The impact of new ($\alpha$, n) reaction rates on the weak s-process in metal-poor massive stars

Abstract

Massive stars are significant sites for the weak s-process (ws-process). $^{22}$Ne and $^{16}$O are, respectively, the main neutron source and poison for the ws-process. In the metal-poor stars, the abundance of $^{22}$Ne is limited by the metallicity, so that the contribution of $^{22}$Ne($\alpha$, n)$^{25}$Mg reaction on the s-process is weaker. Conversely, the $^{17}$O($\alpha$, n)$^{20}$Ne reaction becomes more prominent in these stars due to the most abundant $^{16}$O in all metallicities. In this work, we calculate the evolution of four metal-poor models ($Z=10^{-3}$) for the Zero-Age Main-Sequence (ZAMS) masses of $M ({\rm ZAMS})=$ 15, 20, 25, and 30 M$_{\odot}$ to investigate the effect of reaction rates on the ws-process. We adopt the new $^{17}$O($\alpha$, n)$^{20}$Ne and $^{17}$O($\alpha, \gamma$)$^{21}$Ne reaction rates suggested by Best et al. (2013) and $^{22}$Ne($\alpha$, n)$^{25}$Mg and $^{22}$Ne($\alpha, \gamma$)$^{26}$Mg from Wiescher et al. (2023). The yields of the s-process isotope with updated reaction rates are compared with the results using default reaction rates from JINA REACLIB. We find that the new $^{17}$O+$\alpha$ reaction rates increase the ws-process mainly in all the stages, while the new $^{22}$Ne+$\alpha$ reaction rates only increase the ws-process in C and Ne burning stages. Updating these new reaction rates would increase the production of ws-process isotopes by tens of times. We also note that for more massive stars, the enhancement by new $^{17}$O+$\alpha$ reaction rates become more significant.