Cygnus OB2 – a young globular cluster in the Milky Way
Résumé
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 .4 pc at an assumed distance of 1.7 kpc. 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) × 10 4 M , where the primary uncertainty comes from the unknown lower mass cutoff. Using the radial density profile of the association, the central mass density is determined to ρ 0 =40–150 M 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 reclassify 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.
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