Radio Emission from Ultra-Cool Dwarfs

TL;DR

Radio observations of ultra-cool dwarfs reveal they generate strong magnetic fields, produce energetic plasmas, and exhibit diverse magnetic phenomena, providing insights into their magnetism and potential as exoplanet hosts.

Contribution

This paper synthesizes recent radio observations of ultra-cool dwarfs, highlighting their magnetic properties and the implications for understanding substellar magnetism and planetary analogs.

Findings

UCDs can generate strong magnetic fields (kG)

They produce nonthermal plasmas with MeV electron energies

They exhibit powerful, coherent radio bursts

Abstract

The 2001 discovery of radio emission from ultra-cool dwarfs (UCDs), the very low-mass stars and brown dwarfs with spectral types of ~M7 and later, revealed that these objects can generate and dissipate powerful magnetic fields. Radio observations provide unparalleled insight into UCD magnetism: detections extend to brown dwarfs with temperatures <1000 K, where no other observational probes are effective. The data reveal that UCDs can generate strong (kG) fields, sometimes with a stable dipolar structure; that they can produce and retain nonthermal plasmas with electron acceleration extending to MeV energies; and that they can drive auroral current systems resulting in significant atmospheric energy deposition and powerful, coherent radio bursts. Still to be understood are the underlying dynamo processes, the precise means by which particles are accelerated around these objects, the observed diversity of magnetic phenomenologies, and how all of these factors change as the mass of the central object approaches that of Jupiter. The answers to these questions are doubly important because UCDs are both potential exoplanet hosts, as in the TRAPPIST-1 system, and analogues of extrasolar giant planets themselves.