High‐Resolution X‐Ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association

The Cygnus OB2 Association contains some of the most luminous OB stars in ourGalaxy, the brightest of which are also among the most luminous in X-rays. Wehave obtained a Chandra High Energy Transmission Grating Spectrometer (HETGS)observation centered on Cyg OB2 No. 8a, the most luminous X-ray source in theAssociation. Although our analysis will focus on the X-ray properties of CygOB2 No. 8a, we also present limited analyses of three other OB stars (Cyg OB2Nos. 5, 9, and 12). Applying standard diagnostic techniques as used in previousstudies of early-type stars, we find that the X-ray properties of Cyg OB2 No.8a are very similar to those of other OB stars that have been observed usinghigh-resolution X-ray spectroscopy. From analyses of the He-like ion "fir"emission lines, we derive radial distances of the He-like line emission sourcesand find these fir-inferred radii are consistent with their corresponding X-raycontinuum optical depth unity radii. Contrary to other O-star results, theemission lines of Cyg OB2 No. 8a show a large range in line centroid shifts(roughly -800 to +250 km/s). We discuss the implications of our results inlight of the fact that Cyg OB2 No. 8a is a member of a rather tight stellarcluster, and shocks could arise at interfaces with the winds of these otherstars.Comment: 36 pages (including 4 tables and 12 figures). LaTeX. Submitted to Ap