Astronomers study the atmosphere of a nearby cool brown dwarf

Using the James Webb Space Telescope (JWST), astronomers observed a nearby cool brown dwarf designated WISEPA J182831.08+265037.8 (or WISE 1828 for short). The observations provided important information about the composition of the object’s atmosphere. The new results were published on February 8 on the preprint server arXiv.

Brown dwarfs are substellar objects intermediate between planets and stars whose mass is below the hydrogen burning limit, or about 80 Jupiter masses. A subclass of brown dwarfs (with effective temperatures below 500 K) are known as Y dwarfs and represent the coldest and faintest substellar objects detected so far.

Located just 32.37 light years from Earth, WISE 1828 is an archetypal Y-type brown dwarf. Previous observations suggest it has a radius similar to that of Jupiter, while its mass is estimated at around 10 masses of Jupiter. The effective temperature of this brown dwarf was found to be 378 K.

Recently, a team of astronomers led by Ben Wei Peng Lew of the Bay Area Environmental Research Institute in Moffett Field, California, conducted an extensive atmospheric study of WISE 1828. To this end, they used the JWST near-infrared spectrograph ( NIRSpec ).

“In this study, we exploit observations at moderate spectral resolution (R ∼ 2700) with the Near Infrared Spectrograph (NIRSpec) G395H array on board JWST to characterize the neighboring Y dwarf (9.9 pc) WISEPA J182831.08+ 265037.8,” the researchers wrote.

Observations detected water, carbon monoxide, carbon dioxide, methane, ammonia, and hydrogen sulfide in the atmosphere of WISE 1828. The paper’s authors noted that their study marks the first measurement of the abundance of hydrogen sulfide in the atmosphere of a Y dwarf.

The study found that WISE 1828 has higher oxygen/hydrogen and sulfur/hydrogen abundance ratios than the Sun. The abundance ratio of nitrogen to hydrogen was estimated to be probably close to the sun. The results also suggest a lower than nominal abundance of methane-d1.

Using two complementary atmospheric modeling tools, the researchers managed to measure the metallicity and carbon/oxygen abundance ratio of the WISE 1828 atmosphere. One model indicates that the metallicity is at a level of 0.3 dex, while the other indicates a metallicity of -0.57 dex. The abundance ratios of carbon to oxygen are similar, according to these two models, approximately 0.46 and 0.43 dex, respectively.

The models allowed the team to also calculate the radius and effective temperature of WISE 1828. One model suggests 1.23 Jupiter radii and 534 K, while the other suggests the planet is smaller and colder: 1.03 Jupiter radii and 425 K.

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