On the massive star contents of Cygnus OB2^*

¹ European Southern Observatory, Karl-Schwarzschild-Str.2, 85748 Garching bei München, Germany
² Space Telescope European Coordinating Facility, Karl-Schwarzschild-Str.2, 85748 Garching bei München, Germany e-mail: apasqual@eso.org
³ Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: rodighiero@pd.astro.it
⁴ Institute of Astronomy, Bulgarian Academy of Sciences, 72 Tsarigradsko Shousse, Blvd, 1784 Sofia, Bulgaria e-mail: vall@astro.bas.bg
⁵ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Liverpool CH41 1LD, UK e-mail: bdf@astro.livjm.ac.uk
⁶ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany e-mail: belen@tls-tautenburg.de
⁷ Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain e-mail: mcz@astrax.fis.ucm.es
⁸ ESTEC, Research & Scientific Support Department (SCI-SR), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands e-mail: astankov@rssd.esa.int
⁹ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: gredel@caha.es

Corresponding author: F. Comerón, fcomeron@eso.org

Received: 18 July 2001
Accepted: 25 April 2002

Abstract

We present a near-infrared spectroscopic survey of a large area centered on the Cygnus OB2 association aimed at constraining its massive star contents. Our goal is to establish a nearly complete list of O-type members of the association, both to examine recent claims based on starcounts that suggest a richer content than previously thought, and to provide a suitable database for further studies of the entire high-mass end of one of the richest associations of the Galaxy. The target selection is based on the JHK photometry published in the 2MASS all-sky survey. We identify 46 new early-type candidates, most of them expected to be O-type stars, plus 16 new stars with emission in Brγ and often in other lines as well, characteristic of evolved massive stars undergoing intense mass loss. We also present spectra of three luminous stars with CO overtone emission, one of them having also intense H₂ emission and being associated with compact nebulosity. By considering our findings, those of other authors, and plausible completeness corrections, we estimate the number of O-type stars or stars having evolved from a O-type progenitor to be 90–100, slightly below, but compatible with, most recent starcounts estimates by Knödlseder (2000, A&A, 360, 539). These results support the notion that Cygnus OB2 may be considered as a young globular cluster. The lists of new members that we provide, in particular those with emission lines, should be a useful resource for future investigations of Cygnus OB2 itself, as well as of very massive stellar evolution by providing a nearby, abundant sample of stars sharing a common environment.