Radio emission from flows

Abstract

Many emission line stars are radio sources including early type stars such as ζ Pup and P Cyg, peculiar objects such as MWC 349 (a possible proto-planetary system) and V1016 Cyg, nebular variables such as T Tau, and at least one late M giant. These “radio stars” are characterized by (1) a rising spectrum with spectral index α = 0.6 to 1.1, (2) positional coincidence between star and radio source, and (3) no rapid (less than 107 sec) time variability. These characteristics are in contrast with the non-thermal spectrum and rapid variability of Algol type “radio stars” and the radio emission associated with x-ray sources. The radio component of the spectrum of emission line stars originates in the substantial mass loss flows which are a feature of their outer atmospheres and which, when ionized by stellar radiation, radiate by free-free electron thermal bremsstrahlung. These circumstellar H II regions are, in general, detectable with present radiotelescopes if the characteristic size of the optically thick region of the flow is greater than about 10−7 of the distance to the star.