Extreme Radio-wave Scattering Associated with Hot Stars

We use data on extreme radio scintillation to demonstrate that this phenomenon is associated with hot stars in the solar neighborhood. The ionized gas responsible for the scattering is found at distances up to 1.75 pc from the host star, and on average must comprise ~10^5 distinct structures per star. We detect azimuthal velocities of the plasma, relative to the host star, up to 9.7 km s^(-1), consistent with warm gas expanding at the sound speed. The circumstellar plasma structures that we infer are similar in several respects to the cometary knots seen in the Helix and in other planetary nebulae. There the ionized gas appears as a skin around tiny molecular clumps. Our analysis suggests that molecular clumps are ubiquitous circumstellar features, unrelated to the evolutionary state of the star. The total mass in such clumps is comparable to the stellar mass