Abstract
Many O stars are observable at radio wavelengths, due to thermal emission from the ionized gas in the stellar wind. The emergent radio spectrum has a characteristic spectral index 0.6 (i.e., F[ nu ] ~ lambda({-0.6}}_{) ). However, about 25% of the brightest O stars have a radio spectrum that differs from the thermal radio emission (Bieging, Abbott, & Churchwell 1989). This non-thermal radio emission is believed to be synchrotron radiation from relativistic particles accelerated in shocks (White 1985). Such shocks could come from colliding winds or from the instability of the driving mechanism of the wind. The magnetic field in hot stars needed to produce a synchrotron spectrum is below the current detection limits.
