Brown dwarf aurora may help characterize distant exoplanets

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Artist's impression of LSRJ 1835+3259(Credit: Chuck Carter and Gregg Hallinan/Caltech)

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The discovery of a powerful aurora surrounding a distant failed star may in future aid astronomers in their hunt for habitable planets. The aurora is the first to be discovered around a brown dwarf, known as LSRJ 1835+3259 (LSRJ). It's a type of star that shares many characteristics with known exoplanets, and the technique used to observe the phenomenon could one day be a factor in determining whether a planet could sustain life.

There are untold billions of stars in our Milky Way that exist in a baffling range of shapes and sizes, from enormous red supergiants to tiny yet incredibly dense neutron stars. However, the category of star that most of us are familiar with is the yellow dwarf – the classification of our own Sun.

A brown dwarf is essentially a star that failed to sustain enough mass to kick start the hydrogen nuclear fusion reaction at its core required to make it burn. Due to this evolutionary failure, a brown dwarf exhibits characteristics common with both stars and large planets. Thanks to these similarities, the discovery of an aurora present on a brown dwarf could have implications in the ongoing search for habitable exoplanets.

A paper covering the recent observations of LSRJ asserts that the aurora, the first to ever be discovered around a brown dwarf, should also be present around other very faint analogues of the quasi-star, and that we may in the future be able to use similar radio emissions to characterize distant worlds.

"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," states assistant professor of astronomy at Caltech, Gregg Hallinan.

The aurora on LSRJ, which Hallinan and his team believe to be hundreds of thousands of times more powerful than any phenomena of its kind in our solar system, was detected using a combination of optical and radio telescopes.

The Very Large Array located at the National Radio Astronomy Observatory, New Mexico(Credit: NRAO/AUI (Photographer: Bob Tetro))

The seeds for the discovery were planted back in the early 2000s, when radio waves were first seen emitting from a brown dwarf. This baffled astronomers, as the bodies lack the standard method of radio wave emission exhibited by stars such as our own Sun that creates the waves through the release of charged particles and solar flares.

In 2006, Hallinan revealed that the enigmatic brown dwarf radio emissions seemed to pulse in a similar manner to those from planets in our solar system known to host aurora. The new study employed the Very Large Array (VLA) located at the National Radio Astronomy Observatory, New Mexico, to characterize the radio emissions from LSRJ. Once again, Hallinan observed radio pulsing as the brown dwarf rotated.

Simultaneously, LSRJ was subjected to surveillance from the Palomar Observatory's Hale telescope, which recorded variations in brightness across the h-alpha emission line. The team then employed optical telescopes located at the Keck Observatory, Hawaii, to make timed observations of the brown dwarf's brightness. The results of the study led to one clear conclusion – that the pulsing radio emissions are the signature of an incredibly powerful aurora.

Whilst the discovery of the aurora may answer the riddle of the radio emissions, it does not explain how an aurora is able to form on a brown dwarf. Ordinarily, for an aurora to form, stellar winds from a nearby star carry charged particles into a planet's magnetosphere, allowing them to excite the gas atoms within to produce colorful emissions such as the aurora borealis.

However, with LSRJ there is no nearby star to produce the stellar wind, and so the team are uncertain as to how the aurora is being produced. One possible explanation holds that a planet orbiting the brown dwarf could have whipped up enough of a current to drive charged particles into the magnetosphere, but further observations will be needed to solve the riddle.

In the future, Hallinan and his team hope to use low-frequency radio observations from the newly-constructed Owens Valley Long Wavelength Array, located in California. This would allow them to measure the strength of an exoplanet's magnetic field in much the same way that optical and radio wave observations were used to characterize LSRJ.

"That could be particularly interesting because whether or not a planet has a magnetic field may be an important factor in habitability," explains Hallinan. "I'm trying to build a picture of magnetic field strength and topology and the role that magnetic fields play as we go from stars to brown dwarfs and eventually right down into the planetary regime."

A paper outlining the discovery has been published in the online journal Nature.

Source: Caltech

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