On the mechanism of polarised metrewave stellar emission

TL;DR

This paper analyzes the mechanisms behind low-frequency stellar radio emissions, demonstrating that brightness temperature, duration, and polarization can distinguish between plasma and cyclotron emission in stellar coronae.

Contribution

It applies the theory of coherent emission to low-frequency observations, providing criteria to identify emission mechanisms in stellar radio data.

Findings

Cyclotron emission likely dominates in long-duration, highly polarized, high brightness temperature signals.

The study offers a heuristic for mechanism identification based on observational parameters.

It enhances interpretation of low-frequency stellar radio observations for coronal diagnostics.

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 polarisation fraction. I arrive at the following heuristic: M-dwarf emission that is $\gtrsim \,$hour-long with $\gtrsim 50\%$ circular polarised fraction at brightness temperatures of $\gtrsim 10^{12}\,$K at $\sim 100\,{\rm MHz}$ in canonical M-dwarfs strongly favours a cyclotron maser interpretation.