EDP Sciences
Free Access
Volume 383, Number 2, February IV 2002
Page(s) 636 - 647
Section Diffuse matter in space
DOI https://doi.org/10.1051/0004-6361:20020025

A&A 383, 636-647 (2002)
DOI: 10.1051/0004-6361:20020025

In hot pursuit of the hidden companion of $\eta$ Carinae: An X-ray determination of the wind parameters

J. M. Pittard1 and M. F. Corcoran2, 3

1  Department of Physics & Astronomy, The University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
2  Universities Space Research Association, 7501 Forbes Blvd, Ste 206, Seabrook, MD 20706, USA
3  Laboratory for High Energy Astrophysics, Goddard Space Flight Center, Greenbelt, MD 20771, USA

(Received 28 September 2001 / Accepted 2 January 2002)

We present X-ray spectral fits to a recently obtained Chandra grating spectrum of $\eta$ Carinae, one of the most massive and powerful stars in the Galaxy and which is strongly suspected to be a colliding wind binary system. Hydrodynamic models of colliding winds are used to generate synthetic X-ray spectra for a range of mass-loss rates and wind velocities. They are then fitted against newly acquired Chandra grating data. We find that due to the low velocity of the primary wind ( $\approx$ $500 \rm {\rm\thinspace km}{\rm\thinspace s}^{-1}$), most of the observed X-ray emission appears to arise from the shocked wind of the companion star. We use the duration of the lightcurve minimum to fix the wind momentum ratio at $\eta = 0.2$. We are then able to obtain a good fit to the data by varying the mass-loss rate of the companion and the terminal velocity of its wind. We find that $\dot M_{2} \approx 10^{-5} \;\rm {~M_{\odot}}{\rm\thinspace yr}^{-1}\,$ and $v_{\infty_{2}} \approx 3000 \; \rm {\rm\thinspace km}{\rm\thinspace s}^{-1}$. With observationally determined values of $\approx$500-700 $\; \rm {\rm\thinspace km}{\rm\thinspace s}^{-1}$ for the velocity of the primary wind, our fit implies a primary mass-loss rate of $\dot M_{1} \approx 2.5 \times 10^{-4} \;\rm {~M_{\odot}}{\rm\thinspace yr}^{-1}\,$. This value is smaller than commonly inferred, although we note that a lower mass-loss rate can reduce some of the problems noted by Hillier et al. (2001) when a value as high as $10^{-3} \;\rm {~M_{\odot}}{\rm\thinspace yr}^{-1}\,$ is used. The wind parameters of the companion are indicative of a massive star which may or may not be evolved. The line strengths appear to show slightly sub-solar abundances, although this needs further confirmation. Based on the over-estimation of the X-ray line strengths in our model, and re-interpretation of the HST/FOS results, it appears that the Homunculus nebula was produced by the primary star.

Key words: stars: binaries: general -- stars: early-type -- stars: individual: $\eta$ Carinae -- stars: Wolf-Rayet -- X-rays: stars

Offprint request: J. M. Pittard, jmp@ast.leeds.ac.uk

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