Classical-nova contribution to the milky way's Al26 abundance: Exit channel of the key Al25(p,γ)Si26 resonance

Classical novae are expected to contribute to the 1809-keV Galactic γ-ray emission by producing its precursor Al26, but the yield depends on the thermonuclear rate of the unmeasured Al25(p,γ)Si26 reaction. Using the β decay of P26 to populate the key Jπ=3⁺ resonance in this reaction, we report the first evidence for the observation of its exit channel via a 1741.6±0.6(stat)±0.3(syst) keV primary γ ray, where the uncertainties are statistical and systematic, respectively. By combining the measured γ-ray energy and intensity with other experimental data on Si26, we find the center-of-mass energy and strength of the resonance to be E _r=414.9±0.6(stat)±0.3(syst)±0.6(lit.) keV and ωγ=23±6(stat)-10+11(lit.) meV, respectively, where the last uncertainties are from adopted literature data. We use hydrodynamic nova simulations to model Al26 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 Al26.