Theoretical and Experimental Evaluation of the 16O(p,a)13N Reaction Rate and Its Impact on Astrophysical Models

The nuclear reactions 16O(p,α)13N and 13N(α,p)16O play critical roles in stellar nucleosynthesis within Type Ia (SNIa) and core-collapse supernovae (CCSN). The 16O(p,α)13N reaction influences Ca/S abundance ratios in SNIa, while the inverse 13N(α,p)16O reaction affects 13C production in CCSN. However, the reaction rates for both remain poorly constrained due to limited direct measurements and uncertainties in nuclear structure inputs. This work presents a new direct measurement of the 16O(p,α)13N cross-section using the MUSIC active-target detector over the centre-of-mass energy range Ecm = 5.7−7.1MeV, where previous datasets exhibit significant discrepancies. The resulting cross-sections were used to recalculate the stellar reaction rate under SNIa conditions, revealing a maximum enhancement of only 1.5 times the CF88 rate. The new rate was substantially lower than previous estimates of up to a factor of seven. These findings indicate that 16O(p,α)13N alone cannot account for the observed Ca/S variations, suggesting a need to re-evaluate alternative reactions, such as 16O +12C fusion. To constrain the inverse 13N(α,p)16O rate relevant to CCSN, the low-energy cross section was derived from R-matrix analysis of available 17F state information above α-threshold. By combining the R-matrix derived cross-section at low energies with the new experimental 16O(p,α)13N data at higher energies, an updated 13N(α,p)16O rate was obtained. The new 13N(α,p)16O rate exceeded previous estimates by up to a factor of three in the temperature range T = 0.4−0.7GK. Moreover, the contribution of thermally excited states was assessed. The results show that, for the temperatures of T < 5GK, the cross-section measured for the ground-state transition adequately represents the stellar reaction rate.