Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate

David H. Cohen (1), Marc Gagn'e (2), Maurice A. Leutenegger (3,4), James P. MacArthur (1), Emma E. Wollman (1,5), Jon O. Sundqvist (6), Alex W. Fullerton (7), Stanley P. Owocki (6)

(1) Swarthmore College (2) West Chester University (3) NASA/Goddard Space Flight Center (4) CRESST and University of Maryland, Baltimore County (5) Caltech, Department of Physics (6) University of Delaware, Bartol Research Institute (7) Space Telescope Science Institute

We present analysis of both the resolved X-ray emission line profiles and the broadband X-ray spectrum of the O2 If* star HD 93129A, measured with the Chandra 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% 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 Rstar. 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 broadband X-ray spectrum is fit with a dominant emission temperature of just kT = 0.6 keV along with significant wind absorption. The broadband wind absorption and the line profiles provide two independent measurements of the wind mass-loss rate: Mdot = 5.2_{-1.5}^{+1.8} times 10^{-6} Msun/yr and Mdot = 6.8_{-2.2}^{+2.8} times 10^{-6} Msun/yr, respectively. This is the first consistent modeling of the X-ray line profile shapes and broadband X-ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3 to 4 compared to the standard H-alpha mass-loss rate that assumes a smooth wind. Finally, we also model the H-alpha emission line, using our lower mass-loss rate and a clumping factor of f_V = 0.08. We find that a clump onset radius of 1.05 Rstar is required to fit the data.

Reference: Monthly Notices of the Royal Astronomical Society, in press
Status: Manuscript has been accepted


Comments: 12 pages; 2 tables; 10 figures (5 color).
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