The challenge to understanding 26Al emission from OB associations

From recent models of 26Al synthesis in core-collapse (Type II and Ib/c) supernovae and Wolf-Rayet stars, we have calculated the expected 1.809 MeV line fluxes from the decay of 26Al, produced in nearby OB associations. We find that 26Al synthesis solely in Type II supernovae and Wolf-Rayet stars fails by factors ⩾3 to account for the observed 1.809 MeV line fluxes from the nearby OB associations, Cyg OB2, Vela OB1, and Ori OB1a. However, the expected line fluxes from the six other comparable OB associations are all consistent with the observed upper limits. Thus, we suggest that the line emission has a stochastic component resulting from the very limited number of massive stars in each of these OB associations. As one possible source of this component, we have suggested 26Al synthesis from the previously ignored, binary SNIb/c supernovae, which seem to be able to account for the rarer but much more intense 1.809 MeV flux observed from the three OB associations.