The Galactic 26 Al Problem and the Close Binary Type Ib/c Supernova Solution?

The origin of the long-lived radioactive 26Al, which has been observed in theGalactic interstellar medium from its 1.809 MeV decay gamma-ray line emission,has been a persistent problem for over twenty years. Wolf-Rayet (WR) winds werethought to be the most promising source, but their calculated 26Al yields arenot consistent with recent analyses of the 1.809 MeV emission from the nearestWR star and nearby OB associations. The expected 26Al yield from the WR starexceeds by as much as a factor of 3, that set by the 2-sigma upper limit on the1.809 MeV emission, while the WR yields in the OB associations are only about1/3 of that required by the 1.809 MeV emission. We suggest that a solution tothese problems may lie in 26Al from a previously ignored source: explosivenucleosynthesis in the core collapse SNIb/c supernovae of WR stars that havelost most of their mass to close binary companions. Recent nucleosyntheticcalculations of SNIb/c suggest that their 26Al yields depend very strongly onthe final, pre-supernova mass of the WR star, and that those with final massesaround 6 to 8 solar masses are expected to produce as much as 0.01 solar massesof 26Al per supernova. Such binary SNIb/c make up only a small fraction of thecurrent SNIb/c and only about 1% of all Galactic core collapse supernovae. Theyappear to be such prolific sources that the bulk of the present 26Al in theGalaxy may come from just a few hundred close binary SNIb/c and the intense1.809 MeV emission from nearby OB associations may come from just one or twosuch supernova.Comment: Accepted for publication in Astrophysical Journal Letters, 611,10 August 200