Radio observations of massive stars

TL;DR

This paper reviews radio emissions from massive stars, focusing on thermal free-free emission and non-thermal synchrotron radiation, highlighting their relation to stellar winds, binarity, and the challenges in interpreting observational data.

Contribution

It provides a comprehensive overview of radio emission mechanisms in massive stars and discusses recent insights into wind clumping and binary interactions.

Findings

Thermal radio emission is affected by wind clumping, complicating mass-loss rate measurements.

Non-thermal emission is linked to binarity and wind collisions in massive-star systems.

Modeling non-thermal emission involves complex parameters but offers insights into stellar physics.

Abstract

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.