On the mechanism of polarized metre-wave 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 (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 polarisation fraction. I arrive at the following heuristic: M-dwarf emission that is ≳ hour-long with $\gtrsim 50{{\ \rm per\ cent}}$ circular polarised fraction at brightness temperatures of ≳ 1012 K at ∼100 MHz in M-dwarfs strongly favours a cyclotron maser interpretation.

Large closed-field corona of WX Ursae Majoris evidenced from radio observations

“…Theory allows the brightness temperature of cyclotron maser emission to be many orders of magnitude higher than this value (Treumann 2006;Melrose & Dulk 1982), and hence, the observed value can be readily accommodated. Fundamental plasma emission on the other hand cannot attain brightness temperatures 10 12 K, especially at such low frequencies (Vedantham 2021). As shown in §3.1, the projected source radius for plasma emission is x ≈ 7.4, giving an observed brightness temperature of ∼ 10 11 K. Although we note there is no precedence for plasma emission to be simultaneously occurring at this brightness throughout the corona (at x = 7.4 in this case), such a brightness temperature can be reached by plasma emission based on canonical theory if the plasma is highly turbulent (see Appendix A.1).…”

“…It is therefore in the o-mode if it originates in the polar regions, where the field points away from us, and x mode if it originates in the equatorial regions, where the field points towards us. Fundamental plasma emission is polarised in the o-mode, whereas cyclotron maser emission can be polarised in the x-mode or o-mode depending on the ambient plasma density (Dulk 1985;Vedantham 2021). Because WX UMa has a mostly axisymmetric dipolar field (Morin et al 2010), we use cyclotron maser emission from Jupiter as a benchmark for our discussion for the possibility of cyclotron maser emission from WX UMa (Zarka 1998).…”

Low-frequency monitoring of flare star binary CR Draconis: long-term electron-cyclotron maser emission

“…The most common way to generate plasma emission is via an impulsive injection of heated plasma into the corona (Dulk 1985;Stepanov et al 2001). Assuming a stable hydrostatic density of the corona (Vedantham 2020;Callingham et al 2021), and applying Equations 15 and 22 of Stepanov et al (2001), the brightness temperature reaches ≈0.4×10 12 K if the impulsive event injected into the corona is 20 times the coronal temperature. The assumptions required in such a calculation are accurate to an order of magnitude as, for example, the coronal plasma density is an upper-limit since CR Dra is not resolved in the X-ray observations (Boller et al 2016).…”

“…In a coronal loop, it is possible to set up an unstable losscone distribution of electron energies to drive the ECMI emission (Dulk 1985;Morosan et al 2016;Vedantham 2020). Injection of heated plasma into the loop, combined with magnetic mirroring, sets up the loss-cone distribution.…”

The population of M dwarfs observed at low radio frequencies

“…Therefore, incoherent gyrosynchrotron radiation can be confidently excluded 1 . More detailed information about the emission physics can be found in Vedantham 64 .…”