Classical-Nova Contribution to the Milky Way's ${}^{26}$Al Abundance: Exit Channel of the Key ${}^{25}$Al($p,{\hat{I}}^3$)${}^{26}$Si Resonance

Abstract

Classical novae are expected to contribute to the 1809-keV Galactic ${\hat{I}}^3$-ray emission by producing its precursor ${}^{26}$Al, but the yield depends on the thermonuclear rate of the unmeasured ${}^{25}$Al($p,{\hat{I}}^3$)${}^{26}$Si reaction. Using the ${\hat{I}}^2$ decay of ${}^{26}$P to populate the key $J^{\ddot{I}}€=3^{+}$ resonance in this reaction, we report the first evidence for the observation of its exit channel via a $1741.6\pm 0.6(\text{stat})\pm 0.3(\text{syst})$ keV primary ${\hat{I}}^3$ ray, where the uncertainties are statistical and systematic, respectively. By combining the measured ${\hat{I}}^3$-ray energy and intensity with other experimental data on ${}^{26}$Si, we find the center-of-mass energy and strength of the resonance to be $E_r=414.9\pm 0.6(\text{stat})\pm 0.3(\text{syst})\pm 0.6(\text{lit.})$ keV and $\ddot{I}‰{\hat{I}}^3=23\pm 6(\text{stat}{)}_{-10}^{+11}(\text{lit.})$ meV, respectively, where the last uncertainties are from adopted literature data. We use hydrodynamic nova simulations to model ${}^{26}$Al production showing that these measurements effectively eliminate the dominant experimental nuclear-physics uncertainty and we estimate that novae may contribute up to 30% of the Galactic ${}^{26}$Al.