  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.

Astrophysics