The X-ray View of OB Star Wind Structure and Dynamics

High-resolution X-ray grating spectroscopy enables us to measure the kinematics and spatial distribution of the shock-heated wind plasma in O and early B stars, testing the predictions of the embedded wind shock scenario of massive star Xray production. By fitting models to the resolved, Doppler broadened X-ray emission line profiles measured by the Chandra X-ray Observatory’s grating spectrometer, we find an onset radius of X-ray production of roughly Ro = 1.5R∗ for the O supergiants, ζ Pup and HD 93129A. From the profile fitting we also find that the terminal velocity of the X-ray emitting plasma is consistent with that of the bulk, UV absorbing wind. We also use the X-ray emission line profiles to measure the wind mass-loss rates and break the degeneracy between mass-loss rate and clumping factor that affects traditional Hα and radio free-free diagnostics. We find clumping factors of order fcl = 10, which also agrees with the simulations of the wind instability. And we find that clumping begins very close to the photosphere, significantly lower in the wind than the onset of X-ray production. For lower density B star winds, the X-ray emission lines are much narrower than in the O supergiants, and are inconsistent with the hot plasma sharing the kinematics of the bulk wind.