Gamma-ray observations and massive stars

Gamma-rays from astrophysical sources testify energetic processes such as nucleosynthesis and cosmic ray collisions. Gamma-rays are observable from throughout the Galaxy, unattenuated by interstellar matter, provided their intensity exceeds the current instrumental sensitivity level (similar to10(-5) ph cm(-2)s(-1) at 1 MeV). Massive stars are at the origin of relevant sources: The all-sky image in the 1.809 MeV gamma-ray line from radioactive Al-26 traces nucleosynthesis throughout the Galaxy. The structure of this emission along the plane of the Galaxy suggests massive stars as dominating sources of this radioactivity. Discrimination of the contribution from core collapse supernova against that from WR-wind ejected hydrostatic nucleosynthesis products may be obtained from Fe-60 gamma-ray line observations, or from spatial-profile consequences of the metallicity dependence of Al-26 production in theories for both source sites. As a single source, the nearest WR star in the gamma(2) Vel system is found to eject less Al-26 into interstellar space than current theories predict. However, a more adequate comparison must be based on a time-dependent Al-26 light-curve of the system. Furthermore, continuum gamma-ray production in WR binaries through wind-wind interaction, and constraints on the low-energy cosmic ray origin in WR winds through characteristic nuclear deexcitation line studies are targets of research. Studies stimulated by COMPTEL'S 3-7MeV excess report from the Orion region indicate that the gamma-ray line measurements could separate the origins from supernova ejecta and wind material. The COMPTEL Orion result is now attributed chiefly to an instrumental artifact, and has been withdrawn. Nevertheless, the search for MeV emission from massive star clusters, as well as from interacting binaries such as WR 140, promises a unique test of particle acceleration scenarios related to the source mechanism for cosmic ray production.