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∼ 2Myr 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 ksec Chandra ACIS-I observation pointed at the core of the Cygnus OB2 region. Sources were detected using the PWDetect code and were then positionally correlated with optical and near-IR catalogs from the literature. Source events were then 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 leas t 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. Typ ical X-ray spectral parameters are log NH∼ 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 10 31 erg s −1 for intermediate- and low-mass stars, and 2.5×10 30 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 essentially related to the magnetic ac tivity of low-mass stars (M/M⊙∼0.5 to 3.0) which display X-ray activity levels comparable to those of Orion Nebular Cluster (ONC) sources in the same mass range.