XMM-Newton observations of Zeta Orionis (O9.7 Ib): A Collisional   Ionization Equilibrium model
  We present XMM-Newton observations of the O supergiant Zeta Orionis (O9.7 Ib). The spectra are rich in emission lines over a wide range of ionization stages. The RGS-spectra show for the first time lines of low ion stages such as C VI, N VI, N VII, and O VII. The line profiles are symmetric and rather broad (FWHM approximately 1500 km/s) and show only a slight blue shift. With the XMM-epic spectrometer several high ions are detected in this star for the first time including Ar XVII and S XV. Simultaneous multi-temperature fits and DEM-modeling were applied to the RGS and EPIC spectra to obtain emission measures, elemental abundances and plasma temperatures. The calculations show temperatures in the range of about 0.07-0.6 keV. According to the derived models the intrinsic source X-ray luminosity at a distance of 251 pc Lx=1.37(.03) times 10^{32} ergs/s, in the energy range 0.3-10 keV. In the best multi-temperature model fit, the abundances of C, N, O, and Fe are near their solar values, while the abundances of Ne, Mg, and Si appear somewhat enhanced. The sensitivity of the He-like forbidden and intercombination lines to Zeta Ori's strong radiation field is used to derive the radial distances at which lines are formed. Values of 34 R-star for N VI, 12.5 R-star for O VII, 4.8 R-star for Ne IX, and 3.9 R-star for Mg XI are obtained. 
A.J.J. Raassen, K.A. van der Hucht, N.A. Miller, J.P. Cassinelli