Production of 26Al in oxygen-neon-magnesium novae

In the beta-decay of the ground state of 26 Al (denoted 26g Al, t 1/2 = 7.2 × 10 5 y), a characteristic 1.809 MeV gamma-ray is emitted. This signature of the presence of 26g Al has been widely observed throughout the Galaxy. Indeed, the observation of this gamma-ray proves the ongoing nucleosynthesis of 26g Al in astrophysical environments, given its short half-life on cosmological timescales. Reproduction of the Galactic 26g Al steady-state abundance implied by the observations (~3 [Special characters omitted.] ) provides a powerful constraint on nucleosynthesis model calculations. These calculations may also be used to determine the relative contributions to the 26g Al abundance by different types of astrophysical phenomena.

The amount of 26g Al produced in nova explosions on oxygen-neon-magnesium white dwarfs is thought to be relatively minor (~0.1-0.4 [Special characters omitted.] ). Nuclear uncertainties in the 25 Al(p,[gamma]) 26 Si and 26g Al(p, [gamma]) 27 Si reactions may change this by a factor of ~2, however. A direct study of the 25 Al (p,[gamma]) 26 Si reaction has been proposed and accepted at the TRIUMF-ISAC radioactive beams facility in Vancouver, Canada, and is awaiting the production of a sufficiently-intense 25 Al beam. To both guide this direct study, and to improve the accuracy of the current 25 Al(p,[gamma]) 26 Si calculations (based on indirect measurements), we have made a new measurement of the 26 Si mass. We find the mass excess of 26 Si to be [Delta] ( 26 Si)= -7139.5 ±1 1.0 keV; this new mass leads to a reduction in the = 25 Al(p, [gamma]) 26 Si rate by as much as ~30% at nova temperatures. We have also made new measurements of the energy and strength of a key resonance for the 26g Al (p,[gamma]) 27 Si reaction: we find E R CM = 184 ± 1 keV and [omega][gamma] = 35 ± 4 stat ± 55 sys [mu]eV. These results lead to a decrease in the 26g Al(p, [gamma]) 27 Si rate by as much as ~15% at nova temperatures.

Our measurements of the 26 Si mass and the resonance in 26 Al(p,[gamma]) 27 Si both imply an increase in the 26g Al yield from novae, but still confirm the secondary nature of their contribution to the Galactic abundance of 26g Al.