A Study of Cyg OB 2 : Pointing the Way Towards Finding Our Galaxy ’ s Super Star Clusters

New optical MK classification spectra have been obtained for 14 OB star candidates identified by Comerón et al. (2002) and presumed to be possible members of the Cyg OB2 cluster as recently described by Knödlseder (2000). All 14 candidate OB stars observed are indeed early-type stars, strongly suggesting the remaining 31 candidates by Comerón et al. are also early-type stars. A thorough investigation of the properties of these new candidate members compared to the properties of the Cyg OB2 cluster star have been completed, using traditional as well as newly revised effective temperature scales for O stars. The cooler O-star, effective temperature scale of Martins et al. (2002) gives a very close distance for the cluster (DM = 10.4). However, even using traditional effective temperature scales, Cyg OB2 appears to be slightly closer (DM = 10.8) than previous studies determined (DM = 11.2; Massey & Thompson 1992), when the very young age of the stellar cluster (∼ 2 x 10 6 yrs) is taken into account in fitting the late-O and early-B dwarfs to model isochrones. Of the 14 new OB stars observed for this study, as many as half appear to be significantly older than the previously studied optical cluster, making their membership in Cyg OB2 suspect. So, while some of the newly identified OB stars may represent a more extended halo of the Cyg OB2 cluster, the survey of Comerón et al. also picked up a large fraction of non-members. Presently, estimates of the very high mass of this cluster (M cl ≈ 10 4 M ⊙ and over 100 O stars) first made by Knödlseder (2000) remain higher than this study can support. Despite this, the recognition of Cyg OB2 being a more massive and extensive star cluster than previously realized using 2MASS images, along with the recently recognized candidate super star cluster Westerlund 1 only a few kpc away (Clark & Negueruela 2002), reminds us that we are woefully under-informed about the massive cluster population in our Galaxy. Extrapolations of the locally derived cluster luminosity function indicate 10s to perhaps 100 of these very massive open clusters (M cl ≈ 10 4 M ⊙ , M V ≈ −11) should exist within our galaxy. Radio surveys will not detect these massive clusters if they are more than a few million years old. Our best hope for remedying this shortfall is through deep infrared searches and follow …