Additional resonant contribution to the potential model for the 12C(alpha,gamma)16O reaction

TL;DR

This study investigates the impact of additional resonances on the potential model for the 12C(α,γ)16O reaction, showing they refine the astrophysical S-factors without significantly altering reaction rates at helium burning temperatures.

Contribution

The paper introduces the consideration of additional resonant contributions to the potential model, improving the description of the reaction and scattering data near the alpha threshold.

Findings

Elastic scattering data below 5 MeV are well reproduced by the potential model.

Additional resonances enhance the astrophysical S-factors, especially near the subthreshold 2+ state.

The reaction rates at helium burning temperatures are not significantly affected by these resonances.

Abstract

The additional resonant contribution to the potential model is examined in $\alpha$+$^{12}$C elastic scattering and the low-energy $^{12}$C($\alpha$,$\gamma$)$^{16}$O reaction. The excitation function of elastic scattering below $E_{c.m.}= 5$ MeV seems to be reproduced by the potential model satisfactorily, and it is not profoundly disturbed by the additional resonances. The weak coupling is good enough to describe the $^{16}$O structure in the vicinity of the $\alpha$-particle threshold, especially below $E_{c.m.}= 8$ MeV, corresponding to the excitation energy $E_x \approx 15$ MeV. The additional resonances give the complement of the astrophysical $S$-factors from the simple potential model. The $S$-factor of $^{12}$C($\alpha$,$\gamma$)$^{16}$O at $E_{c.m.}=300$ keV is dominated by the $E$2 transition, which is enhanced by the subthreshold 2$^+_1$ state at $E_x= 6.92$ MeV. The contribution from the subthreshold 1$^-_1$ state at $E_x= 7.12$ MeV is predicted to be small. The additional resonances do not give the large contribution to the thermonuclear reaction rates of $^{12}$C($\alpha$,$\gamma$)$^{16}$O at helium burning temperatures.