Astron. Astrophys. 360, 539-548 (2000)

1. Introduction

The study of galactic OB associations provides the key to a number of astrophysical questions, such as the star formation process and efficiency, the interaction of massive stars with the interstellar medium, the characterisation of the initial mass function at the high-mass end, the study of stellar nucleosynthesis, chemical evolution, and galactic recycling processes, and the evolution of binary systems. The Cygnus OB2 association is a particularly good region to address such questions, since it is extremely rich (e.g. Reddish et al. 1966, hereafter RLP), and contains some of the most luminous stars known in our Galaxy (e.g. Torres-Dodgen et al. 1991).

The most comprehensive study of the size and shape of Cyg OB2 has been performed by RLP who inferred an elliptical shape with major and minor axes of and , respectively (see also Fig. 5). They estimate more than 3000 members of which at least 300 are of OB spectral type, resulting in a total stellar mass between  . For their analysis, RLP performed star counts on the blue and red plates of the Digitized Sky Survey (DSS), reaching limiting magnitudes around . Although this limit assures a reasonable complete census for unobscured associations, the extreme reddening in and around Cyg OB2 hampers the detection of even OB stars. With an estimated distance of 1.7 kpc (e.g. Massey & Thompson 1991) Cyg OB2 is located behind the Great Cygnus Rift, causing visual extinction from to at least . A number of observations suggest that Cyg OB2 could indeed be larger than the RLP estimate, and that the observed morphology is rather an artifact of the particular extinction pattern in the field. The association boundary determined by RLP fits suspiciously well in a region of low CO column density (cf. Fig. 1) and low visual extinction (Dickel & Wendker 1978), indicating that the visual star densities are probably biased by the extinction pattern. There is a considerable number of early-type stars in the obscured area south and south-east of Cyg OB2 that are estimated to lie at the same distance as Cyg OB2, and that could indeed be bright member stars of the association. Examples are the Wolf-Rayet stars WR 145 and WR 146 (Niemela et al. 1998), the potential Luminous Blue Variable star G79.29+0.46 (Higgs et al. 1994), the massive binary system MWC 349 (Cohen et al. 1985), or the recently discovered group of massive stars around the H II region DR 18 (Comerón & Torra 1999).

Fig. 1. Velocity integrated (-10 to 20 km s-1) CO intensity map of the region around Cyg OB2 (from Leung & Thaddeus 1992). The ellipsoid indicates the size of Cyg OB2 as determined by RLP. Massive stars that may be associated to Cyg OB2 but lying outside the classical Cyg OB2 boundary of RLP are indicated by asterisks. Visual extinction estimates are quoted in parentheses.

The availability of the Two Micron All Sky Survey (2MASS) provides now an excellent opportunity to re-address the question on the morphology and stellar content of Cyg OB2. This survey covers the infrared bands J, H, and K which have proven to be an excellent tool for unveiling embedded star clusters due to the reduced impact of dust extinction at longer wavelengths. In the following I will use these data to determine the morphology and stellar content of Cyg OB2. It will turn out that the association is much larger and much richer than previously thought, making it the most massive young stellar association known in our Galaxy.

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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