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Investigating Water Ice Clouds and Weather on the Coldest Brown Dwarf with JWST

Presentation #626.10 in the session Planetary Atmospheres - Directly Imaged Planets and Brown Dwarfs.

Published onApr 03, 2024
Investigating Water Ice Clouds and Weather on the Coldest Brown Dwarf with JWST

Initial modeling efforts have provided foundational insights into the atmospheric behaviors of Y dwarfs. Previous observations with Spitzer and ground-based telescopes suggest the existence of water ice clouds in the atmospheres of Y dwarfs, the coolest brown dwarfs, and temperate giant exoplanets with effective temperatures less than 350 K. Water ice clouds may dampen observed spectral features, change the temperature structure, increase the albedo, and drive variability in brown dwarfs and cold giant planets. With JWST we now have our first opportunity to study these faint, extrasolar worlds in precise detail. Here we present an initial analysis of the first JWST 11-hour time series observation of WISE 0855— the coldest observed Y dwarf with an effective temperature of 250 K. WISE 0855 is a free-floating planetary-mass object, with a mass of ~3-10 Jupiter masses, just 2 pc from Earth. Its low temperature makes it the perfect candidate for probing water-ice cloud behavior. At this temperature, WISE 0855 is just warm enough such that ammonia will not condense into clouds in its atmosphere, so water clouds should be directly observable, unlike in Jupiter and Saturn where they lie underneath a layer of ammonia clouds. We present a set of new self-consistent models that aim to model WISE 0855, including water ice clouds and disequilibrium chemistry. Spectroscopic time series observations can be used to disentangle variability caused by clouds, temperature changes, and chemical changes. Here we present our preliminary model comparison for the time series analysis of the NIRSpec G395M spectra from 2.87 to 5.10 microns, aiming to detect signatures of water clouds and their impact on observed variability.

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