The outskirts of Cygnus OB2 ^*,**

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany e-mail: fcomeron@eso.org
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: pasquali@mpia-hd.mpg.de
³ Departament d'Astronomia i Meteorologia, Universitat de Barcelona, 08028 Barcelona, Spain e-mail: [jordi;cesca]@am.ub.es

Received: 7 April 2008
Accepted: 15 May 2008

Abstract

Context. Cygnus OB2 is one of the richest OB associations in the local Galaxy, and is located in a vast complex containing several other associations, clusters, molecular clouds, and HII regions. However, the stellar content of Cygnus OB2 and its surroundings remains rather poorly known largely due to the considerable reddening in its direction at visible wavelength.

Aims. We investigate the possible existence of an extended halo of early-type stars around Cygnus OB2, which is hinted at by near-infrared color–color diagrams, and its relationship to Cygnus OB2 itself, as well as to the nearby association Cygnus OB9 and to the star forming regions in the Cygnus X North complex.

Methods. Candidate selection is made with photometry in the 2MASS all-sky point source catalog. The early-type nature of the selected candidates is confirmed or discarded through our infrared spectroscopy at low resolution. In addition, spectral classifications in the visible are presented for many lightly-reddened stars.

Results. A total of 96 early-type stars are identified in the targeted region, which amounts to nearly half of the observed sample. Most of them have featureless near-infrared spectra as expected from OB stars at the available resolution. Another 18 stars that display Brackett emission lines can be divided between evolved massive stars (most likely Be stars) and Herbig Ae/Be stars based on their infrared excesses. A component associated with Cygnus OB9/NGC 6910 is clearly identified, as well as an enhancement in the surface density of early-type stars at Cygnus X North. We also find a field population, consisting largely of early B giants and supergiants, which is probably the same as identified in recent studies of the inner circle around Cygnus OB2. The age and large extension of this population discards a direct relationship with Cygnus OB2 or any other particular association.

Conclusions. Earlier claims of the possible large extent of Cygnus OB2 beyond its central, very massive aggregate seem to be dismissed by our findings. The existence of a nearly ubiquitous population of evolved stars with massive precursors suggests a massive star formation history in Cygnus having started long before the formation of the currently observed OB associations in the region.