Constraints on the 12 C( α , γ ) 16 O and 16 O+ 16 O Reaction Rates from Binary Black Holes Detected via Gravitational Wave Signals

Abstract Gravitational-wave observations of binary black hole (BH) mergers provide a novel avenue for testing massive-star evolution and the resulting BH mass spectrum. Recent population analyses under the hierarchical-merger hypothesis have provided evidence for the BH mass gap and inferred its lower edge to ∼44–68 M ⊙ . Motivated by these findings, we compute low-metallicity ( Z = 10 −5 ) helium star models with MESA and systematically explore the effect of uncertainties in the 12 C( α , γ ) 16 O and 16 O+ 16 O reaction rates on the final fate of these massive stars. Varying the 12 C( α , γ ) 16 O reaction rate from −3 σ to +3 σ , we find that the predicted BH mass gap shifts from ∼104–184 M ⊙ to ∼45–135 M ⊙ . In contrast, scaling the 16 O+ 16 O reaction rate by global factors of 0.1, 1, and 10 has only a modest effect on the lower edge of the BH mass gap (less than 5 M ⊙ ), and shifts the upper edge by more than 10 M ⊙ . Using the predictions of our models together with the literature estimates for the lower edge of the BH mass gap, we constrain the astrophysical S factor of 12 C( α , γ ) 16 O reaction at 300 keV of S 300 ≃ 137.6–263.4 keV barn.