Implications for Type Ia Supernova Nucleosynthesis from an Experimentally Constrained $^{16}$O$(p,\alpha)^{13}$N Reaction Rate

TL;DR

This study provides a new, experimentally constrained reaction rate for $^{16}$O$(p,eta)^{13}$N, showing it is about 1.5 times higher than previous estimates, which impacts supernova nucleosynthesis models and challenges previous assumptions of a sevenfold enhancement.

Contribution

The paper presents the first direct measurement of the $^{16}$O$(p,eta)^{13}$N reaction rate at astrophysical energies, reducing uncertainties and revising the enhancement factor needed in supernova models.

Findings

The new reaction rate is approximately 1.5 times higher than the REACLIB rate.

The previously suggested factor of seven enhancement is excluded.

Uncertainties in other oxygen-burning reactions remain significant.

Abstract

The $^{16}$O$(p,\alpha)^{13}$N reaction plays a key role in shaping the $\alpha$-particle abundance during explosive oxygen burning in Type Ia supernovae. By enhancing $\alpha$-production, this reaction directly affects the calcium-to-sulphur (Ca/S) and argon-to-sulphur (Ar/S) ratios, which serves as a tracer of progenitor metallicity. However, recent work suggests that the rate must be enhanced by a factor of up to seven over the standard value to explain observed Ca/S ratios across a range of progenitor metallicities. To explore this impact, available experimental cross-section data for the $^{16}$O$(p,\alpha)^{13}$N reaction have been compiled and critically evaluated. Significant discrepancies are identified in the low-energy region ($E_{\mathrm{cm}}$ = 5.7--7.0 MeV), primarily due to limitations of the activation method. To resolve this, the first direct measurement at astrophysical energies has been performed using the MUSIC active-target detector. The new $^{16}$O$(p,\alpha)^{13}$N thermonuclear reaction rate is found to be approximately 1.5 times higher than the REACLIB rate in the temperature range T = 3--4 GK, with more constrained uncertainties that resolve the previously large spread among existing data. The suggested factor of seven enhancement is excluded and these results indicate that this reaction alone cannot fully explain the variation in the Ca/S and Ar/S ratios observed across different progenitor metallicities. Therefore, future work should focus on reducing the uncertainties in other key oxygen-burning reactions, particularly $^{16}$O+$^{16}$O and $^{12}$C+$^{16}$O. Further reducing the constraints on the $^{16}$O$(p,\alpha)^{13}$N rate is also needed to fully determine to whether a nuclear physics solution to this discrepancy is possible.