[ITAL]Chandra[/ITAL] 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, ZetaOrionis (O9.7 Ib). A 73.4 ks HETGS observation shows a wide range of ionizationstages and line strengths over the wavelength range of 5 to 26 A. The observedemission lines indicate a range in temperature of 2 to 10 MK which isconsistent with earlier X-ray observations of Zeta Ori. Many lines arespectrally resolved showing Doppler broadening of 900 +/- 200 km/s. Theobserved He-like ions (O VII, Ne IX, Mg XI, and Si XIII) provide informationabout the spatial distribution of the X-ray emission. Although the observationssupport a wind distribution of X-ray sources, we find three conflictingresults. First, line diagnostics for SIXIII indicate that this line emissionforms 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 requiredfor the observed ionization level. Second, the strong X-ray line profiles aresymmetric and do not show any evidence of Doppler blue-shifted line centroidswhich are expected to accompany an outwardly moving source in a high densitywind. Third, the observed velocity dispersions do not appear to correlate withthe associated X-ray source radii velocities, contrary to expectations of winddistributed source models. A composite source model involving wind shocks andsome magnetic confinement of turbulent hot plasma in a highly non-symmetricwind, appears to be needed to explain the line diagnostic anomalies.Comment: 7 pages, 3 figures, to appear in ApJ