A Study of Cygnus OB2: Pointing the Way toward Finding Our Galaxy’s Super-Star Clusters

By M. Hanson. 2003

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New optical MK classification spectra have been obtained for 14 OB star candidates identified by Comerón et al. and presumed to be possible members of the Cyg OB2 cluster as recently described by Knödlseder. 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 with the properties of the Cyg OB2 cluster star has 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. 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), when the very young age of the stellar cluster (~2 × 106 yr) 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 nonmembers. Presently, estimates of the very high mass of this cluster (Mcl ≈ 104 M☉ and over 100 O stars) first made by Knödlseder remain higher than this study can support. Despite this, the recognition of Cyg OB2 as 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 kiloparsecs away (Clark & Negueruela), reminds us that we are woefully underinformed about the massive cluster population in our Galaxy. Extrapolations of the locally derived cluster luminosity function indicate that tens to perhaps a hundred of these very massive open clusters (Mcl ≈ 104 M☉, MV ≈ -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-up near-infrared spectroscopic observations, as were used by Comeron et al. to identify candidate members of the Cyg OB2 association.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

On the massive stellar population of the super star cluster Westerlund 1 (2005, unknown): “However subsequent near-IR (Comeron et al. 2002) and optical (Hanson 2003) spectroscopic surveys were unable to support such a conclusion, with Hanson concluding that many of the candidate OB stars are likely to be field contaminants and hence that the total O star census is unlikely to exceed 100.”
TOWARD COMPLETE STATISTICS OF MASSIVE BINARY STARS: PENULTIMATE RESULTS FROM THE CYGNUS OB2 RADIAL VELOCITY SURVEY (2014, background): “…Miczaika (1953); (12) Walborn (1973); (13) Contreras et al. (1997); (14) Rauw et al. (1999); (15) Hall (1974); (16) Romano (1969); (17) De Becker et al. (2004); (18) Nazé et al. (2008); (19) Nazé et al. (2012); (20) Otero (2008a); (21) Hanson (2003); (22) Stroud et al. (2010); (23) Otero (2008b)”
The Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. II. Analysis of 20 More Magellanic Cloud Stars and Results from the Complete Sample (2005, background): “– 32 – temperature scale of O stars do have implications in the interpretation of H-R diagrams and star-formation in clusters; see discussion in Hanson (2003) and Massey (2003).”
Astrophysical supplements to the ASCC‐2.5: Ia. Radial velocities of ∼55000 stars and mean radial velocities of 516 Galactic open clusters and associations (2007, background): “…cluster Melotte 20 (α Per) is also being called the Per OB3 association (Mel’nik & Efremov 1995; de Zeeuw et al. 1999), whereas the Cyg OB2 association is also being studied as a (super-star) cluster (Hanson 2003) or even as a young globular cluster (Knödlseder 2000) based on its mass estimates.”
Parallaxes and proper motions of interstellar masers toward the Cygnus X star-forming complex. I. Membership of the Cygnus X region (2011, methodology): “More recently, a nearer distance of 1.5 kpc was obtained by Hanson (2003) using new MK stellar classification spectra of the stars in Cyg OB 2.”
STAR FORMATION IN MASSIVE CLUSTERS VIA THE WILKINSON MICROWAVE ANISOTROPY PROBE AND THE SPITZER GLIMPSE SURVEY (2009, methodology): “We assign a distanceD = 1.7kpc Hanson (2003) to both bubbles (and to the WMAP sources atl = 76.0, l = 78.6 and 81.1).”
The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X (2007, background): “The number and age of OB stars in Cygnus X are taken to be those of the Cyg OB2 association (Knödlseder 2000; Hanson 2003), increasing uncertainties to take into account the other (smaller) OB associations which have different ages (Uyaniker et al. 2001).”