S -process Nucleosynthesis and the Evolution of Massive Rotating Stars with Low Metallicity during Core Helium Burning

Abstract At very low metallicities, s -process production in massive stars is expected to be negligible in non-rotating models owing to three key factors: the scarcity of the 22 Ne neutron source, the presence of primary neutron poisons, and the declining abundance of iron seeds. By making use of advanced stellar evolution models generated by the Geneva Stellar Evolution Code, we investigate the impact of rapid rotation on s -process nucleosynthesis in low-metallicity massive stars. We have presented nucleosynthesis calculations for a rotating 30 M ⊙ star at metallicity Z = 10 −4 . It is found that about 0.8% of the convective He-burning core mass comprises primary 22 Ne. Iron seeds are consumed to a significant extent in rotating stars and the production of 88 Sr, 89 Y, and 90 Zr is achieved significantly. This can also produce s -process nucleosynthesis, allowing it to reach the 208 Pb neutron-magic peak. On the other hand, the amount of 22 Ne strongly impacts the s -process efficiency and the Sr/Ba ratio. Rotation boosts the s -process in massive stars at low metallicities. There are three primary reasons for this. First, we confirm that rotation-induced mixing leads to a significant overproduction of 22 Ne. More 22 Ne means a higher neutron flux and thus higher s -process efficiency, which leads to the production of more and heavier elements. Second, at low metallicities, the primary production of 22 Ne leads to a much higher neutron-to-seed ratio than in non-rotating stars. The increased availability of neutrons allows the star to consume a much larger fraction of its iron seeds to form heavier s -process elements. Finally, as the metallicity decreases, the production of elements up to the Ba or Pb peak increases at the expense of the Sr-peak elements.