期刊
ASTROPHYSICAL JOURNAL
卷 882, 期 2, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/ab3393
关键词
astrometry; binaries: general; brown dwarfs; infrared: stars; stars: atmospheres
资金
- National Aeronautics and Space Administration
- NASA [NAS 5-26555]
- W. M. Keck Foundation
- NSF [AST-1518339]
- STFC [ST/R003726/1] Funding Source: UKRI
Half of the energy emitted by late-T- and Y-type brown dwarfs emerges at 3.5 <= lambda mu m <= 5.5. We present new L' (3.43 <= lambda mu m <= 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 that 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 similar to 4 mu m observations to models, and find that the model fluxes are too low for brown dwarfs cooler than similar to 700 K. The discrepancy increases with decreasing temperature, and is a factor of similar to 2 at T-eff = 500 K and similar to 4 at T-eff = 400 K. 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 modeled 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.
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