Radio observations of stellar winds from early type stars

FifteenO andB supergiants have been observedwith the Very Large Array (VLA) at 4.85, 8.45, and 14.95 GHz in order to make a detailed comparative study of the mass loss rates evaluated from Hα and radio continuum observations and reveal and quantify possible departures from standardwind conditions. We detected 12 sources, 7 of which for the first time, thus increasing by 30% the total number of detections of OB supergiants in the northern sky. Radio spectral slopes indicate that the radio emission is mainly of thermal origin in all objects with one exception (HD 190603) out of the 12 detections. Our results demonstrate the value of using Hα for mass loss rate determinations, especially for stars that are too distant or too faint to be detectedwith radio techniques. The relationship Ṁ−L for supergiants turns out to be appreciably flatter than commonly reported, i.e. log Ṁ = (1.25± 0.30) logL.