RADIO-CONTINUUM MEASUREMENTS OF SOUTHERN EARLY-TYPE STARS .2. A DISTANCE-LIMITED SAMPLE OF WOLF-RAYET STARS

A distance-limited sample of southern Wolf-Rayet stars within 3 kpc of the Sun has been observed with the Australia Telescope Compact Array at 8.64 and 4.80 GHz. Radio continuum flux densities at one or both fr equencies were obtained for 10 sources and upper limits for 20; four s ources are found to be thermal emitters on the basis of the observed s pectral index. Five sources are classified as nonthermal. One source c ould not be classified. We derive mass-loss rates for the thermal sour ces. After combining them with all existing radio mass-loss rates of W olf-Rayet stars in the northern and southern hemisphere, we perform a comparison with mass-loss rates derived from optical emission lines. T he two methods lead to consistent results, which suggests either that the assumption of a spherically symmetric, stationary, homogeneous ste llar wind is correct or that deviations from this assumption affect bo th methods in the same way. Wolf-Rayet mass-loss rates are surprisingl y uniform across spectral type. We find an average mass-loss rate of 4 x 10(-5) M-circle dot yr(-1) for all types observed, except for WC9 s tars, which have rates that are lower by at least a factor of 2. An al ternative explanation could be partial recombination of helium from He + to He-O in the radio region, which would lead to a reduced number of free electrons, and therefore reduced radio flux for WC9 stars. Mass- loss rates of 8 x 10(-5) M-circle dot yr(-1) for late WN stars favored in recent stellar evolution models disagree with the observations of these subtypes. The results of this survey suggest that similar to 40% of all Wolf-Rayet stars with measured spectral index are nonthermal e mitters at centimeter wavelengths. This percentage is nearly twice as high as that of nonthermal emitters among OB stars and is higher than that previously estimated for WR stars. The nature of the nonthermal e mission is still not fully understood. Possible causes of nonthermal e mission are discussed. In particular, we speculate that nonthermal emi ssion may arise from an interaction between a thermal WR wind and surr ounding material owing to a shell ejected during a previous evolutiona ry stage or owing the wind of a companion star.