We discuss new methods of measuring and interpreting the forbidden-to-intercombination line ratios of heliumlike triplets in the X-ray spectra of O- type stars, including accounting for the spatial distribution of the X-ray-emitting plasma and using the detailed photospheric UV spectrum. Measurements are made for four O stars using archival Chandra HETGS data. We assume an X- ray-emitting plasma spatially distributed in the wind above some minimum radius R-0. We find minimum radii of formation typically in the range of 1.25 < R-0 / R-* < 1.67, which is consistent with results obtained independently from line profile fits. We find no evidence for anomalously low f / i ratios, and we do not require the existence of X-ray-emitting plasmas at radii that are too small to generate sufficiently strong shocks.
M. A. Leutenegger, F. B.S. Paerels, S. M. Kahn, and David H. Cohen.
"Measurements And Analysis Of Helium-Like Triplet Ratios In The X-Ray Spectra Of O-Type Stars".