Radio Continuum Measurements of Southern Early‐Type Stars. II. A Distance‐limited Sample of Wolf‐Rayet Stars

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 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 nonthermal. 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, which suggests either that the assumption of a spherically symmetric, stationary, homogeneous stellar wind is 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 × 10-5 M☉ yr-1 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 He0 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 × 10-5 M☉ 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 ~40% of all Wolf-Rayet stars with measured spectral index are nonthermal emitters 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 emission is still not fully understood. Possible causes of nonthermal emission are discussed. In particular, we speculate that nonthermal emission may arise from an interaction between a thermal WR wind and surrounding material owing to a shell ejected during a previous evolutionary stage or owing the wind of a companion star.