Context. The Cygnus region harbours a huge complex ofmassive stars at a distance of 1.0-2.0kpc from us. About 170 Ostars are distributed over several OB associations, among which the CygOB2 cluster is by far the most important with about 100-120Ostars. These massive stars inject large quantities ofradioactive nuclei into the interstellar medium, such as 26Al and 60Fe, and their gamma-ray line decay signals can provide insight into the physics of massive stars and core-collapse supernovae. Aims. Past studies of the nucleosynthesis activity of Cygnus have concluded that the level of 26Aldecay emission as deduced from CGRO/COMPTEL observations was a factor2-3 above the predictions based on the theoretical yields available atthat time and on the observed stellar content of the Cygnus region. Wereevaluate the situation from new measurements of the gamma-ray decayfluxes with INTEGRAL/SPI (presented in a previous paper) and newpredictions based on recently improved stellar models. Methods. We built a grid of nucleosynthesis yields from recentmodels of massive stars. Compared to previous works, our data includesome of the effects of stellar rotation for the higher mass stars and acoherent estimate of the contribution from SNIb/c. We then developed apopulation synthesis code to predict the nucleosynthesis activity andcorresponding decay fluxes of a given stellar population of massivestars. Results. The observed decay fluxes from the Cygnus complex arefound to be consistent with the values predicted by populationsynthesis at solar metallicity; and yet, when extrapolated to thepossible subsolar metallicity of the Cygnus complex, our predictionsfail to account for the INTEGRAL/SPI measurements. The observed extentof the 1809keV emission from Cygnus is found to be consistentwith the result of a numerical simulation of the diffusion of 26Al inside the superbubble blown by Cyg OB2. Conclusions. Our work indicates that the past dilemma regardingthe gamma-ray line emission from Cygnus resulted from an overestimateof the 1809keV flux of the Cygnus complex, combined with anunderestimate of the nucleosynthesis yields. Our results illustrate theimportance of stellar rotation and SNIb/c in the nucleosynthesis of 26Al and 60Fe. The effects of binarity and metallicity may also be necessary to account for the observations satisfactorily. Key words: gamma rays: stars - gamma rays: ISM - open clustersand associations: individual: Cygnus OB - stars: early-type - ISM:bubbles - nuclear reactions, nucleosynthesis, abundances
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