  X-ray line profile analysis has proved to be the most direct diagnostic of
the kinematics and spatial distribution of the very hot plasma around O stars.
In this paper we apply several analysis techniques to the emission lines in the
Chandra HETGS spectrum of the late-O supergiant zeta Ori (O9.7 Ib), including
the fitting of a simple line-profile model. We show that there is distinct
evidence for blue shifts and profile asymmetry, as well as broadening in the
X-ray emission lines of zeta Ori. These are the observational hallmarks of a
wind-shock X-ray source, and the results for zeta Ori are very similar to those
for the earlier O star, zeta Pup, which we have previously shown to be well-fit
by the same wind-shock line-profile model. The more subtle effects on the
line-profile morphologies in zeta Ori, as compared to zeta Pup, are consistent
with the somewhat lower density wind in this later O supergiant. In both stars,
the wind optical depths required to explain the mildly asymmetric X-ray line
profiles imply reductions in the effective opacity of nearly an order of
magnitude, which may be explained by some combination of mass-loss rate
reduction and large-scale clumping, with its associated porosity-based effects
on radiation transfer. In the context of the recent reanalysis of the
helium-like line intensity ratios in both zeta Ori and zeta Pup, and also in
light of recent work questioning the published mass-loss rates in OB stars,
these new results indicate that the X-ray emission from zeta Ori can be
understood within the framework of the standard wind-shock scenario for hot
stars.
