New estimates of the gamma-ray line emission of the Cygnus region from INTEGRAL/SPI observations

摘要

Context. The Cygnus region harbours a huge complex of massive stars at a distance of 1.0-2.0kpc from us. About 170Ostars are distributed over several OB associations, among which the CygOB2 cluster is by far the largest with about 100-120Ostars. As a consequence of their successive nuclear-burning episodes, these massive stars inject large quantities of radioactive nuclei into the interstellar medium such as 26Al and 60Fe. The gamma-ray line signal from the latter is a solid tracer of ongoing nucleosynthesis. Aims. We want to compare the decay emission from the Cygnus region with predictions of recently improved stellar models. As a first step, we establish observational constraints upon the gamma-ray line emission from 26Al and 60Fe, with particular emphasis placed on separating the emission due to the Cygnus complex from the foreground and background mean Galactic contributions. Methods. We used the high-resolution gamma-ray spectrometer INTEGRAL/SPI to analyse the 26Al and 60Fe decay signal from the Cygnus region. The weak gamma-ray line emissions at 1809 and 1173/1332keV have been characterised in terms of photometry, spectrometry, and source morphology. Results. The 1809keV emission from Cygnus is centred on the position of the Cyg OB2 cluster and has a typical size of9or more. The total 1809keV flux from the Cygnus region is phcm-2s-1, but the flux really attributable to the Cygnus complex reduces to phcm-2s-1. The 1809keV line centroid agrees with expectations considering the direction and distance of the Cygnus complex, and the line width is consistent with typical motions of the interstellar medium. No decay emission from 60Fe has been observed from the Cygnus region and an upper limit of phcm-2s-1 was derived. Conclusions. The determination in a coherent way of the gamma-ray line emission of the clustered OB population of the nearby Cygnus complex will allow a more accurate comparison of observations with theoretical expectations based on the most recent stellar models. Key words: gamma rays: observations - Galaxy: open clusters and associations: individual: Cyg OB - stars: early-type - ISM: bubbles - nuclear reactions, nucleosynthesis, abundances