On the mechanism of polarised metrewave stellar emission

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 ($\nu \sim 10^2\mathrm{MHz}$) 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 ($\nu \sim 10^2\mathrm{MHz}$) 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 $\sim 100\mathrm{MHz}$ in canonical M-dwarfs strongly favours a cyclotron maser interpretation.

Revised version (under review MNRAS)

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• A Review of Solar Type III Radio Bursts
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• Limits on low frequency radio emission from southern exoplanets with the Murchison Widefield Array
• 154 MHz detection of faint, polarized flares from UV Ceti
• A search for circularly polarised emission from young exoplanets
• Prospects for radio detection of stellar plasma beams

Cited by in corpus (13)

• Low-frequency monitoring of flare star binary CR Draconis: Long-term electron-cyclotron maser emission
• V-LoTSS: The Circularly-Polarised LOFAR Two-metre Sky Survey
• Fine structures of radio bursts from flare star AD Leo with FAST observations
• Coherent radio emission from a population of RS Canum Venaticorum systems
• Energetic particle activity in AD Leo: Detection of a solar-like type-IV burst
• Peculiar radio$-$X-ray relationship in active stars
• Radio masers on WX UMa: hints of a Neptune-sized planet, or magnetospheric reconnection?
• Large closed-field corona of WX UMa evidenced from radio observations
• Location and energy of electrons producing the radio bursts from AD Leo observed by FAST in December 2021
• Searching for stellar flares from low mass stars using ASKAP and TESS
• Starspots as the origin of ultrafast drifting radio bursts from an active M dwarf
• A circularly polarized low-frequency radio burst from the exoplanetary system HD 189733
• First detection of variable radio emission originating from the infant planetary system V1298 Tau