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Cygnus OB2 - a young globular cluster in the Milky Way
Received 31 May 2000 / Accepted 23 June 2000
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 in diameter with a half light radius of , corresponding to pc at an assumed distance of 1.7 kpc. The interstellar extinction for member stars ranges from to , 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 , while the number of O stars amounts to . 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 . The total mass of Cyg OB2 is estimated to , 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 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.
Key words: stars: early-type ISM: dust, extinction Galaxy: globular clusters: general Galaxy: open clusters and associations: individual: Cyg OB2 Galaxy: stellar content
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000