Theoretical profile shapes for optically thin X-ray emission lines from spherical stellar winds

One of the major outstanding problems in hot star wind theory is an understanding of the observed X-ray emissions from the early-type B, O, and Wolf-Rayet (WR) stars. The latest X-ray satellites Chandra and XMM-Newton are providing key new observations of resolved emission profiles to advance that understanding. This study presents a derivation of the expected emission-line profiles, assuming optically thin line emission and spherical symmetry, with a proper treatment of the attenuation of X-rays by the dense cool wind component. Examples of line profile variability for a narrow outflowing shell are presented. Then the case of embedded hot gas existing throughout the wind flow is discussed. It is shown that for the special case of constant expansion, emission profile shapes can be derived analytically, and in the limit of strong wind attenuation, the profile achieves a self-similar form. The results of this Letter provide a framework in which to model X-ray line profiles and analytic results to serve as a benchmark for more sophisticated numerical evaluations.