Radio Continuum Measurements of Southern Early-Type Stars.
II. A Distance Limited Sample Wolf-Rayet Stars

(1997) ApJ 481, 898

Abstract. 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 GHz and 4.80 GHz. Radio continuum flux densities at one or both frequencies were obtained for 10 sources, and upper limits for 20; four sources are found to be thermal emitters on the basis of the observed spectral index. Five sources are classified as non-thermal. One source could not be classified.

We derive mass-loss rates for the thermal sources. After combining them with all existing radio mass-loss rates of Wolf-Rayet stars in the northern and southern hemisphere, we perform a comparison with mass-loss rates derived from optical emission lines. The two methods lead to consistent results, suggesting that the assumption of a spherically symmetric, stationary, homogeneous stellar wind is either correct, or that deviations from this assumption affect both methods in the same way.

Wolf-Rayet mass-loss rates are surprisingly uniform across spectral type. We find an average mass-loss rate of 4 x 10^{-5} Myr for all types observed, except for WC9 stars, which have rates that are lower by at least a factor of 2. An alternative explanation could be partial recombination of helium from He^+ to He^o in the radio region, leading to a reduced number of free electrons, and therefore reduced radio flux for WC9 stars. Mass-loss rates of 8 x 10^{-5} Myr for late WN stars favored in recent stellar evolution models disagree with the observations of these subtypes.

The results of this survey suggest that approx. 40% of all Wolf-Rayet stars with measured spectral index are non-thermal emitters at cm wavelengths. This percentage is nearly twice as high as that of non-thermal emitters among OB stars and is higher than previously estimated for WR stars. The nature of the non-thermal emission is still not fully understood. Possible causes of non-thermal emission are discussed. In particular, we speculate that non-thermal emission may arise from an interaction between a thermal WR wind and surrounding material due to a shell ejected during a previous evolutionary stage, or due to the wind of a companion star.

Keywords: surveys - stars: mass loss - stars: Wolf-Rayet - radio continuum: stars