T. A. Th. Spoelstra

Radio observations of candidate magnetic O stars

A number of O stars are suspected to have (weak) magnetic fields because of the observed cyclical variability in their UV wind-lines. However, direct detections of these magnetic fields with optical spectropolarimetry have proven to be very difficult. We have searched for non-thermal radio emission, which would be a strong indication for the presence of a magnetic field. Of our 5 selected candidate magnetic O stars, 3 are detected: xi Per, which we find to have a non-thermal spectrum, and lambda Cep and alpha Cam which show a thermal spectrum. We also find that the fluxes are lower than the expected free-free (thermal) contribution of the stellar wind. This is in agreement with recent findings that the mass-loss rates from O stars using Halpha are overestimated because of clumping in the inner part of the stellar wind.

The Influence of Ionospheric Refraction on Radio Astronomy Interferometry

Observations with radio astronomy interferometers (VLBI and local interferometers) may suffer severe phase errors due to ionospheric refraction and its variations. Techniques to correct these phase errors are discussed. Results of the application of these correction procedures to observations done with the Westerbork Synthesis Radio Telescope are presented.

Radio Monitoring of the Non-Thermal Wolf-Rayet Objects WR125, WR146 and WR147

The radio emission from WR125 (WC7+O9) has faded to a new low, apparently below its stellar wind level, while continuous, variable non-thermal emission has been observed from WR146 (WC6) and WR147 (WN8).

Long-Term Radio Monitoring of Wolf-Rayet Stars With Non-Thermal Emission

Radio observations of Wolf-Rayet stars over a period of up to 20 years show that some of them display variable non-thermal radio emission, some continuously, others at episodes. Two such episodes have been observed from the archetype, the WC7+O4-5 system WR 140 (HD 193793) with a binary period of 2900 d. The non-thermal emission from another WC7+O system, WR 125, faded in the mid-1980s, while continuous, variable non-thermal emission is found in WR 146 (WC6) and WR 147 (WN8). In all four cases the origin of the non-thermal radio emission is thought to originate in the collision of the wind of the Wolf-Rayet star with the wind of a companion or in accretion on the companion itself.

Radio Light-Curves and Circumstellar Mapping of the Colliding Wind Binary WR140

New observations of the colliding wind Wolf-Rayet WC7+O4-5 binary WR140 (HD 193793) at 6 cm and 21 cm with the Westerbork Synthesis Radio Telescope are presented. They show that the 21 cm flux, in particular, rose to a sharp maximum in 1992, lagging about a year behind the 6 cm flux. After maximum, both fell simultaneously. The data are interpreted in terms of the varying circumstellar extinction to a non-thermal source in the wind interaction region due to the winds of both WC and O stars.

A Multi-Wavelength Study of WR140 (HD 193793, WC7+04-5) — A Colliding Wind Binary

This poster addresses a class of Wolf-Rayet stars which show phenomena of colliding supersonic mass-loss winds: variable non-thermal radio emission, variable IR excesses due to episodic dust formation, variable UV P-Cygni profiles due to instabilities in the wind and eclipse effects, variable excessive X-ray emission, and possibly γ-ray emission.