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 \hbox { $ { \thinspace km { \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 \hbox { $ { \dot { M } } $ } _ { 2 } \approx 10 ^ { -5 } \hbox { $ { \hbox { $ { \thinspace M _ { % \odot } } $ } { \thinspace yr } ^ { -1 } } $ } and v _ { \infty _ { 2 } } \approx 3000 \hbox { $ { \thinspace km { \thinspace s } ^ { -1 % } } $ } .
With observationally determined values of \approx 500 - 700 \hbox { $ { \thinspace km { \thinspace s } ^ { -1 } } $ } for the velocity of the primary wind , our fit implies a primary mass-loss rate of \hbox { $ { \dot { M } } $ } _ { 1 } \approx 2.5 \times 10 ^ { -4 } \hbox { $ { \hbox { $ { % \thinspace M _ { \odot } } $ } { \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 } \hbox { $ { \hbox { $ { \thinspace M _ { \odot } } $ } { \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 .