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July 2015 ‘Failed Stars’ Host Powerful Auroral Displays
International team led by Irish astronomer says that brown dwarfs behave more like planets than stars
Brown dwarfs are the mysterious middle children of celestial objects. These relatively cool, dim bodies are difficult to detect, and have remained hard to classify. They are too massive to be planets, yet possess some planet-like characteristics; they are too small to sustain hydrogen fusion reactions at their cores, a defining characteristic of stars, yet they have star-like attributes.
Now, by observing a brown dwarf 20 light-years away using both radio and optical telescopes, a team led by Dr Gregg Hallinan, NUI Galway astronomy PhD graduate and now assistant professor of astronomy at Caltech, has found another feature that makes these so-called failed stars more like supersized planets - they host powerful auroras near their magnetic poles.
The findings were co-authored by scientists around the world, including many Irish-trained astronomers and Dr Ray Butler a lecturer in the School of Physics at NUI Galway, and appear in the July 30 issue of the journal Nature.
“We're finding that brown dwarfs are not like small stars in terms of their magnetic activity; they're like giant planets with hugely powerful auroras,” says Hallinan. “If you were able to stand on the surface of the brown dwarf we observed - something you could never do because of its extremely hot temperatures and crushing surface gravity - you would sometimes be treated to a fantastic light show courtesy of auroras hundreds of thousands of times more powerful than any detected in our solar system.”
In the early 2000s, astronomers began finding that brown dwarfs emit radio waves. At first, everyone assumed that the brown dwarfs were creating the radio waves in basically the same way that stars do - through the action of an extremely hot atmosphere, or corona, heated by magnetic activity near the object’s surface. But brown dwarfs do not generate large flares and charged-particle emissions in the way that our sun and other stars do, so the radio emissions were surprising.
While studying for his PhD at NUI Galway, in 2006, Hallinan discovered that brown dwarfs can actually pulse at radio frequencies. “We see a similar pulsing phenomenon from planets in our solar system,” says Hallinan, “and that radio emission is actually due to auroras”. Since then he has wondered if the radio emissions seen on brown dwarfs might be caused by auroras.
Auroral displays result when charged particles, carried by the stellar wind for example, manage to enter a planet’s magnetosphere, the region where such charged particles are influenced by the planet’s magnetic field. Once within the magnetosphere, those particles get accelerated along the planet's magnetic field lines to the planet’s poles, where they collide with gas atoms in the atmosphere and produce the bright emissions associated with auroras.
Following his hunch, Hallinan and his colleagues recently conducted an extensive observation campaign of a brown dwarf called LSRJ 1835+3259, using the National Radio Astronomy Observatory’s Very Large Array (VLA) in New Mexico, the most powerful radio telescope in the world, as well as giant optical instruments that included Palomar’s Hale Telescope in California and the W. M. Keck Observatory's telescopes in Hawaii.
Using the VLA, they detected a bright pulse of radio waves that appeared as the brown dwarf rotated around. The object rotates every 2.84 hours, so the researchers were able to watch nearly three full rotations over the course of a single night.
Next, the astronomers used the Hale Telescope to observe that the brown dwarf varied optically on the same period as the radio pulses. Focusing on one of the spectral lines associated with excited hydrogen - the H-alpha emission line - they found that the object's brightness varied periodically.
Finally, Hallinan and his colleagues used the Keck telescopes to precisely measure the brightness of the brown dwarf over time—no simple feat given that these objects are intrinsically extremely faint, many thousands of times less luminous than our own sun. Hallinan and his team were able to establish that this hydrogen emission is a signature of auroras near the surface of the brown dwarf.
“As the electrons spiral down toward the atmosphere, they produce radio emissions, and then when they hit the atmosphere, they excite hydrogen in a process that occurs at Earth and other planets, albeit tens of thousands of times more intense”, explains Hallinan. “We now know that this kind of auroral behavior is extending all the way from planets up to brown dwarfs.”
In the case of brown dwarfs, charged particles cannot be driven into their magnetosphere by a stellar wind, as there is no stellar wind to do so. Hallinan says that some other source, such as an orbiting planet moving through the brown dwarf’s magnetosphere, may be generating a current and producing the auroras. “But until we map the aurora accurately, we won't be able to say where it's coming from”, he says.
He notes that brown dwarfs offer a convenient stepping stone to studying exoplanets, planets orbiting stars other than our own sun. “For the coolest brown dwarfs we've discovered, their atmosphere is pretty similar to what we would expect for many exoplanets, and you can actually look at a brown dwarf and study its atmosphere without having a star nearby that's a factor of a million times brighter obscuring your observations,” says Hallinan.
The work, ‘Magnetospherically driven optical and radio aurorae at the end of the main sequence’, was supported by funding from the National Science Foundation in the US.
In all, five of the authors are connected with NUI Galway. Ray Butler is a lecturer in the School of Physics; Aaron Golden is on extended leave from his lecturer position in the School of Mathematics, Applied Mathematics and Statistics; Leon Harding did his PhD under the joint supervision of Drs Butler and Golden; and Stephen Bourke and the lead author Gregg Hallinan both did their PhDs under Dr Golden.
NUI Galway’s Ray Butler adds: “The key roles played by so many Irish-trained astronomers, in making the discoveries to produce this Nature publication, demonstrate that we have the skills and ideas to compete with the world’s best in this field. For example, I worked on planning the spectroscopy observations, and developing the methods to analyse them in order to extract the subtle signature of the brown dwarf’s rotation. The selection of this particular brown dwarf followed work by our co-author Leon Harding during his time as my PhD student, when he used GUFI (the Galway Ultra Fast Imager), an instrument that we built ourselves, to observe its optical variability with unprecedented accuracy.
Today’s major breakthrough and the successes of Irish astronomers abroad underline the compelling arguments for the government to reintroduce policies to fund this kind of basic research here in Ireland.”