High-resolution radio observations have revealed that non-thermal radio
emission in WR stars arises where the stellar wind of the WR star collides with
that of a binary companion. These colliding-wind binary (CWB) systems offer an
important laboratory for investigating the underlying physics of particle
acceleration. Hydrodynamic models of the binary stellar winds and the
wind-collision region (WCR) that account for the evolution of the electron
energy spectrum, largely due to inverse Compton cooling, are now available.
Radiometry and imaging obtained with the VLA, MERLIN, EVN and VLBA provide
essential constraints to these models. Models of the radio emission from WR146
and WR147 are shown, though these very wide systems do not have defined orbits
and hence lack a number of important model parameters. Multi-epoch VLBI imaging
of the archetype WR+O star binary WR140 through a part of its 7.9-year orbit
has been used to define the orbit inclination, distance and the luminosity of
the companion star to enable the best constraints for any radio emitting CWB
system. Models of the spatial distribution of relativistic electrons and ions,
and the magnetic energy density are used to model the radio emission, and also
to predict the high energy emission at X-ray and gamma-ray energies. It is
clear that high-energy facilities e.g. GLAST and VERITAS, will be important for
constraining particle acceleration parameters such as the spectral index of the
energy spectrum and the acceleration efficiency of both ions and electrons, and
in turn, identify unique models for the radio spectra. This will be especially
important in future attempts to model the spectra of WR140 throughout its
complete orbit. A WCR origin for the synchrotron emission in O-stars, the
progenitors of WR stars, is illustrated by observations of Cyg OB2 No. 9.Comment: Invited review at the 8th EVN Symposium, Torun September 26-29, 2006.
11 pages, 12 figure
