The impact of new ($α$, 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($α$, n)$^{25}$Mg reaction on the s-process is weaker. Conversely, the $^{17}$O($α$, 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($α$, n)$^{20}$Ne and $^{17}$O($α, γ$)$^{21}$Ne reaction rates suggested by Best et al. (2013) and $^{22}$Ne($α$, n)$^{25}$Mg and $^{22}$Ne($α, γ$)$^{26}$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+$α$ reaction rates increase the ws-process mainly in all the stages, while the new $^{22}$Ne+$α$ 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+$α$ reaction rates become more significant.

Figures (15)

• Figure 1: The ($\alpha$, n)/($\alpha, \gamma$) ratio as a function of temperature for $^{22}$Ne+$\alpha$ (top) and $^{17}$O+$\alpha$ (bottom) reactions.
• Figure 2: The central temperature against the central density for the evolution of stars with $M ({\rm ZAMS}) =$ 15, 20, 25 and 30 M$_{\odot}$. The grey dashed lines show the ignition lines of C burning, Ne burning, O burning and Si burning, where the energy generation rate by nuclear burning equals the energy loss rate by neutrino emissions. In the region on the left of the black line, stars are dynamically unstable due to the electron-positron pair creation (indicated as "pair instability") (2009ApJ...706.1184O), general relativistic effects ("GR instability") (see, e.g., 1966PASJ...18..384O), and the photo-disintegration of matter in nuclear statistical equilibrium (NSE) at $Y_{\rm e}=$ 0.5 ("photo-disintegration") (2009ApJ...706.1184O).
• Figure 3: The Kippenhahn diagram of the star with $M {\rm (ZAMS)}$ = 25 M$_{\odot}$ The inner part of $M_r = 0 - 14$ M$_{\odot}$ is shown. The blue, grey, and pink represent the convection, overshoot, and semiconvection regions, respectively. The orange line is the isotherm line of 0.2 GK, and the red line shows the location of $M_r=1.84$ M$_\odot$. Between these two lines, the hatched region indicates where the ws-process is taken into consideration.
• Figure 4: The mass distribution of the main isotopes at $t=t_{\rm final}$ from MESA. The two grey regions indicate where $M_r \le 1.84$ M$_\odot$ and $T\le 0.2$ GK, respectively.
• Figure 5: The mass distribution of the $Y_e$ (red) and log $\rho$ (blue) at $t=t_{\rm final}$.. The two black lines indicate where $M_r = 1.84$ M$_\odot$ and $T = 0.2$ GK, respectively.