From astrobites: “Auroral Displays at Brown Dwarfs”

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Title: Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence
Authors: G.Hallinan, S. P. Littlefair, G. Cotter, et al.
First Author’s Institution: California Institute of Technology
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Status: Published in Nature (2015), open access

Auroras are the spectacular light shows visible in the polar regions at Earth and other planets. In 2015 they were detected for the first time outside of the solar system. Brown dwarfs are objects often described as “failed stars”, meaning they are insufficiently massive to ignite hydrogen fusion in their cores. Today’s paper reports on the remarkable discovery that a particular brown dwarf plays host to auroral displays far more powerful than those found anywhere in the solar system.

Brown dwarfs

Artist's concept of a Brown dwarf [not quite a] star. NASA/JPL-Caltech
Artist’s concept of a Brown dwarf [not quite a] star. NASA/JPL-Caltech

Brown dwarfs occupy the region between giant planets and the lowest mass stars. It is generally accepted that they form in a manner similar to stars, i.e. the gravitational collapse of interstellar gas, but never reaching a mass sufficient to sustain hydrogen fusion in the core. As such, brown dwarfs are extremely cool, faint objects, making their detection much more difficult than ordinary stars. However, they provide an excellent opportunity to for us to better understand the physics that differentiates the stellar and planetary domains. Since their discovery many surveys have been performed which have revealed, amongst other things, the existence of complex weather systems and strong global magnetic fields.

Auroras

Understanding the interaction of the magnetic field at a brown dwarf with its nearby space environment is a key scientific goal. At Earth, space scientists observe the aurora as a means of revealing the structure and dynamics of the magnetic field, and the plasma which interacts with it. Before turning to auroras at brown dwarfs we shall briefly review at what we know about auroras from our studies at Earth and other solar system planets.

The vibrant displays that we see are a result of charged particles (i.e. electrons and ions) from the plasma population around the Earth raining down along magnetic field lines, and colliding with molecules in the atmosphere. These collisions excite the atmospheric constituents to a higher energy state, causing the emission of a photon as they return to their original state.

Auroral emissions aren’t just confined to Earth; they are found at other magnetised planets in the solar system, with Jupiter being a particularly spectacular example.

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JUNE 30, 2016: Astronomers are using NASA’s Hubble Space Telescope to study auroras — stunning light shows in a planet’s atmosphere — on the poles of the largest planet in the solar system, Jupiter. The auroras were photographed during a series of Hubble Space Telescope Imaging Spectrograph far-ultraviolet-light observations taking place as NASA’s Juno spacecraft approaches and enters into orbit around Jupiter. The aim of the program is to determine how Jupiter’s auroras respond to changing conditions in the solar wind, a stream of charged particles emitted from the sun. Auroras are formed when charged particles in the space surrounding the planet are accelerated to high energies along the planet’s magnetic field. When the particles hit the atmosphere near the magnetic poles, they cause it to glow like gases in a fluorescent light fixture. Jupiter’s magnetosphere is 20,000 times stronger than Earth’s. These observations will reveal how the solar system’s largest and most powerful magnetosphere behaves. The full-color disk of Jupiter in this image was separately photographed at a different time by Hubble’s Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project that annually captures global maps of the outer planets.
Date 30 June 2016
Source http://hubblesite.org/newscenter/archive/releases/2016/24
Author NASA, ESA, and J. Nichols (University of Leicester)

Neither are they confined only to the visible part of the spectrum; auroral emissions occur from radio frequencies through to UV and X-ray.

Now we return to brown dwarfs. Since 2006 it has been known that a handful of brown dwarfs emit very regular and persistent radio bursts. These burst are pulsed at the rotation period of the dwarf, leading some researchers to suggest that they may be caused by auroras that are generated in a similar manner to Jupiter’s main auroral oval. The pulsing in this case may be due to the magnetic axis being tilted from the spin axis, so that as the dwarf rotates the auroral emission cones into our line of sight. This motivated the authors of today’s paper to target a particular brown dwarf, LSR J1835 + 3259, with simultaneous radio and optical observation, pursuing a possible relation between the two.

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LSR J1835 + 3259. Image: http://images.zeit.de/ http://www.theweeklyobserver.com/ailed-star-shows-dazzling-display-of-northern-lights/5575/

Radio observations were made using the Very Large Array (VLA) radio telescope, while simultaneously, optical measurements were made with the 5.1 m Hale telescope at the Palomar Observatory with follow-up observations from the 10 m Keck telescope.

NRAO/VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA
NRAO/VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

Caltech Palomar 200 inch Hale Telescope, at Mt Wilson, CA, USA
Caltech Hale Telescope at Palomar interior
Caltech Palomar 200 inch Hale Telescope, at Mt Wilson, CA, USA

Keck Observatory, Mauna Kea, Hawaii, USA
Keck Observatory Interior
Keck Observatory, Mauna Kea, Hawaii, USA

The results of the observations are shown in Figure 1, where the light curve from the optical measurements (Fig 1a) shows a clear periodicity of 2.84 h. Observations of the radio emission (Fig 1b) show the same periodicity, with a slight offset in phase causing it to lag slightly behind the optical emission. The authors attribute their findings to auroras which are driven by strong electric currents flowing in the magnetosphere of the dwarf.

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Figure 1: (a) Optical measurements of Balmer line emission of LSR J1835 made using the Hale telescope. (b) Corresponding radio observations of the same object made using the VLA radio telescope. [Figure 1 from Hallinan et al. 2015]

With this discovery many open questions are presented. What is the mechanism driving the auroras? It may be interaction with the interstellar medium, analogous to the process of the Earth’s magnetosphere interacting with the solar wind. Or it could be due to a continuously replenishing source of plasma mass outflow from within a closed magnetosphere, analogous to the mechanism producing Jupiter’s main auroral oval. Additionally, the source of the required plasma population is unknown, with the cool temperatures (∼2000 K) of brown dwarfs being unable to support significant ionisation of their atmospheres, and the lack of nearby stars restricting the possibility ionisation by stellar irradiation.

Ultimately it is an exciting prospect that this discovery, along with the arrival of even more sensitive radio telescopes (e.g. the Square Kilometre Array), may pave the way towards detecting auroras at exoplanets.

SKA Square Kilometer Array

This which would add a novel technique to the exoplanet-detectors toolkit, and enable us to learn about the magnetic fields and plasma populations around those objects.

See the full article here .

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