Webb observes possible aurorae on brown dwarf
Using James Webb, a team of astronomers have observed infrared emission from methane in the atmosphere of a brown dwarf. This unexpected discovery is most likely driven by energy in the upper atmosphere; however, the source of such energy is currently a mystery. It has been proposed that processes generating aurorae are driving such emission.
Brown dwarfs, also known as failed stars, are objects in between a planet and a star. They are larger than gas giants such as Jupiter and Saturn, but smaller than stars. Despite this, they form in a way similar to stars, but don’t have sufficient mass to trigger hydrogen fusion. The brown dwarf showing such aurorae is called W1935. However, W1935 is an isolated brown dwarf with no companion star, meaning that there is no obvious source for energy in the upper atmosphere.
The team, led by Jackie Faherty, an astronomer at the American museum of natural history in New York turned to our solar system for an answer to this mystery. Methane emission can commonly be observed in solar system gas giants, where heating of the upper atmosphere which powers the emission is connected to aurorae. On earth, aurorae more well known as the northern or southern lights are created when energetic particles from the sun are captured by Earths magnetic field. The particles cascade down into the atmosphere along magnetic field lines near the poles. Ultimately, there are collisions between such particles and gas molecules, which results in the magnificent and colourful aurorae.
Similar events are seen to occur on Jupiter and Saturn, where the processes involve interacting with the solar wind, whilst also having contributions from nearby active moons like Io and Enceladus. The team have suggested that the emission on the brown dwarf may be explained by unaccounted for internal processes like the atmospheric phenomena of Jupiter and Saturn, or external interactions with interstellar plasma or a nearby active moon.
Faherty was awarded JWST time to observe 12 cold brown dwarfs. Included in the 12 was W1935, first discovered by citizen scientist Dan Caselden, who worked with the backyard worlds Zooniverse project. Additionally, W2220 was another brown dwarf in the sample, discovered with NASA’s Wide Field Infrared Survey Explorer. Thanks to Webb, it was revealed that the two objects were almost identical in composition, sharing a similar brightness, temperature and spectral features of water, ammonia, carbon monoxide and carbon dioxide. However, the main difference was W1935’s methane emission. This contrasted with the absorption feature seen for the same molecule in W2220, observed at a particular infrared wavelength in which Webb is sensitive.
“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.
The team used computer models to figure out what could be driving the emission. From these models, W2220 had an expected distribution of energy throughout the atmosphere, with temperatures decreasing with increasing altitude. W1935 had the opposite trend. A model which involved a temperature inversion was favoured, where the atmosphere increases in temperature at higher altitudes. “This temperature inversion is really puzzling,” said Ben Burningham, a co-author from the University of Hertfordshire in England and lead modeler on the work. “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.”
Similar temperature inversions are seen in Jupiter and Saturn, so can be used as a way to understand what is happening in the atmospheres of these distant failed stars. Processes behind the inversion aren’t yet well established, however, the leading theories involve external heating by aurorae as well as internal energy transport from deeper areas of the atmosphere.
Previously, radio emission has been detected from several warmer brown dwarfs, the most likely explanation being aurorae. An attempt to further characterise the phenomenon was made through ground-based telescope observations, searching for infrared signatures. However, results were inconclusive. This makes W1935 the first auroral candidate outside the solar system with a signature of methane emission. Additionally, it is the coldest of the candidates with a temperature of 200°C
“With W1935, we now have a spectacular extension of a solar system phenomenon without any stellar irradiation to help in the explanation.” Faherty noted. “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,” she added.
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Cover image: NASA, ESA, CSA, and L. Hustak (STScI)