On the mechanism of polarized metre-wave stellar emission

Abstract

ABSTRACT 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 ($\nu \sim 10^2\, {\rm MHz}$) 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 ($\nu \sim 10^2\, {\rm MHz}$) 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 ${\gtrsim}50{{\ \rm per\ cent}}$ circular polarized fraction at brightness temperatures of ${\gtrsim}10^{12}\,$K at ${\sim}100\, {\rm MHz}$ in M-dwarfs strongly favours a cyclotron maser interpretation.