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

Abstract

Gamma-ray and microwave observations of the Cygnus region reveal an intense signal of 1.809 line emission, attributed to radioactive decay of , that is closely correlated with 53 GHz free-free emission, originating from the ionised interstellar medium. We modelled both emissions using a multi-wavelength evolutionary synthesis code for massive star associations that we applied to the known massive star populations in Cygnus. For all OB associations and young open clusters in the field, we determined the population age, distance, and richness as well as the uncertainties in all these quantities from published photometric and spectroscopic data. We propagate the population uncertainties in model uncertainties by means of a Bayesian method. The young globular cluster Cyg OB2 turns out to be the dominant nucleosynthesis and ionisation source in Cygnus. Our model reproduces the ionising luminosity of the Cygnus region very well, yet it underestimates production by about a factor of 2. We attribute this underestimation to shortcomings of current nucleosynthesis models, and suggest the inclusion of stellar rotation as possible mechanism to enhance production. We also modelled nucleosynthesis in the Cygnus region, yet the small number of recent supernova events suggests only little production. Consequently, a detection of the 1.137 and 1.332 decay lines of from Cygnus by the upcoming INTEGRAL observatory is not expected.