Exploiting Isospin Symmetry to Study the Role of Isomers in Stellar Environments

Abstract

Proton capture on the excited isomeric state of ${}^{26}\mathrm{Al}$ strongly influences the abundance of ${}^{26}\mathrm{Mg}$ ejected in explosive astronomical events and, as such, plays a critical role in determining the initial content of radiogenic ${}^{26}\mathrm{Al}$ in presolar grains. This reaction also affects the temperature range for thermal equilibrium between the ground and isomeric levels. We present a novel technique, which exploits the isospin symmetry of the nuclear force, to address the long-standing challenge of determining proton-capture rates on excited nuclear levels. Such a technique has in-built tests that strongly support its veracity and, for the first time, we have experimentally constrained the strengths of resonances that dominate the astrophysical ${}^{26m}\mathrm{Al}(p,\gamma ){}^{27}\mathrm{Si}$ reaction. These constraints demonstrate that the rate is at least a factor $\sim 8$ lower than previously expected, indicating an increase in the stellar production of ${}^{26}\mathrm{Mg}$ and a possible need to reinvestigate sensitivity studies involving the thermal equilibration of ${}^{26}\mathrm{Al}$.