RADIO-CONTINUUM MEASUREMENTS OF SOUTHERN EARLY-TYPE STARS

We report the results of a pilot project to measure radio continuum fl ux densities of early-type stars with the Australia Telescope Compact Array. A sample of 12 stars comprising six Wolf-Rayet stars, three B h ypergiants, two Of stars, and one luminous blue variable has been obse rved at 8.64 GHz and 4.80 GHz. Eleven objects have been detected at 8. 64 GHz and seven at 4.80 GHz, respectively. Ail objects except the lum inous blue variable HD 168625 were unresolved at an angular resolution of 1'', as expected if the radio flux originates in dense, ionized st ellar winds. HD 168625 is clearly resolved; we detect an incomplete ci rcumstellar ring whose morphology is rather similar to the H alpha mor phology of the nebula discovered by Hutsmekers et al. The radio spectr um between 8.64 GHz and 4.80 GHz of all sources detected at both frequ encies is consistent with thermal emission from an optically thick win d expanding at constant velocity. The radio fluxes are used to derive accurate mass-loss rates. We find very similar mass-loss rates for the six Wolf-Rayet stars of type WNL in our sample: M approximate to 10(- 4.3+/-0.15) M. yr(-1), supporting previous results of a very small dis persion among the mass-loss rates of WNL stars. Comparison with rates determined from optical recombination lines suggests excellent agreeme nt. This result makes it unlikely that distance-dependent density inho mogeneities are present in the winds. Our data essentially double the number of luminous B stars detected in the radio. The mass-loss rates of the three B hypergiants are among the most accurate ever derived fo r B stars. Their rates are not correlated with luminosity. The massive O3 V((f)) star HD 93250 has only been detected at 8.64 GHz. The mass- loss rate required to account for the measured radio flux is M approxi mate to 10(-4.3+/-0.15) M yr(-1). This rate is several times higher th an expected on the basis of its H alpha luminosity. We speculate that most of the 8.64 GHz flux is nonthermal and that the true mass-loss ra te, for this source, is lower than implied by thermal emission alone.