Constraints on key ${}^{17}$O($\hat{I}\pm ,{\hat{I}}^3$)${}^{21}$Ne resonances and impact on the weak s-process

Abstract

The efficiency of the slow neutron-capture process in massive stars is strongly influenced by neutron-capture reactions on light elements. At low metallicity, ${}^{16}$O is an important neutron absorber, but the effectiveness of ${}^{16}$O as a light-element neutron poison is modified by competition between subsequent ${}^{17}$O$(\hat{I}\pm ,n{)}^{20}$Ne and ${}^{17}$O$(\hat{I}\pm ,{\hat{I}}^3{)}^{21}$Ne reactions. The strengths of key ${}^{17}$O$(\hat{I}\pm ,{\hat{I}}^3{)}^{21}$Ne resonances within the Gamow window for core helium burning in massive stars are not well constrained by experiment. This work presents more precise measurements of resonances in the energy range $E_{c.m.}=612-1319$ keV. We extract resonance strengths of $\ddot{I}‰{\hat{I}}_{638}^3=4.85\pm 0.79$ $\hat{I}¼$eV, $\ddot{I}‰{\hat{I}}_{721}^3=13.0_{-2.4}^{+3.3}$ $\hat{I}¼$eV, $\ddot{I}‰{\hat{I}}_{814}^3=7.72\pm 0.55$ meV and $\ddot{I}‰{\hat{I}}_{1318}^3=136\pm 13$ meV, for resonances at $E_{c.m.}=$ 638, 721, 814 and 1318 keV, respectively. We also report an upper limit for the 612 keV resonance of $\ddot{I}‰{\hat{I}}^3<140$ neV ($95\%$ c.l.), which effectively rules out any significant contribution from this resonance to the reaction rate. From this work, a new ${}^{17}$O$(\hat{I}\pm ,{\hat{I}}^3{)}^{21}$Ne thermonuclear reaction rate is calculated and compared to the literature. The effect of present uncertainties in the ${}^{17}$O$(\hat{I}\pm ,{\hat{I}}^3{)}^{21}$Ne reaction rate on weak s-process yields are then explored using post-processing calculations based on a rotating $20M_{\odot }$ low-metallicity massive star. The resulting ${}^{17}$O$(\hat{I}\pm ,{\hat{I}}^3{)}^{21}$Ne reaction rate is lower with respect to the pre-existing literature and found to enhance weak s-process yields in rotating massive star models.

Accepted in Physical Review C