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HD~193793 = WR~140 is a WR+O binary system with a 7.9 year, highly elliptical ($e=0.85$) orbit. It shows widely variable, nonthermal radio emission over its orbital period that is explained as synchrotron radiation from relativistic electrons accelerated in the shock between the WR and O winds. Although we now know of quite a few WR$+$O star binary systems, only HD~193793 is well studied across the entire electromagnetic spectrum; hence it affords us the best opportunity to test various models for the system against a wealth of observational data. HD~193793 is an ideal laboratory for studying the properties of hot star winds and the physics of particle acceleration by strong shocks in those winds.
In this paper we present the results of 8 years of monitoring the radio flux density from HD~193793 with the VLA. This database is unique both in terms of its dense coverage of an entire binary cycle and because it extends the radio coverage to 2~cm wavelength, a shorter wavelength than previously available. With this data we are able to simultaneously solve for the time-dependent attenuation in the system and the intrinsic radio luminosity. The standard model of spherically symmetric colliding winds faces severe difficulties in explaining the observations. We conclude that the radio data are most readily interpreted if we adopt a new model of the system in which the WR star wind is confined to a disk. A disk model for the WR wind also provides a natural explanation for the sudden formation of dust that causes an infrared outburst just after periastron.
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