2021MNRAS.500.3898V

Two coherent radio emission mechanisms operate in stellar coronae: plasma emission and cyclotron emission. They directly probe the electron density and magnetic field strength respectively. Most stellar radio detections have been made at cm-wavelengths where it is often not possible to uniquely identify the emission mechanism, hindering the utility of radio observations in probing coronal conditions. In anticipation of stellar observations from a suite of sensitive low-frequency (ν∼10² MHz^) radio telescopes, here I apply the general theory of coherent emission in non-relativistic plasma to the low-frequency case. I consider the recently reported low-frequency emission from dMe flare stars AD Leo and UV Ceti and the quiescent star GJ 1151 as test cases. My main conclusion is that unlike the cm-wave regime, for reasonable turbulence saturation regimes, the emission mechanism in metre-wave observations (ν∼10² MHz) can often be identified based on the observed brightness temperature, emission duration, and polarization fraction. I arrive at the following heuristic: M-dwarf emission that is >= hour-long with ≳50 per cent circular polarized fraction at brightness temperatures of ≳10¹² K at ∼100 MHz in M-dwarfs strongly favours a cyclotron maser interpretation.