Improved Direct Measurement of the 64.5 keV Resonance Strength in the 17O(p,a)14N Reaction at LUNA

The ${}^{17}$O(p,$\alpha$)${}^{14}$N reaction plays a key role in various astrophysical scenarios, from asymptotic giant branch stars to classical novae. It affects the synthesis of rare isotopes such as ${}^{17}$O and ${}^{18}$F, which can provide constraints on astrophysical models. A new direct determination of the $E_{\mathrm{R}}=64.5$~keV resonance strength performed at the Laboratory for Underground Nuclear Astrophysics accelerator has led to the most accurate value to date, $\omega \gamma =10.0\pm {1.4}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {0.7}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}}$~neV, thanks to a significant background reduction underground and generally improved experimental conditions. The (bare) proton partial width of the corresponding state at $E_{\mathrm{x}}=5672$~keV in ${}^{18}$F is $\Gamma_{\rm p} = 35 \pm 5_{\rm stat} \pm 3_{\rm syst}$~neV. This width is about a factor of 2 higher than previously estimated thus leading to a factor of 2 increase in the ${}^{17}$O(p,$\alpha$)${}^{14}$N reaction rate at astrophysical temperatures relevant to shell hydrogen-burning in red giant and asymptotic giant branch stars. The new rate implies lower ${}^{17}$O/${}^{16}$O ratios, with important implications on the interpretation of astrophysical observables from these stars.