Production of 26Al in Oxygen-Neon-Magnesium Novae

Aluminium 26



In the beta-decay of the ground state of 26Al (denoted 26gAl, t1/2 = 7.2 x 105 y), a characteristic 1.809 MeV gamma-ray is emitted. This signature of the presence of 26gAl has been widely observed throughout the Galaxy. Indeed, the observation of this gamma-ray proves the ongoing nucleosynthesis of 26gAl in astrophysical environments, given its short half-life on cosmological timescales. Reproduction of the Galactic 26gAl steady-state abundance implied by the observations (~ 3 M) provides a powerful constraint on nucleosynthesis model calculations. These calculations may also be used to determine the relative contributions to the 26gAl abundance by different types of astrophysical phenomena.

The amount of 26gAl produced in nova explosions on oxygen-neon-magnesium white dwarfs is thought to be relatively minor (~ 0.1 - 0.4 M). Nuclear uncertainties in the 25Al(p,γ)26Si and 26gAl(p,γ)27Si reactions may change this by a factor of ~2, however. A direct study of the 25Al(p,γ)26Si 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 25Al beam. To both guide this direct study, and to improve the accuracy of the current 25Al(p,γ)26Si calculations (based on indirect measurements), we have made a new measurement of the 26Si mass. We find the mass excess of 26Si to be(26Si) = -7139.5 ± 1.0 keV; this new mass leads to a reduction in the 25Al(p,γ)26Si 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 26gAl(p,γ)27Si reaction: we find ERCM = 184 ± 1 keV and ωγ = 35 ± 4stat ± 5sys μeV. These results lead to a decrease in the 26gAl(p,γ)27Si rate by as much as ~15% at nova temperatures.

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

Author: Parikh, A. R.