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

Abstract

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 13 C, which subsequently makes neutrons via during core He burning. Neutron capture can last up to (∼3 × 10 5 yr) with the peak central neutron density ranging from ∼10 7 to 10 8 . 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.