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

II. The Cygnus region

¹ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, BP 4346, 31028 Toulouse Cedex 4, France
² LAEFF (INTA) Apdo. 50727, Madrid 28080, Spain
³ Observatoire de Genève, 1290 Sauverny, Switzerland
⁴ Observatoire Midi-Pyrénées, 14, avenue Édouard Belin, 31400 Toulouse, France

Corresponding author: J. Knödlseder, knodlseder@cesr.fr

Received: 22 March 2002
Accepted: 22 May 2002

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.