1995A&A...298..445D

The recent nucleosynthesis activity in the Galaxy can be traced through the 1.809MeV gamma-ray line from radioactive ²⁶Al with its decay time of one million years. The COMPTEL imaging telescope aboard the Compton Observatory performed a survey of the Galaxy. This yields a map of 1.809MeV emission along the Galactic plane with structured emission over a wide longitude range and a marked asymmetry relative to the Galactic Centre. It suggests that the emission originates in rather localised regions, not necessarily concentrated in the inner disc of the Galaxy. Smooth intensity distributions as expected for a nova origin are very hard to reconcile with the data. In fact, none of the classical Galaxy-wide source distributions resembles the observations. The intermittent nature of both massive-star formation and ²⁶Al line emission, each on a time scale of a few million years, probably determines to a large extent the structured appearance of the 1.809MeV sky. Nearby sources of ²⁶Al may therefore contribute significantly to the observed intensity distribution. The Vela region in particular shows evidence for a single close-by ²⁶Al source, the Vela supernova remnant. Our estimated Galaxy-wide total ²⁶Al mass of ≤1M_☉ from diffuse 1.809MeV emission reduces the extreme requirements imposed on the candidate sources if a large-scale Galactic origin would apply. Such a reduced mass results in a Galaxy-wide isotope ratio estimate consistent with the value predicted from massive star nucleosynthesis in recent chemical evolution models for the Galaxy. We favour an ²⁶Al origin from massive stars, presumably core-collapse supernovae or Wolf-Rayet stars.