Radio continuum measurements of southern early-type stars. III. Nonthermal emission from Wolf-Rayet stars

The Australia Telescope Compact Array (ATCA) has been used to search for radio continuum emission at 2.4 and 1.4 GHz from a sample of 36 southern Wolf-Rayet stars. Seven Wolf-Rayet stars were detected at 2.4 GHz, of which two were also detected at 1.4 GHz. We have identified six Wolf-Rayet stars, WR 14, 39, 48, 90, 105, and 112, that have nonthermal emission. The ATCA data confirm that at least 40% of Wolf-Rayet stars with measured spectral indices have nonthermal emission at centimeter wavelengths. Properties of each of the six sources are discussed. The measured spectral indices are between 0 and -1.0, and the radio luminosities are of order 10(29) ergs s(-1). So far 10 confirmed sources of nonthermal emission are known, including the six ATCA detections and four previously known cases, WR 125, 140, 146, and 147. In all cases, the nonthermal radio emission almost certainly originates from an interaction between the Wolf-Rayet stellar wind and the wind from a massive companion star. The radio observations agree well with theoretical predictions for colliding winds. Synchrotron emission occurs from relativistic electrons accelerated in strong shocks. The nonthermal spectral indices are likely to be close to -0.5. For WR 39, the detected radio emission is offset by similar to 3 " from the optical position of WR 39 and by similar to 2 " from the optical position of WR 38B. We suggest that the radio emission may originate from a wind-wind interaction between WR 39 and WR 38B, although this is not confirmed. For WR 11, the radio spectral index increases from +0.3 between 3 and 6 cm to +1.2 between 13 and 20 cm. This is interpreted as evidence for a highly attenuated nonthermal component that originates well within the ionized wind of the W-R star from an interaction with the wind of the O9 companion star.