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

Abstract

We present X-ray spectral fits to a recently obtained Chandra grating spectrum of η 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 (≈500 kms-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 η = 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 Ṁ2 ≈ 10-5 M⊙ yr-1 and v∞2 ≈ 3000 kms-1. With observationally determined values of ≈500-700 kms-1 for the velocity of the primary wind, our fit implies a primary mass-loss rate of Ṁ1 ≈ 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 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.