Constraints on key ${}^{17}$O($\alpha ,\gamma$)${}^{21}$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$(\alpha ,n{)}^{20}$Ne and ${}^{17}$O$(\alpha ,\gamma {)}^{21}$Ne reactions. The strengths of key ${}^{17}$O$(\alpha ,\gamma {)}^{21}$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 $\omega {\gamma }_{638}=4.85\pm 0.79$ $\mu$eV, $\omega {\gamma }_{721}=13.0_{-2.4}^{+3.3}$ $\mu$eV, $\omega {\gamma }_{814}=7.72\pm 0.55$ meV and $\omega {\gamma }_{1318}=136\pm 13$ 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 $\omega \gamma <140$ 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$(\alpha ,\gamma {)}^{21}$Ne thermonuclear reaction rate is calculated and compared to the literature. The effect of present uncertainties in the ${}^{17}$O$(\alpha ,\gamma {)}^{21}$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$(\alpha ,\gamma {)}^{21}$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.

arXiv commentAccepted in Physical Review C

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