Chandra detection of Doppler-shifted X-ray line profiles from the wind of ζ Puppis (O4 f)

We report on a 67 ks High-Energy Transmission Grating observation of the optically brightest early O star zeta Puppis (O4 f). Many resolved X-ray lines are seen in the spectra over a wavelength range of 5-25 Angstrom. Chnndra has sufficient spectral resolution to study the velocity structure of isolated X-ray line profiles and to distinguish the individual forbidden, intercombination, and resonance (fir) emission lines in several He-like ions, even where the individual components are strongly Doppler-broadened. In contrast to X-ray line profiles in other hot stars, zeta Pup shows blueshifted and skewed line profiles, providing the dearest and most direct evidence that the X-ray sources are embedded in the stellar wind. The broader the line, the greater the blueward centroid shift tends to be. The N VII line at 24.78 Angstrom is a special case, showing a flat-topped profile. This indicates that it is formed in regions beyond most of the wind attenuation. The sensitivity of the He-like ion fir lines to a strong UV radiation field is used to derive the radial distances at which lines of S XV, Si XIII, Mg XI, Ne Ix, and O VII originate. The formation radii correspond well with a continuum optical depth of unity at the wavelength of each line complex, indicating that the X-ray line emission is distributed throughout the stellar wind. However, the S XV emission lines form deeper in the wind than expected from standard wind-shock models.