2006A&A...458..855S

The Cygnus X region is one of the richest star formation sites in the Galaxy. There is a long-standing discussion about whether the region is a chance superposition of several complexes along the line of sight or a single coherent complex at a distance of 1.5 to 2kpc. Combining a ¹³CO 2-1 survey taken with the KOSMA 3m telescope with mid-IR images from MSX provides a way to improve our understanding of the spatial structure of the complex. The physical properties of the molecular gas can be derived in more detail as it was done in former studies. Cygnus X has been mapped in ¹³CO J =2-1 (10.8deg²) at an angular resolution of 130'', as well as for smaller areas in ¹²CO and ¹³CO J=3-2 (90''), using the KOSMA 3m submm-telescope. We identified 91 clumps in ¹³CO 2-1 that have a typical excitation temperature of 10-30K, an average density of 1.3x10³cm^–3, radii of 1-8pc, and masses of a few hundred to several ten thousand M_☉. The main cloud complexes, the northern part (M≃2.8x10⁵M_☉) including DR21 and W75N and the southern region (M≃4.5x10⁵M_☉) with IC 1318 b/c and AFGL2591, show differences in their physical properties. The ¹³CO emission is closely associated with mid-IR emission seen with MSX. We find evidence that Cygnus OB2 and Cygnus OB9 are affecting the molecular material in Cygnus X. Since essentially all molecular cloud complexes in Cygnus X form groups that are connected by molecular emission (visible in channel and position-velocity maps) and partly show evidence of interaction with UV radiation, we conclude that most of the objects seen in this region are located at the same distance, i.e., that of the OB2 cluster at ∼1.7kpc, which is also consistent with the distances of other OB associations (OB9, OB1) in Cygnus X.