EDP Sciences
Free access
Volume 383, Number 2, February IV 2002
Page(s) 636 - 647
Section Diffuse matter in space
DOI http://dx.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

SIMBAD Objects

© ESO 2002

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access.

An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account.
    In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.