Self-consistent Models of Y Dwarf Atmospheres with Water Clouds and Disequilibrium Chemistry

Abstract

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.

main text: 27 pages, 19 figures, 4 tables; appendix + references: 13 pages, 3 figures, 4 tables; model grid available on zenodo https://doi.org/10.5281/zenodo.7779180

References in corpus (55)

• The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
• Atomic and Molecular Opacities for Brown Dwarf and Giant Planet Atmospheres
• ExoMol molecular line lists XXXV: a rotation-vibration line list for hot ammonia
• ExoMol molecular line lists - XVI: The rotation-vibration spectrum of hot H${}_2$S
• The 2020 release of the ExoMol database: molecular line lists for exoplanet and other hot atmospheres
• Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets
• ExoMol line lists XVIII. The high temperature spectrum of VO
• ExoMol line lists -- XXXIX. Ro-vibrational molecular line list for CO${}_2$
• Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D
• Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun
• Spitzer/IRAC Photometry of M, L, and T Dwarfs
• Water Clouds in Y Dwarfs and Exoplanets
• A hybrid line list for CH${}_4$ and hot methane continuum
• The Field Substellar Mass Function Based on the Full-sky 20-pc Census of 525 L, T, and Y Dwarfs
• Discovery of 11 New T Dwarfs in the Two Micron All-Sky Survey, Including a Possible L/T Transition Binary
• A Systematic Study of Departures from Chemical Equilibrium in the Atmospheres of Substellar Mass Objects
• ExoMol molecular line lists V: The ro-vibrational spectra of NaCl and KCl
• CFBDS J005910.90-011401.3: reaching the T-Y Brown Dwarf transition?
• A very cool brown dwarf in UKIDSS DR1
• The discovery of an M4+T8.5 binary system
• Exploring the substellar temperature regime down to ~550K
• Exploring Biases of Atmospheric Retrievals in Simulated JWST Transmission Spectra of Hot Jupiters
• Hubble Space Telescope Spectroscopy of Brown Dwarfs Discovered with the Wide-field Infrared Survey Explorer
• The full infrared spectrum of molecular hydrogen
• The Solar Neighborhood. XXI. Parallax Results from the CTIOPI 0.9m Program: 20 New Members of the 25 Parsec White Dwarf Sample
• Optical Albedo Theory of Strongly-Irradiated Giant Planets: The Case of HD 209458b
• Observations of Disequilibrium CO Chemistry in the Coldest Brown Dwarfs
• A Search for L/T Transition Dwarfs With Pan-STARRS1 and WISE. II. L/T Transition Atmospheres and Young Discoveries
• Atmospheric Circulation of Brown Dwarfs: Jets, Vortices, and Time Variability
• Fifteen new T dwarfs discovered in the UKIDSS Large Area Survey
• Near-infrared photometry of Y dwarfs: low ammonia abundance and the onset of water clouds
• Spectral Variability from the Patchy Atmospheres of T and Y Dwarfs
• ExoMol molecular line lists XIX: high accuracy computed hot line lists for H${}_2$${}^{18}$O and H${}_2$${}^{17}$O
• The Luminosities of the Coldest Brown Dwarfs
• The Hawaii Infrared Parallax Program. IV. A Comprehensive Parallax Survey of L0-T8 dwarfs with UKIRT
• Molecular Cross Sections for High Resolution Spectroscopy of Super Earths, Warm Neptunes and Hot Jupiters
• A Uniform Retrieval Analysis of Ultracool Dwarfs. III. Properties of Y-Dwarfs
• The Sonora Substellar Atmosphere Models. II. Cholla: A Grid of Cloud-free, Solar Metallicity Models in Chemical Disequilibrium for the JWST Era
• Marvel analysis of the measured high-resolution rovibrational spectra of H2S
• Probing the Extent of Vertical Mixing in Brown Dwarf Atmospheres with Disequilibrium Chemistry
• Pressure-dependent water absorption cross sections for exoplanets and other atmospheres
• Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project
• A H13CN/HN13C linelist, model atmospheres and synthetic spectra for carbon stars
• Expanding the Y Dwarf Census with Spitzer Follow-up of the Coldest CatWISE Solar Neighborhood Discoveries
• A theoretical room-temperature line list for 15NH3
• A model of rotating convection in stellar and planetary interiors: I - convective penetration
• Discovery of a Low-Luminosity, Tight Substellar Binary at the T/Y Transition
• Discovery of a new Y dwarf: WISE J030449.03-270508.3
• WISEA J083011.95+283716.0: A Missing Link Planetary-Mass Object
• A 1.46-2.48 $\mu$m Spectroscopic Atlas of a T6 Dwarf (1060 K) Atmosphere with IGRINS: First Detections of H${}_2$S and H${}_2$, and Verification of H${}_2$O, CH${}_4$, and NH${}_3$ Line Lists
• WISE2150-7520AB: A very low mass, wide co-moving brown dwarf system discovered through the citizen science project Backyard Worlds: Planet 9
• A New Parallax Measurement for the Coldest Known Brown Dwarf
• An Improved Near-Infrared Spectrum of the Archetype Y Dwarf WISEP J182831.08+265037.8
• Microphysics of Water Clouds in the Atmospheres of Y Dwarfs and Temperate Giant Planets
• Improved infrared photometry and a preliminary parallax measurement for the extremely cold brown dwarf CWISEP J144606.62$-$231717.8

Cited by in corpus (15)

• The Sonora Substellar Atmosphere Models. IV. Elf Owl: Atmospheric Mixing and Chemical Disequilibrium with Varying Metallicity and C/O Ratios
• A temperate super-Jupiter imaged with JWST in the mid-infrared
• Methane Emission From a Cool Brown Dwarf
• Water depletion and 15NH3 in the atmosphere of the coldest brown dwarf observed with JWST/MIRI
• Three-dimensional dynamical evolution of cloud particle microphysics in sub-stellar atmospheres I. Description and exploring Y-dwarf atmospheric variability
• Silicate Precursor Silane detected in Cold Low-Metallicity Brown Dwarf
• Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of $\alpha$ Cen A. I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits
• PICASO 4.0: Clouds and Photochemistry in Climate Models of Brown Dwarfs and Exoplanets
• Microphysical Prescriptions for Parameterized Water Cloud Formation on Ultra-cool Substellar Objects
• Diversity of Cold Worlds: A Near Complete Spectral Energy Distribution for 2MASS J04151954-0935066 using JWST
• Direct Images of CO2 Absorption in the Atmosphere of a Super-Jupiter: Enhanced Metallicity Suggestive of Formation in a Disk
• Time-resolved JWST Retrieval Analysis of the Coldest Brown Dwarf: Temperature and Chemical Variations in WISE 0855-07
• Panchromatic View of the Frigid Jovian Exoplanet COCONUTS-2 b
• Dynamics of Y Dwarf Atmospheres
• Panchromatic characterization of the Y0 brown dwarf WISEP J173835.52+273258.9 using JWST/MIRI