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: 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.
Keywords: stars: early-type; stars: individual: HD 93129A; stars: mass-loss; stars: winds, outflows; X-rays: stars
Journal Article. 9185 words. Illustrated.
Subjects: Astronomy and Astrophysics
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