In a study led by Nick Wright we derived precise proper motions for 873 well known members of the associations, previously discovered using xray or spectroscopic observations.
The velocity dispersion among these members clearly shows that the associations is gravitationally unbound. It does not seem to be expandind, which argues against the association being an expanded star cluster disrupted by process such as residual gas expulsion or tidal heating. The presence of sub-structures in the phase (spatial and velocity) space suggests that the association is dynamically unevolved.
These results allow us to set important conclusions on the origin and initial conditions of the formation of this massive star forming region: Cyg OB2 was most likely born highly substructured and globally unbound, with the individual subgroups born in (or close to) virial equilibrium, and has not experienced significant dynamical evolution since then.
Figure 1 shows the proper motion vector map for 798 X-ray and spectroscopically selected stars towards Cygnus OB2 including 16 O-type stars, 34 B-type stars, and 748 X-ray selected stars. The 75 most extreme kinematic outliers, as noted in the text, have been removed. The dots show the current position of the stars, while the vectors shown the proper motions, colour-coded based on their direction of motion to highlight the kinematic substructure. The grey box shows the border of the X-ray observations used to identify members of Cyg OB2 and an empty black star symbol marks the centre of mass of the association. A representative 10 mas/yr vector is shown in the top left corner and a colour wheel showing the relationship between colour and position angle is shown in the top right corner. The background is a Spitzer 8 μm image.