  XMM-Newton observations of the O supergiant zeta Orionis (O9.7 Ib) extend
knowledge of its high-resolution spectrum beyond the CVI line at 33.7 Angstroms
and suggest a new framework for the interpretation of the X-ray spectra of
single hot stars. All the lines are broad and asymmetric with similar velocity
profiles. X-rays probably originate in the wind's terminal velocity regime in
collisionless shocks controlled by magnetic fields rather than in cooling
shocks in the acceleration zone. During post-shock relaxation, exchange of
energy between ions and electrons is so slow that electron heating does not
take place before hot gas is quenched by the majority cool gas. The observed
plasma is not in equilibrium and the electron bremsstrahlung continuum is weak.
Charge exchange, ionization and excitation are likely to be produced by
protons. Fully thermalized post-shock velocities ensure high cross-sections and
account for the observed line widths, with some allowance probably necessary
for non-thermal particle acceleration. In general, the form of X-ray spectra in
both single and binary stars is likely to be determined principally by the
amount of post-shock electron heating: magnetically confined X-ray plasma in
binary systems can evolve further towards the higher electron temperatures of
equilibrium while in single stars this does not take place. The long mean-free
path for Coulomb energy exchange between fast-moving ions may also inhibit the
development of line-driven instabilities.
