Unveiling the Cygnus OB2 stellar population with Chandra

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 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 10³⁰ and 10³¹ erg s^-1 for intermediate- and low-mass stars, and 2.5×10³⁰ and between 6.3×10³³ 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.

Tables 1-3 are only available in electronic form at http://www.aanda.org