A&A 486, 453–466 (2008) Astronomy DOI: 10.1051/0004-6361:200809917 & c ESO 2008 Astrophysics The outskirts of Cygnus OB2, F. Comerón1,, A. Pasquali2, F. Figueras3, and J. Torra3 1 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany e-mail:
[email protected] 2 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail:
[email protected] 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.