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 Orionis (O9.7 Ib). A 73.4 ks HETGS observation shows a wide range of ionization stages and line strengths over the wavelength range of 5 to 26 A. The observed emission lines indicate a range in temperature of 2 to 10 MK which is consistent with earlier X-ray observations of Zeta Ori. Many lines are spectrally resolved showing Doppler broadening of 900 +/- 200 km/s. 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 SIXIII indicate that this line emission forms very close to the stellar surface, where the density is of order 10^{12} 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 blue-shifted 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 non-symmetric wind, appears to be needed to explain the line diagnostic anomalies. 
Wayne L. Waldron and Joseph P. Cassinelli