New s-process Mechanism in Rapidly Rotating Massive Population II Stars

We report a new mechanism for the s-process 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 s-process. Rotation-induced mixing results in primary production of 13C, which subsequently makes neutrons via during core He burning. Neutron capture can last up to (∼3 × 105 yr) with the peak central neutron density ranging from ∼107 to 108 . Depending on the rotation speed and the mass loss rate, a strong s-process can occur with production of elements up to Bi for progenitors with initial metallicities of [Z] ≲ −1.5. This result suggests that rapidly rotating massive metal-poor stars are likely the first sites of the main s-process. We find that these stars can potentially explain the early onset of the s-process and some of the carbon-enhanced metal-poor (CEMP-s and CEMP-r/s) stars with strong enrichment attributed to the s-process or a mixture of the r-process and the s-process.