The X-ray spectrum of delta Orionis observed by LETGS aboard Chandra

Abstract

We analyze the high-resolution X-ray spectrum of the supergiant O-star delta Orionis (O9.5II) with line ratios of He-like ions and a thermal plasma model, and we examine its variability. The O-supergiant delta Ori was observed in the wavelength range 5-175 Angstrom 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 N VI and C V implies ionization stratification with the hottest plasma to be found within a few stellar radii 3R* (Mg XI) and the coolest farther out, far beyond the acceleration zone, up to 49R* (N VI) and 75R* (C V). The observed temperatures cover a range from about 0.1 to 0.7 keV, i.e., 1-8 MK. The X-ray luminosity L is about 1.5 x 10e+32 erg/s in the range from 0.07 to 3 keV covered by LETGS. Velocity widths of about 1040 km/s have been determined.