Cohen, David H. and Gagné, Marc and Leutenegger, Maurice A. and MacArthur, James P. and Wollman, Emma E. and Sundqvist, Jon O. and Fullerton, Alex W. and Owocki, Stanley P. (2011) Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate. Monthly Notices of the Royal Astronomical Society, 415 (4). pp. 3354-3364. ISSN 0035-8711 http://resolver.caltech.edu/CaltechAUTHORS:20110927-105006935
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We present an analysis of both the resolved X-ray emission-line profiles and the broad-band X-ray spectrum of the O2 If^* star HD 93129A, measured with the Chandra High Energy Transmission Grating Spectrometer (HETGS). This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind-shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10 per cent of the X-ray flux. We find results consistent with the predictions of numerical simulations of the line-driving instability, including line broadening indicating an onset radius of X-ray emission of several tenths of R_*. Helium-like forbidden-to-intercombination line ratios are consistent with this onset radius, and inconsistent with being formed in a wind-collision interface with the star’s closest visual companion at a distance of 100 au. The broad-band X-ray spectrum is fitted with a dominant emission temperature of just kT= 0.6 keV along with significant wind absorption. The broad-band wind absorption and the line profiles provide two independent measurements of the wind mass-loss rate: M = 5.2^(+1.8)_(-1.5) x 10^(-6) and 6.8^(+2.8)_(−2.2) × 10^(−6) M_⊙ yr^(−1), respectively. This is the first consistent modelling of the X-ray line-profile shapes and broad-band X-ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3–4 compared to the standard Hα mass-loss rate that assumes a smooth wind.
|Additional Information:||© 2011 The Authors Monthly. Notices of the Royal Astronomical Society © 2011 RAS. Accepted 2011 April 21. Received 2011 April 20; in original form 2010 March 17. Article first published online: 13 Jun. 2011. Support for this work was provided by the National Aeronautics and Space Administration through Chandra award numbers AR7-8002X and GO0-11002B to Swarthmore College. EEW was supported by a Lotte Lazarsfeld Bailyn Summer Research Fellowship, and JPM was supported by a Surdna Summer Research Fellowship, both from the Provost’s Office at Swarthmore College. MAL is supported by an appointment to the NASA Postdoctoral Programme at Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. JOS and SPO acknowledge support from NASA award ATP NNX11AC40G to the University of Delaware. The authors thank Véronique Petit for her careful reading of the manuscript and several useful suggestions.|
|Subject Keywords:||stars: early-type; stars: individual: HD 93129A; stars: mass-loss; stars: winds, outflows; X-rays: stars|
|Official Citation:||Cohen, D. H., Gagné, M., Leutenegger, M. A., MacArthur, J. P., Wollman, E. E., Sundqvist, J. O., Fullerton, A. W. and Owocki, S. P. (2011), Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate. Monthly Notices of the Royal Astronomical Society, 415: 3354–3364. doi: 10.1111/j.1365-2966.2011.18952.x|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||27 Sep 2011 18:34|
|Last Modified:||27 Sep 2011 18:34|
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