AAS 197, January 2001
Session 97. Evolved Stars
Oral, Wednesday, January 10, 2001, 1:30-3:00pm, Pacific One

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[97.04] HST FGS1R Results On the Association Between Binary Wolf-Rayet Stars and Non-Thermal Radio Emission

D. J. Wallace, D. R. Gies (CHARA), E. Nelan, C. Leitherer (STScI)

Two separate models have been proposed to explain the non-thermal emission detected in some Wolf-Rayet (WR) stars. In models based on single WR stars, this emission is proposed to arise via synchrotron radiative processes in the outer (intrinsically unstable) WR wind (e.g. White & Chen 1995). In models based on WR~+~O systems, this non-thermal radio emission is suggested to arise from the WR wind colliding with the wind of a companion (e.g. Williams et al.\ 1990). In order to be observed, the colliding winds region is believed to occur in wide binaries where the interaction zone is outside the WR radio photosphere (\approx30~AU based on spherically symmetric uniform wind models).

HST FGS1R observations of 9 non-thermal and 9, as a control group, purely thermal radio emitting stars attempted to verify the theory that this non-thermal emission is always a result of binary interactions. If the binary model is correct, then most or all of our non-thermal targets should have companions with projected separations of 0\farcs01With a single exception, WR 48, {\em no} companions were found. While this result does not rule out the role of companions in producing the non-thermal radio emission, it does not support the companion hypothesis. These results do strongly suggest that a wind interaction region, if occurring, must lie closer to the WR star than previously believed.

Support for this work was provided by NASA through grant number GO-8309 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.

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