Constraints on key O 17 (α,γ) Ne 21 resonances and impact on the weak s process

The efficiency of the slow neutron-capture process in massive stars is strongly influenced by neutron-capture reactions on light elements. At low metallicity, O16 is an important neutron absorber, but the effectiveness of O16 as a light-element neutron poison is modified by competition between subsequent O17(α,n)Ne20 and O17(α,γ)Ne21 reactions. The strengths of key O17(α,γ)Ne21 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 Ec.m.=612-1319 keV. We extract resonance strengths of ωγ638=4.85±0.79μeV, ωγ721=13.1-2.4+3.2μeV, ωγ814=7.72±0.55meV, and ωγ1318=136±13meV, for resonances at Ec.m.=638, 721, 814, and 1318 keV, respectively. We also report an upper limit for the 612 keV resonance of ωγ