Brown Dwarfs

Self-consistent Models of Y Dwarf Atmospheres With Water Clouds And Disequilibrium Chemistry

By Keith Cowing
Status Report
March 29, 2023
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Self-consistent Models of Y Dwarf Atmospheres With Water Clouds And Disequilibrium Chemistry
Relative contributions of major opacity sources in Y dwarf atmospheres. 400 K and 1 bar correspond to the photosphere for our clear, equilibrium model with Teff = 350, log10(g) = 4.25, and Z/Z = 1. CO is negligible at 400 K, but we have enhanced its abundance in the figure, as one might expect in cases of strong vertical mixing, in order to illustrate which wavelengths are affected. — astro-ph.EP

Y dwarfs are the coolest spectral class of brown dwarf. They have effective temperatures less than 500 K, with the coolest detection as low as ~250 K. Their spectra are shaped predominantly by gaseous water, methane, and ammonia.

At the warmer end of the Y dwarf temperature range, spectral signatures of disequilibrium carbon monoxide have been observed. Cooler Y dwarfs could host water clouds in their atmospheres. Since they make up the low-mass tail of the star formation process, and are a valuable analogue to the atmospheres of giant gaseous exoplanets in a temperature range that is difficult to observe, understanding Y dwarf atmospheric compositions and processes will both deepen our understanding of planet and star formation, and provide a stepping stone towards characterizing cool exoplanets.

JWST spectral observations are anticipated to provide an unprecedented level of detail for these objects, and yet published self-consistent model grids do not accurately replicate even the existing HST and ground-based observations.

In this work, we present a new suite of 1-d radiative-convective equilibrium models to aid in the characterization of Y dwarf atmospheres and spectra. We compute clear, cloudy, equilibrium-chemistry and disequilibrium-chemistry models, providing a comprehensive suite of models in support of the impending JWST era of panchromatic Y dwarf characterization.

Comparing these models against current observations, we find that disequilibrium CH4-CO and NH3-N2 chemistry and the presence of water clouds can bring models and observations into better, though still not complete, agreement.

Brianna Lacy, Adam Burrows

Comments: main text: 27 pages, 19 figures, 4 tables; appendix + references: 13 pages, 3 figures, 4 tables; model grid available on zenodo this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2303.16295 [astro-ph.EP] (or arXiv:2303.16295v1 [astro-ph.EP] for this version)
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Submission history
From: Brianna Lacy
[v1] Tue, 28 Mar 2023 20:18:57 UTC (22,797 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻