Name/Affiliation: Gregg Hallinan (Caltech)
The Radio and Optical Aurorae of Brown Dwarfs
Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are generated by powerful, magnetospheric current systems that lead to downward precipitation of energetic particles into the high-latitude regions of the upper atmosphere. In the case of the gas giant planets, these aurorae include intense radio emission at kHz and MHz frequencies produced by the downward precipitating electrons, and a myriad of continuum and line emission in the infrared, optical and ultraviolet associated with the collisional excitation and ionization of the hydrogen-dominated atmosphere. I will present radio and optical data of a low-mass dwarf of spectral type M8.5, located right at the boundary between stars and brown dwarfs, from which we have detected both radio and optical auroral signatures that bear striking resemblance to those detected from gas giant planets. The dissipated power associated with these aurorae is 10^6 times larger than those produced in the Jovian magnetosphere, revealing such emissions to be a scale-independent and potentially ubiquitous signature of magnetospheric current systems. As well as signalling a transition in magnetic activity at the end of the main sequence, auroral currents may play a causal role in modifying atmospheric opacity in near infrared bands, relating to recent reports of weather phenomena on cool brown dwarfs. In addition, I will present results from recent radio (VLA) and optical surveys (Keck) that extend studies of the auroral phenomenon into the L and T dwarf regime. In particular, the detection of radio emission from multiple candidates requires the presence of magnetic field strengths that are challenging for current dynamo theories.