Radio observations of massive stars
  Detectable radio emission occurs during almost all phases of massive star
evolution. I will concentrate on the thermal and non-thermal continuum emission
from early-type stars. The thermal radio emission is due to free-free
interactions in the ionized stellar wind material. Early ideas that this would
lead to an easy and straightforward way of measuring the mass-loss rates were
thwarted by the presence of clumping in the stellar wind. Multi-wavelength
observations provide important constraints on this clumping, but do not allow
its full determination. Non-thermal radio emission is associated with binarity.
This conclusion was already known for some time for Wolf-Rayet stars and in
recent years it has become clear that it is also true for O-type stars. In a
massive-star binary, the two stellar winds collide and around the shocks a
fraction of the electrons are accelerated to relativistic speeds. Spiralling in
the magnetic field these electrons emit synchrotron radiation, which we detect
as non-thermal radio emission. The many parameters that influence the resulting
non-thermal radio fluxes make the modelling of these systems particularly
challenging, but their study will provide interesting new insight into massive
stars.
