Chandra discovers a very high density X-ray plasma on the O star zeta Orionis

We report on a Chandra line spectrum observation of the O supergiant zeta O rionis (O9.7 Ib). A 73.4 ks High-Energy Transmission Grating Spectrometer o bservation shows a wide range of ionization stages and line strengths over the wavelength range of 5-26 Angstrom. The observed emission lines indicate a range in temperature of 2-10 MK, which is consistent with earlier X-ray observations of zeta Ori. Many lines are spectrally resolved showing Dopple r broadening of 900 +/- 200 km s(-1). The observed He-like ions (O VII, Ne IX, Mg XI, and Si XIII) provide information about the spatial distribution of the X-ray emission. Although the observations support a wind distributio n of X-ray sources, we find three conflicting results. First, line diagnost ics for Si XIII indicate that this line emission forms very close to the st ellar surface, where the density is of order 10(12) cm(-3), but the velocit y there is too small to produce the shock jump required for the observed io nization level. Second, the strong X-ray line profiles are symmetric and do not show any evidence of Doppler-blueshifted line centroids, which are exp ected to accompany an outwardly moving source in a high-density wind. Third , the observed velocity dispersions do not appear to correlate with the ass ociated X-ray source radii velocities, contrary to expectations of wind-dis tributed source models. A composite source model involving wind shocks and some magnetic confinement of turbulent hot plasma in a highly nonsymmetric wind appears to be needed to explain the line diagnostic anomalies.