HST FGS1R Observations of 18 Radio Emitting Wolf-Rayet Stars

Abstract

Non-thermal radio emission in Wolf-Rayet (WR) stars is hypothesized to be synchrotron emission from shocks in the wind. For single star models, the shocks arise from instabilities in the WR wind itself. In binary models, the shocks form at the wind-wind interaction zone between a WR star and a massive companion. In Niemela et al. (1998), we used WFPC2 PC imaging to investigate the binary theory. For two WR stars we linked the non-thermal emission with the colliding wind region between binary companions. These observations support the conclusion that non-thermal radio emission arises in a wind collision zone between binary companions outside of the radio photosphere of the WR star.However, not all WR stars exhibiting non-thermal radio emission are known to have a binary companion. To conclusively link non-thermal emission to binarity, we must demonstrate that all non-thermal emitters are binary, and that all thermal emitters are either single stars or binary systems with separations either too wide or too close to result in a wind-wind interaction that produces shocks.In an effort to accomplish this goal we used the FGS1R interferometer on board the Hubble Space Telescope (HST) to observe 9 non-thermal emitting WR stars as well as a control sample of 9 thermal emitting WR stars. The FGS can resolve angular separations as small as 0”.007 within the currently accepted size of the radio photosphere for the relatively nearby stars in our sample. We found only one binary among the non-thermal emitting WR stars; WR 48 has a companion at a separation of 40 mas which is probably responsible for colliding winds non-thermal emission. However, the lack of evidence for companions among the other non-thermal emitters casts some doubt on the applicability of the colliding winds model for all non-thermal WR stars.KeywordsWolf-RayetStars-Binaries-Non-thermalRadioEmission