The Chandra HETGS X-Ray Grating Spectrum of η Carinae

η Carinae may be the most massive and luminous star in the Galaxy and is suspected to be a massive colliding-wind binary system. The Chandra X-ray observatory has obtained a calibrated high-resolution X-ray spectrum of the star, uncontaminated by the nearby extended soft X-ray emission. Our 89 ks Chandra observation with the High-Energy Transmission Grating Spectrometer (HETGS) shows that the hot gas near the star is nonisothermal. The temperature distribution may represent the emission on either side of the colliding-wind bow shock, effectively "resolving" the shock. If so, the preshock wind velocities are ~700 and ~1800 km s-1 in our analysis, and these velocities may be interpreted as the terminal velocities of the winds from η Carinae and from the hidden companion star. The forbidden-to-intercombination (f/i) line ratios for the He-like ions of S, Si, and Fe are large, indicating that the line-forming region lies far from the stellar photosphere. The iron fluorescent line at 1.93 Å, first detected by ASCA, is clearly resolved from the thermal iron line in the Chandra grating spectrum. The Fe fluorescent line is weaker in our Chandra observation than in any of the ASCA spectra. The Chandra observation also provides an uninterrupted, high time resolution light curve of the stellar X-ray emission from η Carinae and suggests that there was no significant coherent variability during the Chandra observation. The η Carinae Chandra grating spectrum is unlike recently published X-ray grating spectra of single massive stars in significant ways and is generally consistent with colliding-wind emission in a massive binary.