0.5$. The efficiency of this \textsl{s}-process is sensitive to the initial enhancement of C (or more generally, CNO) and mass of the star, with the yield increasing approximately linearly with the initial Fe abundance. The \textsl{s}-process in CEMP stars of $\gtrsim 20{\mathrm{M}}_{\odot }$ with $[\mathrm{C}\mathrm{N}\mathrm{O}]\gtrsim -1.5$ can be an important source for heavy elements in the Early Galaxy.