July 11-15, 2011
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X-ray Spectroscopy of the O2 If* star HD 93129A: Embedded Wind Shocks and a Mass-Loss Rate Measurement

David Cohen (Swarthmore College)

Marc Gagne (West Chester University) Maurice Leutenegger (GSFC) Jon Sundqvist (Bartol Research Institute, U. Delaware) Stan Owocki (Bartol Research Institute, U. Delaware) Alex Fullerton (STScI)

HD 93129A is one of the earliest O stars in the Galaxy and is also a strong source of hard X-rays. We present high-resolution X-ray spectroscopy and medium-resolution broadband X-ray spectroscopy that both indicate that the dominant X-ray production mechanism is embedded wind shocks, with only a small possible contribution from colliding wind shocks. The hardness of the observed X-rays is not due to high intrinsic emission temperatures, but rather to very strong attenuation of the soft X-ray emission by the wind. This wind attenuation is evident in both the X-ray line profiles' skewed and blue shifted shapes and in the broadband spectral energy distribution. We model both effects independently to derive a mass-loss rate. Both determinations give consistent values of about 6 X 10^-6 Msun/yr, representing a modest factor of 3 or 4 reduction over the traditional unclumped H-alpha mass-loss rate, and consistent with the observed H-alpha when we take clumping into account. Interestingly, the clumping onset must be very close to the photosphere in order to match the H-alpha profile, while the X-ray onset is several tenths of a stellar radius into the wind flow in order to explain the X-ray emission line properties.
(to be confirmed by the SOC)