Auroral Radio Emission from Ultracool Dwarfs: a Jovian Model

Abstract

A number of fast-rotating ultra cool dwarfs (UCDs) emit pulsed coherent radiation, attributed to the electron cyclotron maser instability, a phenomenon that occurs in the solar system at planets with strong auroral emission. In this paper we examine magnetosphere-ionosphere coupling currents in UCDs, adopting processes used in models of Jovian emission. We consider the angular velocity gradient arising from a steady outward flux of angular momentum from an internal plasma source, as analogous to the jovian main oval current system, as well as the interaction of a rotating magnetosphere with the external medium. Both of these mechanisms are seen in the solar system to be responsible for the production of radio emission. We present the results of an investigation over a range of relevant plasma and magnetosphere-ionosphere coupling parameters to determine regimes consistent with observed UCD radio luminosities. Both processes are able to explain observed UCD luminosities with ionospheric Pedersen conductances of ~1-2 mho, either for a closed magnetosphere with a plasma mass outflow rate of ~10${}^5$ kg/s, i.e. a factor of ~100 larger than that observed at Jupiter's moon Io, or for a dwarf with an open magnetosphere moving through the interstellar medium at ~50 km/s and a plasma mass outflow rate of ~1000 kg/s. The radio luminosity resulting from these mechanisms have opposing dependencies on the magnetic field strength, a point which may be used to discriminate between the two models as more data become available.

12 pages, 10 figures

References in corpus (11)

• Simulations of dynamo action in fully convective stars
• Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs
• Magnetic field generation in fully convective rotating spheres
• Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf
• Predicting low-frequency radio fluxes of known extrasolar planets
• Rotational Modulation of the Radio Emission from the M9 Dwarf TVLM 513-46546: Broadband Coherent Emission at the Substellar Boundary?
• The Arecibo Detection of the Coolest Radio-flaring Brown Dwarf
• Wideband Dynamic Radio Spectra of Two Ultra-cool dwarfs
• Discovery of Radio Emission from the Tight M8 Binary: LP 349-25
• A Mini-survey of Ultracool Dwarfs at 4.9 GHz
• A LOFAR mini-survey for low-frequency radio emission from the nearest brown dwarfs

Cited by in corpus (15)

• Low-frequency monitoring of flare star binary CR Draconis: Long-term electron-cyclotron maser emission
• Resolved imaging of an extrasolar radiation belt around an ultracool dwarf
• Direct radio discovery of a cold brown dwarf
• The CARMENES search for exoplanets around M dwarfs: Stable radial-velocity variations at the rotation period of AD~Leonis -- A test case study of current limitations to treating stellar activity
• Fine structures of radio bursts from flare star AD Leo with FAST observations
• Brown dwarfs as ideal candidates for detecting UV aurora outside the Solar System: Hubble Space Telescope observations of 2MASS J1237+6526
• The TESS View of LOFAR Radio-Emitting Stars
• Polarised radio pulsations from a new T dwarf binary
• Radio Emission from Binary Ultracool Dwarf Systems
• The Occurrence Rate of Quiescent Radio Emission for Ultracool Dwarfs using a Generalized Semi-Analytical Bayesian Framework
• Differences in radio emission from similar M dwarfs in the binary system Ross 867-8
• Location and energy of electrons producing the radio bursts from AD Leo observed by FAST in December 2021
• Transitions in magnetic behavior at the substellar boundary
• Starspots as the origin of ultrafast drifting radio bursts from an active M dwarf
• Simulations of Electron Beam Interactions in Brown Dwarf Atmospheres