Astronomy and Astrophysics, volume 464, 211-227 (2007/3-2)
Unveiling the Cygnus OB2 stellar population with Chandra.
ALBACETE COLOMBO J.F., FLACCOMIO E., MICELA G., SCIORTINO S. and DAMIANI F.
Abstract (from CDS):
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. We analyzed a 97.7ks 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. 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≃7.0mag. 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 logNH∼22.25(cm–2) and kT∼1.35keV with 1σ dispersion of 0.2dex and 0.4keV, respectively. Variable and flaring sources have harder spectra with median kT=3.3 and 3.8keV, respectively. OB stars are typically softer (kT∼0.75keV). X-ray luminosities range between 1030 and 1031erg/s for intermediate- and low-mass stars, and 2.5x1030 and between 6.3x1033erg/s for OB stars. 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.
stars: formation - stars: early-type - stars: pre-main-sequence - X-rays: stars - Galaxy: open clusters and associations: individual: Cygnus OB2
VizieR on-line data:
<Available at CDS (J/A+A/464/211): table1.dat table2.dat table3.dat>
Table 1, col(1): [AFM2007] NNNN (Nos 1-1003) = Table 1, col(2): CXOCyg JHHMMSS.ss+DDMMSS.s N=1003.
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