Confirmation of a new resonance in 26Si and contribution of classical novae to the galactic abundance of 26Al

Abstract

The 25Al(p ,γ ) reaction has long been highlighted as a possible means to bypass the production of 26Al cosmic γ rays in classical nova explosions. However, uncertainties in the properties of key resonant states in 26Si have hindered our ability to accurately model the influence of this reaction in such environments. We report on a detailed γ -ray spectroscopy study of 26Si and present evidence for the existence of a new, likely ℓ =1 , resonance in the 25Al + p system at Er=153.9 (15 ) keV. This state is now expected to provide the dominant contribution to the 25Al(p ,γ ) stellar reaction rate over the temperature range, T ≈0.1 −0.2 GK. Despite a significant increase in the rate at low temperatures, we find that the final ejected abundance of 26Al from classical novae remains largely unaffected even if the reaction rate is artificially increased by a factor of 10. Based on new, galactic chemical evolution calculations, we estimate that the maximum contribution of novae to the observed galactic abundance of 26Al is ≈0.2 M⊙ . Finally, we briefly highlight the important role that super-asymptotic giant branch stars may play in the production of 26Al.