Webb discovers potential aurorae on brown dwarf

by Haygen Warren

Silently floating through the universe are cosmic objects that are both too large to be planets and too small to be stars. Called brown dwarfs, these substellar objects are among the most captivating objects in the universe, and their low surface temperatures mean that most of the light they emit is infrared and thus can only be observed and characterized using infrared telescopes.

Fortunately, the joint NASA/European Space Agency/Canadian Space Agency James Webb Space Telescope is an infrared-sensitive observatory, with the telescope’s powerful suite of instruments primarily observing in the near-infrared and mid-infrared regions of the electromagnetic spectrum. Recently, a team of astronomers used Webb to observe a brown dwarf called W1935 and found an infrared emission from methane in the upper atmosphere of the brown dwarf. While this isn’t an uncommon detection, W1935 does not orbit a star — meaning there isn’t an obvious source behind the emission.

The team does have theories as to what could be causing the methane emission and one of the leading theories involves the production of aurorae in the upper atmosphere of W1935.  The team believes that excess energy within the brown dwarf’s upper atmosphere could be what’s causing the emission, and after investigating the upper atmospheric environments of Jupiter and Saturn, the team found that the upper atmospheric heating that powers methane emissions from Jupiter and Saturn is linked to the production of aurorae.

Graph showing the spectra of W1935 and W2220. Note the methane emission on the W1935 graph. (Credit: NASA/ESA/CSA/L. Hustak (STScI))

If aurorae are indeed present on W1935, they wouldn’t be the same as aurorae seen on Earth. Aurorae produced within Earth’s atmosphere are created when Earth’s magnetic field interacts with solar wind that is ejected into space by the Sun. The energetic particles that make up the solar wind are caught by Earth’s magnetosphere and fall magnetic lines near Earth’s poles. The collision of these particles with Earth’s atmosphere is what creates the iconic green swirls of Earth’s aurora.

However, earth is not the only planet within our solar system that experiences aurorae. As mentioned, Jupiter and Saturn regularly experience aurorae at their poles. The processes by which Jovian and Saturnian aurorae are created are similar to those of Earth, with surrounding moons, such as Jupiter’s Io and Saturn’s Enceladus, contributing to the gas giants’ aurorae.

However, in the case of W1935, there is no stellar object to produce stellar wind that could interact with the brown dwarf’s atmosphere to produce aurorae or excess atmospheric energy. The team explains that, for W1935’s methane emission to make sense, either unknown internal atmospheric phenomena or external interactions with interstellar plasma/material have to occur.

So, what is so intriguing about this methane emission? Why is the team investigating its source?

W1935 was investigated as part of a project led by Jackie Faherty to use Webb to investigate 12 brown dwarfs. Another brown dwarf investigated by Faherty et al., called W2220, was found to be a near clone of W1935 in composition, brightness, temperatures, and spectral features of water, ammonia, carbon monoxide, and carbon dioxide. However, the only major difference between the two brown dwarfs was the methane emission from W1935, with W2220 showing an absorption feature.

“We expected to see methane because methane is all over these brown dwarfs. But instead of absorbing light, we saw just the opposite: The methane was glowing. My first thought was, what the heck? Why is methane emission coming out of this object?” said Faherty.

To further investigate the methane emission, Faherty et al. utilized computer models that modeled both W1935 and W2220. The W2220 model showed an — expected — distribution of energy throughout the entire atmosphere of the brown dwarf, wherein the atmosphere gets increasingly colder with increasing altitude. However, the W1935 model results showed the exact opposite — an unexpected and surprising result. On W1935, atmospheric temperatures get warmer and warmer with increasing altitude; a phenomenon known as temperature inversion.

“This temperature inversion is really puzzling. We have seen this kind of phenomenon in planets with a nearby star that can heat the stratosphere, but seeing it in an object with no obvious external heat source is wild,” said co-author Ben Burningham of the University of Hertfordshire in England.

As mentioned, given brown dwarves’ similarities to gas giants like Jupiter, the team turned to Jupiter and Saturn to investigate possible causes for the temperature inversion within W1935. Faherty et al. found that temperature inversions are prominent within Jupiter and Saturn, and current theories suggest that external heating from aurorae and internal energy transport are responsible for them.

Interestingly, the aurorae theory for W1935 is not the first time aurorae has been used to explain observations of brown dwarfs. From warmer brown dwarfs, scientists have detected radio emissions and used aurorae to explain the emissions. No telescope is as sensitive to infrared light as Webb, though, and thus further observations of these radio-emitting brown dwarfs to characterize the potential aurorae have been inconclusive.

However, W1935 is the first brown dwarf auroral candidate to feature the methane emission signature. What’s more, it’s also the coldest auroral candidate, with a temperature of about 200 degrees Celsius, which is around 316 degrees Celsius warmer than Jupiter.

Future observations of W1935 and other auroral candidate brown dwarfs with Webb will allow scientists to better understand aurorae on brown dwarfs. In the case of W1935, further investigation into how aurorae could form without stellar wind is needed.

“With W1935, we now have a spectacular extension of a solar system phenomenon without any stellar irradiation to help in the explanation. With Webb, we can really ‘open the hood’ on the chemistry and unpack how similar or different the auroral process may be beyond our solar system,” Faherty said.

Faherty et al.’s results were presented at the 243rd meeting of the American Astronomical Society in New Orleans in early January.

(Lead image: artist’s concept of W1935 with aurorae. Credit: NASA/ESA/CSA/L. Hustak (STScI))

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