Blomme, R.

published in Massive Stars and High-Energy Emission in OB Associations, pp. 45-48 (2005)

Abstract: The non-thermal radio emission of O-type stars is due to synchrotron radiation from relativistic electrons. These electrons are accelerated in shocks. Most probably, these shocks are due to colliding winds in a binary or multiple system. We present new VLA observations of the non-thermal radio emitter Cyg OB2 No. 8A, which was only recently discovered to be a binary. For this star, the radio fluxes show a good correlation with the phase in the binary period. This result is actually quite surprising because the high free-free optical depth should prevent us from detecting the synchrotron radiation emitted by the colliding-wind region. We can only explain the observations by introducing porosity in the model. We also present a complete series of archive VLA observations on the triple system HD 167971. The radio lightcurve shows a more complicated behaviour than for Cyg OB2 No. 8A: it is difficult to detect the binary period in the observations, but we found a clear variation that occurs on a ˜ 20-year timescale.

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