Chandra Discovers a Very High Density X-Ray Plasma on the O Star ζ Orionis

We report on a Chandra line spectrum observation of the O supergiant ζ Orionis (O9.7 Ib). A 73.4 ks High-Energy Transmission Grating Spectrometer observation shows a wide range of ionization stages and line strengths over the wavelength range of 5-26 Å. The observed emission lines indicate a range in temperature of 2-10 MK, which is consistent with earlier X-ray observations of ζ Ori. Many lines are spectrally resolved showing Doppler 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 distribution of X-ray sources, we find three conflicting results. First, line diagnostics for Si XIII indicate that this line emission forms very close to the stellar surface, where the density is of order 1012 cm-3, but the velocity there is too small to produce the shock jump required for the observed ionization level. Second, the strong X-ray line profiles are symmetric and do not show any evidence of Doppler-blueshifted line centroids, which are expected to accompany an outwardly moving source in a high-density wind. Third, the observed velocity dispersions do not appear to correlate with the associated X-ray source radii velocities, contrary to expectations of wind-distributed 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.