Brown Dwarfs Feature Powerful Auroras Like Planets

Published on July 31st, 2015 | by

July 31st, 2015 by

In a finding that shows that brown dwarfs, often referred to as failed stars, have more in common with regular planets than might otherwise be expected, researchers have found that brown dwarfs are home to powerful auroral phenomena near their poles.

The findings are the result of a 20-year study that used radio + optical telescopes to watch a brown dwarf over that timeframe.


“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,” stated researcher 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.”

With the daicovery over the last w decades that brown dwarfs emit radio waves, speculation as to the cause/mechanism has been near constant.

“We see a similar pulsing phenomenon from planets in our solar system,” noted Hallinan, “and that radio emission is actually due to auroras.”

Could something similar be occurring on brown dwarfs? That question is what set off the recent research.

A recent press release provides more:

Following his hunch, Hallinan and his colleagues conducted an extensive observation campaign of a brown dwarf called LSRJ 1835+3259, using the National Radio Astronomy Observatory’s Very Large Array (VLA), the most powerful radio telescope in the world, as well as optical instruments that included Palomar’s Hale Telescope and the WM Keck Observatory’s telescopes.

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 measure precisely the brightness of the brown dwarf over time–no simple feat given that these objects are many thousands of times fainter 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.”

The new findings are detailed in a paper published in the journal Nature.

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