We report a new mechanism for the \textsl{ s} in rotating massive metal-poor stars. Our models show that above a critical rotation speed, such stars evolve in a quasi-chemically-homogeneous fashion, which gives rise to a prolific \textsl{ s}-process. Rotation-induced mixing results in primary production of $^{ 13}$C, which subsequently makes neutrons via $^{ 13}\mathrm{ C}(\alpha,\mathrm{ n})^{ 16}\mathrm{ O}$ during core He burning. Neutron capture can last up to $\sim 10^{ 13}\,\mathrm{ s}$ ($\sim 3\times 10^{ 5}$~yr) with the peak central neutron density ranging from $\sim10^7$ to $10^{ 8}\,\mathrm{ cm}^{ -3}$. Depending on the rotation speed and the mass loss rate, a strong \textsl{ s} can occur with production of elements up to Bi for progenitors with initial metallicities of $[Z]\lesssim -1.5$. This result suggests that rapidly-rotating massive metal-poor stars are likely the first site for the main \textsl{ s}-process. We find that these stars can potentially explain the early onset of the \textsl{ s}-process and some of the carbon-enhanced metal-poor (CEMP-\textsl{ s} and CEMP-\textsl{ r/s}) stars with strong enrichment attributed to the \textsl{ s} or a mixture of the \textsl{ r}-process and the \textsl{ s}-process.
Categories: astro-ph.SR