Unveiling the Cygnus OB2 stellar population with Chandra^*

INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: facundo@astropa.unipa.it

Received: 25 July 2006
Accepted: 10 October 2006

Abstract

Aims.The aim of this work is to identify the so far unknown low mass stellar population of the ~2 Myr old Cygnus OB2  star forming region, and to investigate the X-ray and near-IR stellar properties of its members.

Methods.We analyzed a 97.7 ks Chandra  ACIS-I observation pointed at the core of the Cygnus OB2  region. Sources were detected using the PWDetect code and were positionally correlated with optical and near-IR catalogs from the literature. Source events were extracted with the Acis Extract package. X-ray variability was characterized through the Kolmogorov-Smirnov test and spectra were fitted using absorbed thermal plasma models.

Results.We detected 1003 X-ray sources. Of these, 775 have near-IR counterparts and are expected to be almost all associated with Cygnus OB2  members. From near-IR color–color and color–magnitude diagrams we estimate a typical absorption toward Cygnus OB2  of A_v ≈ 7.0 mag. Although the region is young, very few stars (~4.4%) show disk-induced excesses in the near-IR. X-ray variability is detected in ~13% of the sources, but this fraction increases, up to 50%, with increasing source statistics. Flares account for at least 60% of the variability. Despite being generally bright, all but 2 of the 26 detected O- that early B-type stars are not significantly variable. Typical X-ray spectral parameters are log  (cm^-2) and  keV with 1σ dispersion of 0.2 dex and 0.4 keV, respectively. Variable and flaring sources have harder spectra with median and 3.8 keV, respectively. OB stars are typically softer ( keV). X-ray luminosities range between 10³⁰ and 10³¹ erg s^-1 for intermediate- and low-mass stars, and and between  erg s^-1 for OB stars.

Conclusions.The Cygnus OB2  region has a very rich population of low-mass X-ray emitting stars. Circumstellar disks seem to be very scarce. X-ray variability is related to the magnetic activity of low-mass stars ( to 3.0) display X-ray activity levels comparable to those of Orion Nebular Cluster (ONC) sources in the same mass range.