Astronomy and Astrophysics, volume 360, 539-548 (2000/8-2)
Cygnus OB2 - a young globular cluster in the Milky Way.
Abstract (from CDS):
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 Rh
=6.4pc at an assumed distance of 1.7kpc. The interstellar extinction for member stars ranges from AV
, 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)x104
, 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
. 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.
stars: early-type - ISM: dust, extinction - Galaxy: globular clusters: general - Galaxy: open clusters and associations: individual: Cyg OB2 - Galaxy: stellar content
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