26Al in the interstellar medium

Several different lines of physical reasoning have converged on the importance of the radioactive nucleus ²⁶Al. The sciences of meteoritics, nucleosynthesis, gamma-ray astronomy, galactic chemical evolution, solar system formation, and interstellar chemistry all place this nucleus in a central position with possible profound implications. Perhaps more importantly the study of this radioactivity can unite these diverse fields in a complicated framework which will benefit all of them. This review traces the evolution of ideas concerning ²⁶Al in the context of these disciplines.²⁶Al was first discussed for the possibility that its decay energy could melt meteorite parent bodies, and its daughter, ²⁶Mg, was later found in meteorites with enhanced abundance. It was also among the first radioactivities expected to be synthesized in interestingly large quantities in nucleosynthetic events. The first definitive detection of gamma-rays from an interstellar radioactivity is that of 1.809 MeV gamma-rays from ²⁶Al. This discovery has many implications, some of which are outlined here. The whole problem of isotopic anomalies in meteorites is greatly influenced by the specific issues surrounding excess ²⁶Mg, whether it represents in situ decay of ²⁶Al or memory of conditions of the ISM. The relationships among these ideas and their implications are examined.