A MICROWAVE SURVEY OF SOUTHERN EARLY-TYPE STARS

A multi-epoch survey with the Parkes telescope of a complete distance- limited sample of 57 stars earlier than F6 has detected possible 8.4-G Hz emission from 16 stars. Single-epoch partial synthesis observations with the Australia Telescope Compact Array (ATCA) at 4.8 GHz on 27 st ars from the same sample (including the possible Parkes detections) fo und no emission at the stellar positions above a flux density limit of 1.2-1.9 mJy, but the maps show that the Parkes detections are not mer ely the results of confusion of sources within the Parkes beam. Three early F stars with UV and/or X-ray emission were observed simultaneous ly at 4.8 and 8.4 GHz in 12-h syntheses with the 6-element ATCA. Two o f these stars were from the above sample and the third was the supergi ant Alpha Carinae. We detected only alpha Car with flux densities of 3 00 +/- 65 and 140 +/- 65 mu Jy at 4.8 and 8.6 GHz(S proportional to nu (-1.3+1.3)). We discuss the legitimacy of the Parkes 3-6 sigma detecti ons and show that, although none has been detected by synthesis observ ations, there is no compelling reason for rejecting them on the intern al evidence. The power emitted by the supergiant alpha Car is similar to that of the 16 possible Parkes detections, although its activity in dex is orders of magnitude lower. We show that this emission cannot be thermal bremsstrahlung from the 10(7.2)-K corona of the star but is p robably synchrotron emission from a magnetically maintained corona.