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Using the James Webb Space Telescope (JWST), astronomers have observed a young brown dwarf known as Cha Hα 1. As a result, they found that the disk around this brown dwarf is hydrocarbon-rich. The finding was published August 7 on the pre-print server arXiv.

Brown dwarfs (BDs) are substellar objects between planets and stars, with masses between 13 and 80 Jupiter masses (0.012 and 0.076 solar masses). Therefore, they have insufficient mass to sustain hydrogen fusion, but enough to trigger deuterium burning.

Observations show that brown dwarfs often possess protoplanetary disks composed of gas and dust. Studies of these disks could provide important clues about the formation of planetary systems. However, the chemistry of BD disks remains largely unexplored, mainly due to their faintness.

Chamaeleon Hα 1, or Cha Hα 1 for short, is a brown dwarf of spectral type M7.5, with an effective temperature of 2,805 K, and a mass of about 0.04–0.05 solar masses. The brown dwarf is a member of the Chamaeleon I South star-forming region at a distance of 625 light years, which is estimated to be 1.4–2.4 million years old.

Previous observations of Cha Hα 1 have found that it hosts an optically thick, flared protoplanetary disk. However, very little is known regarding the properties of this disk and its chemical composition.

That is why a team of astronomers led by María Morales-Calderón of the Spanish Astrobiology Center in Madrid, Spain, have employed the Mid-Infrared Instrument (MIRI) onboard JWST to investigate Cha Hα 1 and its disk. MIRI allowed them to probe the inner, dense, and warm region of the disk, where small planets may form.

The observations found that the disk of Cha Hα 1 contains a plethora of hydrocarbons, including acetylene, methane, ethylene, diacetylene, ethane or benzene. All in all, the results indicate the presence of 12 carbon-bearing molecules in the disk.

The collected data indicate that warm water is present alongside all the hydrocarbons in the disk. Carbon dioxide, molecular hydrogen and hydrogen cyanide have also been identified, but carbon monoxide and hydroxyl radicals turned out to be absent in the spectrum.

The astronomers note that the hydrocarbons in the disk exhibit temperatures between 225–450 K, which suggests that they originate from a shared reservoir. The images indicate that dust in the disk is dominated by large (about 4 µm in size) amorphous silicates. The results suggest that grain growth began about 1.5 million years ago.

Summing up the new findings, the authors of the paper conclude that the disk of Cha Hα 1 could be transitioning from a very young one, rich in water, to a more evolved one, with a high carbon-to-oxygen ratio. They underline that this disk shows the most diverse chemistry seen to date in a brown dwarf protoplanetary disk.

"The rich molecular environment presents a unique opportunity to test our understanding of disk chemistry and its influence on potential planet formation. Gaining insight into the reservoirs of the detected molecules can further illuminate the composition of future planets forming around brown dwarfs," the scientists write.

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