Red Sky for Brown Dwarf

Astronomers have discovered a new failed star with unusually red, dusty skies.

brown dwarf's atmosphere
Some brown dwarfs seem to have wind-driven cloud structures in their atmospheres, reminiscent of weather on planets.
NASA / JPL-Caltech
Weather forecasts for alien worlds have been a hot topic lately — from the cloudy forecast for gas giant exoplanets to mapping a brown dwarf’s visible surface. But the weather on the L type brown dwarf ULAS J222711–004547 stays the same: red.

An international team of scientists from institutions in the UK and Italy discovered this “unusually red L dwarf” (URL) using the United Kingdom Infrared Deep Sky Survey (UKIDSS) Large Area Survey data. Though Caltech’s Dwarf Archive currently lists more than 900 known L dwarfs, only nine of these fit the mysterious “unusually red” category. And the newfound ULAS J222711–004547 is far brighter at longer infrared wavelengths than other L dwarfs, making it one of the reddest of this rare population.

Usually, an L dwarf is redder because it’s young. By “redder,” astronomers mean that the brown dwarf emits more of its energy at a wavelength of 2.1 microns relative to 1.2 microns than the average L dwarf of the same spectral type. (Although brown dwarfs do not have enough mass to sustain hydrogen fusion in their cores and so don’t shine like our Sun, they still radiate thermally as they cool and contract over time.)

Young brown dwarfs tend to be redder because they’re puffier than old brown dwarfs. The lower pressure means that less energy is absorbed by hydrogen molecules in the failed star’s atmosphere, allowing more light to pass through.

However, ULAS J222711–004547 doesn’t seem to be a young L dwarf. Recent studies have found older brown dwarfs sometimes also have this youthful blush, a red herring (pun intended) that has prompted further investigation.

To explore the strange nature of this URL, the research team analyzed its spectral data.

“It was a bit like when you take a picture with your phone's camera, and then you use different filters using Instagram to make it look nicer,” explained lead author Federico Marocco (University of Hertfordshire, UK). But instead of a picture, they were trying to clean up the brown dwarf’s emission spectrum. “Different minerals and grain sizes give you different ‘filters.’ Finally, we chose the ‘filter’ that made our brown dwarf look more similar (actually, almost identical) to normal brown dwarfs.”

Based on this analysis, the team estimated the characteristic size of the dust grains populating the brown dwarf’s clouds: 0.5 microns.

It is important to note that the atmospheric models for brown dwarfs are still not mature enough to handle real-world complexity — so take that size with a grain of salt.

“Modeling the grain size in a brown dwarf atmosphere hadn't been done this way before, as far as I know,” says Davy Kirkpatrick (Caltech). “Unfortunately, though, finding that these particular objects are dustier than other L dwarfs still doesn't tell us why these particular objects are cloudier.” Knowing that a dusty dwarf can mimic a young, hotter one will hopefully help astronomers figure out what’s going on inside these objects’ atmospheres.

URLs also present an opportunity to study the dividing line between giant planets and brown dwarf. Although astronomers now have a clear line between star and brown dwarf, the dwarf-planet line is blurry at best. Objects with planet-like masses on the scale of tens of Jupiters show near-infrared coloring similar to that of URLs, and theorists think condensates in their atmospheres influence these objects’ colors as well. Because of these parallels, URLs can serve as test beds for planetary atmospheric models sans the glare of a nearby host star.

Reference: F. Marocco et al. "The extremely red L dwarf ULAS J222711?004547 — dominated by dust." Accepted to Monthly Notices of the Royal Astronomical Society.

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