Hydrogen cyanide and acetylene detected in a brown dwarf atmosphere for the first time

Using the James Webb Space Telescope (JWST), an international team of astronomers has explored the atmosphere of a nearby brown dwarf binary designated WISE J045853.90+643451.9. As a result, they detected hydrogen cyanide and acetylene in the atmosphere of this binary, marking the first time these two species have been identified in the atmosphere of a brown dwarf. The finding was reported Feb. 19 on the arXiv pre-print server.

Brown dwarfs are intermediate objects between planets and stars. Astronomers generally agree that they are substellar objects occupying the mass range between 13 and 80 Jupiter masses. One subclass of brown dwarfs (with effective temperatures between 500 and 1,500 K) is known as T-dwarfs, and represents the coolest and least luminous substellar objects so far detected.

Located just 30.1 light years away, WISE J045853.90+643451.9 (or WISE-0458) is a binary composed of two T-dwarfs of spectral type T8.5 and T9, with effective temperatures of 600 and 500 K, respectively. The pair has a semi-major axis of approximately 5.0 AU.

Given that the T-dwarfs of WISE-0458 have similar spectral types and temperatures, it is assumed that they have nearly identical atmospheres.

A group of astronomers led by Elisabeth C. Matthews of the Max Planck Institute for Astronomy in Heidelberg, Germany, decided to explore the atmosphere of WISE-0458 in detail. For this purpose, they employed the Medium Resolution Spectrometer (MRS) of the Mid-Infrared Instrument (MIRI) onboard JWST.

"We observed WISE-0458 with JWST/MIRI instrument on November 21, 2022, using the MRS. Observations were collected as part of the European MIRI GTO [guaranteed time observations] consortium," the researchers write in the paper.

The observations suggest that the atmosphere of WISE-0458 is cloud-free and molecule-rich. The model used by Matthews' team includes the presence of methane, carbon dioxide, carbon monoxide, water and ammonia in the atmosphere of WISE-0458—therefore species expected in cold atmospheres.

Furthermore, the observations detected hydrogen cyanide and acetylene in the atmosphere of WISE-0458. The astronomers explained that both species are indicators of disequilibrium chemistry, and likely form together deeper in the brown dwarf atmosphere. This is the first discovery of these two species in any brown dwarf atmosphere.

The study also found that each brown dwarf of WISE-0458 has a radius of about 0.81 Jupiter radii and the total mass of the pair is approximately 132 Jupiter masses. The distance to the binary was confirmed to be 30.12 light years.

The authors of the paper concluded that their findings demonstrate the power of MRS to characterize cold brown dwarfs. They added that future studies should investigate hydrogen cyanide and acetylene in more detail, and determine whether these species are present in other cold brown dwarfs of similar temperature to WISE-0458.

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