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

Abstract

We present X-ray spectral ts to a recently obtained Chandra grating spectrum of 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 tted against newly acquired Chandra grating data. We nd that due to the low velocity of the primary wind (500 km 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 x the wind momentum ratio at =0 :2. We are then able to obtain a good t to the data by varying the mass-loss rate of the companion and the terminal velocity of its wind. We nd that _ M 2 10 5 M yr 1 and v12 3000 km s 1 . With observationally determined values of500{700 km s 1 for the velocity of the primary wind, our t implies a primary mass-loss rate of _ M 1 2:5 10 4 M 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 M 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 conrmation. 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.