2000A&A...360..539K

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° in diameter with a half light radius of 13', corresponding to R_h=6.4pc at an assumed distance of 1.7kpc. The interstellar extinction for member stars ranges from A_V≃5^m to 20^m, 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 Γ=-1.6±0.1. The total mass of Cyg OB2 is estimated to (4-10)x10⁴M_☉, 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 ρ₀=40-150M_☉/pc³. 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 (1966PROE....5..111R). Cygnus OB2 would then be the first object of this class in the Milky Way.