Unveiling the Cygnus OB2 stellar population with Chandra

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 Av ≈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 N H ∼ 22.25 (cm-2) and kT ∼ 1.35 keV with 1σ dispersion of 0.2 dex and 0.4 keV, respectively. Variable and flaring sources have harder spectra with median kT = 3.3 and 3.8 keV, respectively. OB stars are typically softer (kT ∼ 0.75 keV). X-ray luminosities range between 10 30 and 1031 erg s-1 for intermediate- and low-mass stars, and 2.5 × 1030 and between 6.3 × 10 33 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 (M/M⊙ ∼ 0.5 to 3.0) display X-ray activity levels comparable to those of Orion Nebular Cluster (ONC) sources in the same mass range. © ESO 2007.