New \textsl{s}-process Mechanism in Rapidly-Rotating Massive Pop II Stars

Abstract

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 $\sim 10^7$ 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.

13 pages, 9 figures, 1 table

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