High Resolution X-ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association

Abstract

The Cygnus OB2 Association contains some of the most luminous OB stars in our Galaxy, the brightest of which are also among the most luminous in X-rays. We have obtained a Chandra High Energy Transmission Grating Spectrometer (HETGS) observation centered on Cyg OB2 No. 8a, the most luminous X-ray source in the Association. Although our analysis will focus on the X-ray properties of Cyg OB2 No. 8a, we also present limited analyses of three other OB stars (Cyg OB2 Nos. 5, 9, and 12). Applying standard diagnostic techniques as used in previous studies 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 using high-resolution X-ray spectroscopy. From analyses of the He-like ion "fir" emission lines, we derive radial distances of the He-like line emission sources and find these fir-inferred radii are consistent with their corresponding X-ray continuum optical depth unity radii. Contrary to other O-star results, the emission 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 in light of the fact that Cyg OB2 No. 8a is a member of a rather tight stellar cluster, and shocks could arise at interfaces with the winds of these other stars.

arXiv comment36 pages (including 4 tables and 12 figures). LaTeX. Submitted to ApJ

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• On the evolution and fate of supermassive stars
• Measuring mass-loss rates and constraining shock physics using X-ray line profiles of O stars from the Chandra archive
• An Extensive Collection of Stellar Wind X-ray Source Region Emission Line Parameters,Temperatures, Velocities, and Their Radial Distributions as Obtained from Chandra Observations of 17 OB Stars
• Unveiling the Cygnus OB2 stellar population with Chandra
• Measurements and analysis of helium-like triplet ratios in the X-ray spectra of O-type stars
• An X-ray Census of Young Stars in the Massive Southern Star-Forming Complex NGC 6357
• TeV gamma-rays and neutrinos from photo-disintegration of nuclei in Cygnus OB2
• Evidence for the importance of resonance scattering in X-ray emission line profiles of the O star $\zeta$ Puppis
• A multi-epoch XMM-Newton campaign on the core of the massive Cyg OB2 association
• A Coordinated X-ray and Optical Campaign of the Nearby Massive Binary $\delta$ Orionis Aa: II. X-ray Variability