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The Auroral Phenomenon: Tracing Magnetic Fields from Brown Dwarfs to Exoplanets

Colloquium: Astronomy Colloquia | March 20 | 4:10 p.m. | 1 LeConte Hall

Greg Hallinan, Caltech

Department of Astronomy

At the lower end of the main sequence, near the boundary between stars and brown dwarfs, a major transition is observed in magnetic activity. The high temperature, X-ray emitting coronae surrounding solar-type stars fade rapidly and have virtually disappeared for the coolest spectral classes, L, T and Y. Magnetic field topologies undergo a dramatic transformation to large-scale, stable configurations and rotation rates increase by an order of magnitude, indicating stellar wind assisted magnetic braking is severely reduced or absent. I will present results that show that this transition also signals the arrival of a new group of electromagnetic phenomena, completely distinct from those traditionally associated with stellar activity. In fact, these emissions are much more analogous to the aurorae detected from the magnetized planets in our Solar System, generated by powerful, magnetospheric current systems that lead to downward precipitation of energetic particles into the high-latitude regions of the upper atmosphere. The resulting emissions from planets, and now brown dwarfs, include intense radio emission 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 upper atmosphere. In the case of optical (Keck) and radio (VLA) observations of one M9 dwarf, the dissipated power associated with these aurorae is 1e6 times larger than those produced in the Jovian magnetosphere. 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 temperature and opacity in near infrared bands, relating to recent reports of weather phenomena on cool brown dwarfs. Notably, the radio emission is a particularly powerful diagnostic and has most recently allowed us to extend magnetic field strength measurements into the late L and T dwarf regime, with potential to extend this further into the exoplanetary regime. To this end, my group has built a new radio telescope at Caltech's Owens Valley Radio Observatory, involving the full cross-correlation of 288 dipole antennas, that allows us to image the entire viewable hemisphere every second in the search for similar auroral radio emission from extrasolar planets., 510-642-5275