Impacts of Water Latent Heat on the Thermal Structure of Ultra-Cool Objects: Brown Dwarfs and Free-Floating Planets


Model spectra (resolving power of 500) in near-infrared (NIR; panels a and c) and mid-infrared (mid-IR; panels b and d) spectral regions for Teff of 250 and 200 K (top and bottom rows, respectively) with log g = 4.0. The JWST MIRI MRS filter regions are shown at the bottom of (b) and (d). Key opacity regions for CH4, H2O, NH3, and H2-H2 collision-induced absorption (CIA) are indicated.

Brown dwarfs are essential targets for understanding planetary and sub-stellar atmospheres across a wide range of thermal and chemical conditions. As surveys continue to probe ever deeper, and as observing capabilities continue to improve, the number of known Y dwarfs -- the coldest class of sub-stellar objects, with effective temperatures below about 600 K -- is rapidly growing.

Critically, this class of ultra-cool objects has atmospheric conditions that overlap with Solar System worlds and, as a result, tools and ideas developed from studying Earth, Jupiter, Saturn and other nearby worlds are well-suited for application to sub-stellar atmospheres. To that end, we developed a one-dimensional (vertical) atmospheric structure model for ultra-cool objects that includes moist adiabatic convection, as this is an important process for many Solar System planets. Application of this model across a range of effective temperatures (350, 300, 250, 200 K), metallicities ([M/H] of 0.0, 0.5, 0.7, 1.5), and gravities (log g of 4.0, 4.5, 4.7, 5.0) demonstrates strong impacts of water latent heat release on simulated temperature-pressure profiles.

At the highest metallicities, water vapor mixing ratios reach an Earth-like 3%, with associated major alterations to the thermal structure in the atmospheric regions where water condenses. Spectroscopic and photometric signatures of metallicity and moist convection should be readily detectable at near- and mid-infrared wavelengths, especially with James Webb Space Telescope observations, and can help indicate the formation history of an object.

Shih-Yun Tang (1 and 2), Tyler D. Robinson (2, 3 and 4), Mark S. Marley (5), Natasha E. Batalha (6), Roxana Lupu (7), L. Prato (1 and 2) ((1) Lowell Observatory, (2) Department of Astronomy and Planetary, Northern Arizona University, (3) Habitability, Atmospheres, and Biosignatures Laboratory, Northern Arizona University, (4) NASA Astrobiology Institute's Virtual Planetary Laboratory, University of Washington, (5) Department of Planetary Sciences and Lunar and Planetary Laboratory, The University of Arizona, (6) Space Sciences Division, NASA Ames Research Center, (7) BAER Institute, NASA Ames Research Center, Naval Air Station)

Comments: 16 pages, 8 figures, submitted to ApJ, comments are welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2105.07000 [astro-ph.EP] (or arXiv:2105.07000v1 [astro-ph.EP] for this version)
Submission history
From: Shih-Yun Tang
[v1] Fri, 14 May 2021 18:00:00 UTC (1,119 KB)
https://arxiv.org/abs/2105.07000
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