Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence
Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.
Additional Information© 2015 Macmillan Publishers Limited. All rights reserved. Received 21 August 2014, Accepted 26 May 2015, Published online 30 July 2015. We are grateful to T. Readhead, S. Kulkarni and J. McMullin for working to ensure that simultaneous Palomar and VLA observations could occur. We thank the staff of the Palomar Observatory, the W.M. Keck Observatory and the National Radio Astronomy Observatory for their support of this project. The W.M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The VLA is operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Armagh Observatory is grant-aided by the Northern Ireland Department of Culture, Arts and Leisure. G.H. acknowledges the generous support of D. Castleman and H. Rosen. This material is based upon work supported by the National Science Foundation under grant number AST-1212226/DGE-1144469. G.C. acknowledges support from the University of Oxford and from STFC grant ST/M006190/1. We thank J. Linsky and P. Goldreich for comments on this manuscript. G.H., S.B., M.P.R., A.A., A.G., A.K., M.M.K. and J.G.D. proposed, planned and conducted the radio observations. G.H. and S.B. reduced the VLA data and the dynamic spectrum was output by S.B. G.H. interpreted the dynamic radio spectra. G.H., S.P.L., G.C., R.P.B., J.S.P. and L.K.H. proposed and conducted the Keck observations. G.C. carried out the Palomar observations and reduced the publication data. S.P.L. and G.C. reduced the Keck spectroscopic data, with the final publication data delivered by S.P.L., G.H., G.C. and S.B. G.H., S.P.L. and J.S.P. developed the interpretation of the optical data. S.P.L. carried out the detailed model fitting of the Keck spectra. G.B. analysed high-resolution archival spectra and provided insight on interpretation of the optical data. S.V.B. coordinated contemporaneous spectropolarimetry with the observations presented in this paper. S.P.L. and G.H. wrote the Supplementary Information. All authors discussed the result and commented on the manuscript. Corresponding author: G. Hallinan.
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