RADIO-CONTINUUM MEASUREMENTS OF SOUTHERN EARLY-TYPE STARS

We report the results of a pilot project to measure radio continuum flux densities of early-type stars with the Australia Telescope Compact Array. A sample of 12 stars comprising six Wolf-Rayet stars, three B hypergiants, two Of stars, and one luminous blue variable has been observed 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 luminous blue variable HD 168625 were unresolved at an angular resolution of 1'', as expected if the radio flux originates in dense, ionized stellar winds. HD 168625 is clearly resolved; we detect an incomplete circumstellar ring whose morphology is rather similar to the H alpha morphology of the nebula discovered by Hutsmekers et al. The radio spectrum between 8.64 GHz and 4.80 GHz of all sources detected at both frequencies is consistent with thermal emission from an optically thick wind 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 dispersion among the mass-loss rates of WNL stars. Comparison with rates determined from optical recombination lines suggests excellent agreement. This result makes it unlikely that distance-dependent density inhomogeneities 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 for 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 approximate to 10(-4.3+/-0.15) M yr(-1). This rate is several times higher than 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 rate, for this source, is lower than implied by thermal emission alone.