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

The origin of the long-lived (1.07 Myr mean life) radioactive ²⁶Al, which has been observed in the Galactic interstellar medium from its 1.809 MeV decay gamma-ray line emission, has been a persistent problem for over 20 years. Wolf-Rayet (W-R) winds were thought to be the most promising source, but their calculated ²⁶Al yields are not consistent with recent analyses of the 1.809 MeV emission from the nearest W-R star and nearby OB associations. The expected ²⁶Al yield from the W-R star exceeds, by as much as a factor of 3, that set by the 2 σ upper limit on the 1.809 MeV emission, while the W-R yields in the OB associations are only about of that required by the 1.809 MeV emission. We suggest that a solution to these problems may lie in ²⁶Al from a previously ignored source: explosive nucleosynthesis in the core-collapse Type Ib/c supernovae (SNe Ib/c) of W-R stars that have lost most of their mass to close binary companions. Recent nucleosynthetic calculations of SNe Ib/c suggest that their ²⁶Al yields depend very strongly on the final pre-SN mass of the W-R star and that those with final masses around 6-8 M_☉ are expected to produce as much as 10^-2 M_☉ of ²⁶Al per SN. Such binary SNe Ib/c make up only a small fraction of the current SNe Ib/c and only about 1% of all Galactic core-collapse SNe. But they appear to be such prolific sources that the bulk of the present ²⁶Al in the Galaxy may come from just a few hundred close binary SNe Ib/c, and the intense 1.809 MeV emission from nearby OB associations may come from just one or two such SNe. More extensive SN Ib/c calculations of the ²⁶Al yields versus pre-SN mass are clearly needed to test this possibility.