2013A&A...550A..55R

We analyze the high-resolution X-ray spectrum of the supergiant O-star δ Orionis (O9.5II) with line ratios of He-like ions and a thermal plasma model, and we examine its variability. The O-supergiant δ Ori was observed in the wavelength range 5-175Å by the X-ray detector HRC-S in combination with the grating LETG aboard Chandra. We studied the He-like ions in combination with the UV-radiation field to determine local plasma temperatures and to establish the distance of the X-ray emitting ions to the stellar surface. We measured individual lines by means of Gaussian profiles, folded through the response matrix, to obtain wavelengths, line fluxes, half widths at half maximum (HWHM) and line shifts to characterize the plasma. We consider multitemperature models in collisional ionization equilibrium (CIE) to determine temperatures, emission measures, and abundances. Analysis of the He-like triplets extended to NVI and CV implies ionization stratification with the hottest plasma to be found within a few stellar radii 3R_* (MgXI) and the coolest farther out, far beyond the acceleration zone, up to 49R_* (NVI) and 75R_* (CV). The observed temperatures cover a range from about 0.1 to 0.7keV, i.e., 1-8MK. The X-ray luminosity (L_x) is ∼1.5x10³²erg/s in the range from 0.07 to 3keV covered by LETGS. Velocity widths of about 1040km/s have been determined.