Unveiling the Cygnus OB2 stellar population with Chandra*
INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: email@example.com
Accepted: 10 October 2006
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 (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.
Key words: stars: formation / stars: early-type / stars: pre-main-sequence / X-rays: stars / Galaxy: open clusters and associations: individual: Cygnus OB2
© ESO, 2007