The analysis of 2MASS data from the Cygnus region has revealed that Cygnus OB2 is considerably larger and more massive than previously thought. Previous surveys in the visible wavelength range, such as the extensive work of RLP, were substantially biased by the local extinction pattern in the field, which is caused by a foreground molecular cloud structure, known as the Great Cygnus Rift. The infrared data reveal now a spherically shaped association, with stellar extinctions reaching in the southern part of the field. Several highly reddened early-type objects that formerly were situated outside the association boundary are now lying within the association, indicating that they possibly are members of Cyg OB2. Examples are the massive binary system MWC349 (Cohen et al. 1985), the potential Luminous Blue Variable star G79.29+0.46 (Higgs et al. 1994), the Wolf-Rayet stars WR 145 and WR 146 (Niemela et al. 1998), and the recently discovered group of massive stars around the H II region DR 18 (Comerón & Torra 1999). Deep spectroscopic surveys of the stars in the highly obscured regions should help to consolidate the stellar population of Cyg OB2, and will improve our knowledge about the evolutionary state of the association.
RLP already recognised from their analysis that Cyg OB2 is unusually massive and compact, and hence suggested that it should be regarded as a new globular cluster of stars, similar to the blue globular clusters discovered by Hodge (1961)in the Large Magellanic Cloud (LMC). Indeed, the comparison of the association parameters, summarised in Table 2, to typical parameters for galactic OB associations, young open clusters, and globular clusters leads inevitably to this conclusion. For an OB association, Cyg OB2 is simply too compact. The typical mass density of OB associations is well below 0.1 pc-3 (Blaauw 1964) and the mean size amounts to 137 pc (Garmany 1994). Cyg OB2, however, shows an average mass density of pc-3 within the inner 10 pc and a diameter of only 60 pc. Hence, in contrast to classical OB association, Cyg OB2 should be a gravitationally bound system (Blaauw 1964). Garmany & Stencel (1992)suggest that Cyg OB2 might be the nucleus of an OB association, hence it could be considered as an open cluster. However, for an open cluster (as well as for an OB association) Cyg OB2 is by far too massive. Typical masses for open clusters are at most and do not exceed (Bruch & Sanders 1983) while the total mass of Cyg OB2 amounts to . Such a high mass is more representative for a small globular cluster.
In particular, the parameters of Cyg OB2 (as listed in Table 2) correspond fairly well to the typical parameters derived for young globular clusters in the LMC (e.g. Elson et al. 1987; Fischer et al. 1992). They have masses , central densities pc-3, and extend to radii pc. Little or no mass segregation is observed in these objects and they show only a small age spread (Elson et al. 1989). The same is true for Cyg OB2. The determination of radial profiles for different intrinsic luminosities did not reveal any mass segregation, and the analysis of a sub-sample of massive stars in Cyg OB2 by Massey & Thompson (1991)did not suggest a considerable age spread. Thus I believe it is reasonable to conclude that Cygnus OB2 shows the same properties as the system of young globular clusters in the LMC - hence Cygnus OB2 should be considered as a member of this population.
Cyg OB2 is the first object of this class that has been identified within our own Galaxy. Young globular clusters (or young populous clusters as termed by Hodge (1961)) seem not to be rare objects. They were first identified in the LMC but are now also found using the Hubble Space Telescope in a variety of extragalactic star forming regions (Ho 1997). Although young globulars are often observed in association with starburst phenomena, they also appear to form in more quiescent environments, such as circumnuclear rings in relatively undisturbed galaxies (e.g. Maoz et al. 1996). It seems therefore plausible that such a system can also form in our own Galaxy, although the formation conditions are yet to be explored.
In a review about young globulars in the LMC, Freeman (1980)asked why these systems "form in the LMC and not in the Galaxy"? Apparently, they do also form in our Galaxy, but heavy obscuration through the interstellar gas and dust make them difficult to detect - or may leave them unrecognised. Infrared surveys may help to improve the detectibility of such systems, although the sensitivity and confusion limits restrict such studies to within a few kpc. Alternatively, one may search for the fingerprints that these systems leave on the surrounding interstellar medium (ISM). An example is the thermal radio emission related to the ionisation of the ISM by the numerous massive stars. Indeed, the 120 O stars in Cyg OB2 produce a Lyman continuum flux of roughly photons s-1, making it probably the most important source of ionisation in the Cygnus X complex. It has been suggested that the Cygnus X complex could be a large Strömgren sphere powered by Cyg OB2 (Landecker 1984 and references therein), hence the search for similar complexes in surveys of thermal radio emission could provide hints for further young globulars in the Galaxy.
Another tracer may come from the emerging field of gamma-ray line spectroscopy. The massive stars in Cyg OB2 are an important source of radioisotopes that are expelled through stellar winds and supernova explosions into the interstellar medium. These isotopes eventually emit monoenergetic gamma-ray photons that arise from nuclear de-excitations following the radioactive decay. An example is the 1.809 MeV gamma-ray line arising from the decay of radioactive 26Al, an isotope with a lifetime of years (see Prantzos & Diehl 1996 for a review). Indeed, besides the central galactic radian, the Cygnus X region is the most intense source of 1.809 MeV photons known in the Galaxy, which can be well explained by 26Al production in massive stars within Cyg OB2 (Knödlseder et al., in preparation). A more detailed study of the galactic plane in the 1.809 MeV gamma-ray line by the upcoming INTEGRAL mission (Vedrenne et al. 1999) could lead to the identification of further regions of high star forming activity, and hence provide a unique tool to unveil young globular clusters in our own Galaxy.
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000