Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate
Abstract
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: 5.2+1.8-1.5 × 10-6 M⊙ yr-1 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- August 2011
- DOI:
- 10.1111/j.1365-2966.2011.18952.x
- arXiv:
- arXiv:1104.4786
- Bibcode:
- 2011MNRAS.415.3354C
- Keywords:
-
- stars: early-type;
- stars: individual: HD 93129A;
- stars: mass-loss;
- stars: winds;
- outflows;
- X-rays: stars;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in Monthly Notices of the Royal Astronomical Society. 12 pages, 10 figures (incl. 5 color)