COMPTEL OBSERVATIONS OF GALACTIC AL-26 EMISSION

The recent nucleosynthesis activity in the Galaxy can be traced throug h the 1.809 MeV gamma-ray line from radioactive Al-26 With its decay t ime of one million years. The COMPTEL imaging telescope aboard the Com pton Observatory performed a survey of the Galaxy. This yields a map o f 1.809 MeV emission along the Galactic plane with structured emission over a wide longitude range and a marked asymmetry relative to the Ga lactic Centre. It suggests that the emission originates in rather loca lised regions, not necessarily concentrated in the inner disc of the G alaxy. Smooth intensity distributions as expected for a nova origin ar e very hard to reconcile with the data. In fact, none of the classical Galaxy-wide source distributions resembles the observations. The inte rmittent nature of both massive-star formation and Al-26 line emission , each on a time scale of a few million years, probably determines to a large extent the structured appearance of the 1.809 MeV sky. Nearby sources of Al-26 may therefore contribute significantly to the observe d intensity distribution. The Vela region in particular shows evidence for a single close-by Al-26 source, the Vela supernova remnant. Our e stimated Galaxy-wide total Al-26 mass of less than or equal to 1 M. fr om diffuse 1.809 MeV 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 Al-26 origin from massive stars, presumably core-collapse supernovae or Wol f-Rayet stars.