Gamma-ray line emission from OB associations and young open clusters

OB associations and young open clusters constitute the most prolific nucleosynthesis sites in our Galaxy. The combined activity of stellar winds and core-collapse supernovae ejects significant amounts of freshly synthesised nuclei into the interstellar medium. Radioactive isotopes, such as $26Al$ or $60Fe,$ that have been co-produced in such events may eventually be observed by gamma-ray instruments through their characteristic decay-line signatures. However, due to the sensitivity and angular resolution of current (and even future) γ-ray telescopes, only integrated γ-ray line signatures are expected for massive star associations. In order to study such signatures and to derive constraints on the involved nucleosynthesis processes, we developed a multi-wavelength evolutionary synthesis model for massive star associations. This model combines latest stellar evolutionary tracks and nucleosynthesis calculations with atmosphere models in order to predict the multi-wavelength luminosity of a given association as function of its age. We apply this model to associations and clusters in the well-studied Cygnus region for which we re-determined the stellar census based on photometric and spectroscopic data. In particular we study the relation between 1.809 MeV γ-ray line emission and ionising flux, since the latter has turned out to provide an excellent tracer of the global galactic 1.809 MeV emission. We compare our model to COMPTEL 1.8 MeV γ-ray line observations from which we derive limits on the relative contributions from massive stars and core-collapse supernovae to the actual $26Al$ content in this region. Based on our model we make predictions about the expected $26Al$ and $60Fe$ line signatures in the Cygnus region. These predictions make the Cygnus region a prime target for the future INTEGRAL mission.