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.Comment: 10 pages, 8 figures, accepted for publication in A&
