3.8um Imaging of 400-600K Brown Dwarfs and Orbital Constraints for WISEP J045853.90+643452.6AB

Abstract

Half of the energy emitted by late-T- and Y-type brown dwarfs emerges at 3.5 < lambda um < 5.5. We present new L' (3.43 < lambda um < 4.11) photometry obtained at the Gemini North telescope for nine late-T and Y dwarfs, and synthesize L' from spectra for an additional two dwarfs. The targets include two binary systems which were imaged at a resolution of 0.25". One of these, WISEP J045853.90+643452.6AB, shows significant motion, and we present an astrometric analysis of the binary using Hubble Space Telescope, Keck Adaptive Optics, and Gemini images. We compare lambda ~4um observations to models, and find that the model fluxes are too low for brown dwarfs cooler than ~700K. The discrepancy increases with decreasing temperature, and is a factor of ~2 at T_eff=500K and ~4 at T_eff=400K. Warming the upper layers of a model atmosphere generates a spectrum closer to what is observed. The thermal structure of cool brown dwarf atmospheres above the radiative-convective boundary may not be adequately modelled using pure radiative equilibrium; instead heat may be introduced by thermochemical instabilities (previously suggested for the L- to T-type transition) or by breaking gravity waves (previously suggested for the solar system giant planets). One-dimensional models may not capture these atmospheres, which likely have both horizontal and vertical pressure/temperature variations.

Accepted for publication in The Astrophysical Journal, July 17 2019. This revision includes changes to the Appendix only. Additional new W1 photometry is given and Figure 11 is updated; Figure 13 contained erroneous data and is corrected; Figure 14 is new and shows the new relationships derived for transforming from ground-based L or M magnitudes to Spitzer [3.6] and [4.5]

References in corpus (27)

• The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
• The Gemini Planet Imager Exoplanet Survey: Giant Planet and Brown Dwarf Demographics From 10-100 AU
• The unWISE Catalog: Two Billion Infrared Sources from Five Years of WISE Imaging
• The 0.8-14.5 micron Spectra of Mid-L to Mid-T Dwarfs: Diagnostics of Effective Temperature, Grain Sedimentation, Gas Transport, and Surface Gravity
• Fingering convection and cloudless models for cool brown dwarf atmospheres
• The Hawaii Infrared Parallax Program. II. Young Ultracool Field Dwarfs
• 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
• Water Clouds in Y Dwarfs and Exoplanets
• Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs I: Characterizing Benchmarks, Gl570D and HD3651B
• Vertical Atmospheric Structure in a Variable Brown Dwarf: Pressure-dependent Phase Shifts in Simultaneous Hubble Space Telescope-Spitzer Light Curves
• The discovery of an M4+T8.5 binary system
• Physical parameters of two very cool T dwarfs
• Keck Laser Guide Star Adaptive Optics Monitoring of 2MASS J1534-2952AB: First Dynamical Mass Determination of a Binary T Dwarf
• 3.6-7.9 um Photometry of L and T Dwarfs and the Prevalence of Vertical Mixing in their Atmospheres
• 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
• A Uniform Retrieval Analysis of Ultracool Dwarfs. III. Properties of Y-Dwarfs
• Thermo-compositional diabatic convection in the atmospheres of brown dwarfs and in Earth's atmosphere and oceans
• New Evidence for a Substellar Luminosity Problem: Dynamical Mass for the Brown Dwarf Binary Gl 417BC
• The Young L Dwarf 2MASS J11193254-1137466 is a Planetary-Mass Binary
• A Survey For Planetary-mass Brown Dwarfs in the Taurus and Perseus Star-forming Regions
• Observed Variability at 1um and 4um in the Y0 Brown Dwarf WISEP J173835.52+273258.9
• The Mass-Luminosity Relation in the L/T Transition: Individual Dynamical Masses for the New J-Band Flux Reversal Binary SDSSJ105213.51+442255.7AB
• Effects of Latent Heating on Atmospheres of Brown Dwarfs and Directly Imaged Planets
• A reduced chemical scheme for modelling warm to hot hydrogen-dominated atmospheres
• Cloud Atlas: Rotational Spectral Modulations and potential Sulfide Clouds in the Planetary-mass, Late T-type Companion Ross 458C

Cited by in corpus (11)

• A new set of atmosphere and evolution models for cool T-Y brown dwarfs and giant exoplanets
• A lack of evolution in the very bright-end of the galaxy luminosity function from z = 8-10
• Observations of Disequilibrium CO Chemistry in the Coldest Brown Dwarfs
• Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: effects of rotation
• Self-consistent Models of Y Dwarf Atmospheres with Water Clouds and Disequilibrium Chemistry
• Uniform Forward-Modeling Analysis of Ultracool Dwarfs. II. Atmospheric Properties of 55 Late-T Dwarfs
• 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
• Cloud Atlas: Unraveling the vertical cloud structure with the time-series spectrophotometry of an unusually red brown dwarf
• Direct-imaging Discovery of a Substellar Companion Orbiting the Accelerating Variable Star, HIP 39017
• MIRI-LRS spectrum of a cold exoplanet around a white dwarf: water, ammonia, and methane measurements
• Idealised 3D simulations of diabatically-forced Ledoux convection. Application to the atmosphere of hot rocky exoplanets