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Observations reveal the properties of a nearby cold brown dwarf

Using the James Webb Space Telescope (JWST), the Hubble Space Telescope (HST) and the Gemini Observatory, European astronomers have observed a nearby cold brown dwarf known as WISE 1738. Results of the observational campaign, July 16 on the arXiv preprint server, deliver important insights into the physical properties and atmospheric chemistry of this object.

Brown dwarfs (BDs) are intermediate substellar objects between planets and stars with masses below the hydrogen burning limit-about 80 Jupiter masses. BDs with effective temperatures lower than 500 K are known as Y dwarfs, and therefore are the coolest and least luminous substellar objects so far identified.

WISEP J173835.52+273258.9, or WISE 1738 for short, is a Y dwarf discovered in 2011. It is one of the first ultracool dwarfs identified by NASA's Wide-field Infrared Survey Explorer (WISE). Previous observations of WISE 1738 have found that it has an effective temperature of about 350–400 K.

However, the physical properties of WISE 1738 remain uncertain as some studies suggest that this object is about half the size of the Earth and some five times more massive than Jupiter, while others point to a size of approximately 1.2 Jupiter radii and a much higher mass—at a level of 59 Jupiter masses.

In order to resolve this discrepancy in the results, a team of astronomers led by Malavika Vasist of the University of Liège in Belgium decided to employ JWST's Mid-Infrared Instrument (MIRI) to perform medium-resolution spectroscopic observations of WISE 1738. Their observations were complemented by data from HST and Gemini Observatory.

"Using a Mid-Infrared Instrument (MIRI) medium-resolution spectrum (5–18 µm), combined with near-infrared spectra (0.98–2.2 µm) from Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3) and Gemini Observatory's Near-Infrared Spectrograph (GNIRS), we aim to accurately characterize the atmospheric chemistry and bulk physical parameters of WISE 1738," the researchers wrote in the paper.

The new observations found that WISE 1738 has a radius of approximately 1.14 Jupiter radii, while its mass is about 13 Jupiter masses. It turned out that the effective temperature of this Y dwarf is 402 K and its age was estimated to be 1–4 billion years. The rotation period of WISE 1738 was measured to be about six hours and its distance was calculated to be 23.9 light years.

Furthermore, the observations disclosed essential information regarding the atmosphere of WISE 1738. By analyzing the data, the astronomers found evidence of disequilibrium chemistry in the atmosphere of this object due to vertical mixing. The study also determined the carbon-to-oxygen abundance ratio, which was found to be approximately 1.35, and metallicity, at a level of 0.34.

The researchers added that further observations of WISE 1738, focusing on oxygen sequestration processes and formation scenarios, are necessary in order to explain such a high carbon-to-oxygen ratio and metallicity.