Cygnus OB2 - a young globular cluster in the Milky Way

The morphology and stellar content of the Cygnus OB2 association has been determined using 2MASS infrared observations in the J, H, and K bands. The analysis reveals a spherically symmetric association of 2 deg in diameter with a half light radius of 13', corresponding to Rh = 6.4 pc at an assumed distance of 1.7 kpc. The interstellar extinction for member stars ranges from Av = 5m to 20m, which led to a considerable underestimation of the association size and richness in former optical studies. From the infrared colour-magnitude diagram, the number of OB member stars is estimated to 2600 +/- 400, while the number of O stars amounts to 120 +/- 20. This is the largest number of O stars ever found in a galactic massive star association. The slope of the initial mass function has been determined from the colour-magnitude diagram to Gamma=-1.6 +/- 0.1. The total mass of Cyg OB2 is estimated to (4-10)*10^4 Msol, where the primary uncertainty comes from the unknown lower mass cut-off. Using the radial density profile of the association, the central mass density is determined to rho0=40-150 Msol pc^-3. Considering the mass, density, and size of Cyg OB2 it seems untenable to classify this object still as OB association. Cygnus OB2 more closely resembles a young globular cluster like those observed in the Large Magellanic Cloud or in extragalactic star forming regions. It is therefore suggested to re-classify Cygnus OB2 as young globular cluster - an idea which goes back to Reddish et al. (1966). Cygnus OB2 would then be the first object of this class in the Milky Way.