Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence

Abstract

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.

EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

“…Despite this, other diagnostics of EGP ionospheres could prove to be more promising in the near future, such as the detection of radio emissions from magnetosphere-ionosphere coupling (Nichols 2011). Although these observations have yet to succeed for EGPs, radio emissions emanating from a brown dwarf have recently been detected (Hallinan et al 2015).…”

First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved

“…If the same mechanism is responsible for radio bursts in ultra-cool dwarfs at frequencies of few GHz, the magnetic field of a few kilo-Gauss is required on the dwarf surface (Hallinan et al 2006), i.e., as high as those possessed by earlier-type classical M-type flare stars. There is also evidence that the radio pulses correlate with bright regions seen in the optical light curves (Harding et al 2013;Hallinan et al 2015). The question of whether these bright spots are magnetic, as some models suggest (Kuznetsov et al 2012), remains to be answered.…”

“…Other evidence for magnetic activity in ultra-cool dwarfs is the presence of both quiescent and flaring non-thermal radio emission (e.g., Berger et al 2001;Berger 2006;Hallinan et al 2006Hallinan et al , 2007Antonova et al 2007). A number of them exhibit transient events with periodic radio bursts and aurora-like, hydrogen and metal line optical emission (Hallinan et al , 2015 but no X-ray corona as in warmer, more massive, active red dwarf stars (Neuhäuser et al 1999;Preibisch et al 2005;Grosso et al 2007). The radio luminosity of some ultracool M dwarfs and brown dwarfs is even higher than that of warmer, earlier-type active M dwarfs, and it continues to increase beyond spectral type M8 (Berger 2006), the border where both cool, old low-mass stars and warm, young brown dwarfs can be found.…”

L-BAND SPECTROSCOPY WITH MAGELLAN-AO/Clio2: FIRST RESULTS ON YOUNGLOW-MASS COMPANIONS

“…If the increase in our spectrum is physical, the larger-thanexpected flux in the spectral region corresponding to methane opacity could indicate an atmospheric inversion or possibly an aurora (e.g., Hallinan et al 2015). However, strong + H 3 emission, often associated with aurorae (e.g., Brown et al 2003), is not present at 3.5 and 3.7 μm.…”