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Cloudless Atmospheres for L/T Dwarfs and Extrasolar Giant Planets

Abstract : The admitted, conventional scenario to explain the complex spectral evolution of brown dwarfs (BDs) since their first detection 20 years ago has always been the key role played by micron-size condensates, called “dust” or “clouds,” in their atmosphere. This scenario, however, faces major problems, in particular the J-band brightening and the resurgence of FeH absorption at the L to T transition, and a physical first-principle understanding of this transition is lacking. In this Letter, we propose a new, completely different explanation for BD and extrasolar giant planet (EGP) spectral evolution, without the need to invoke clouds. We show that, due to the slowness of the CO/CH4 and N2/NH3 chemical reactions, brown dwarf (L and T, respectively) and EGP atmospheres are subject to a thermo-chemical instability similar in nature to the fingering or chemical convective instability present in Earth oceans and at the Earth core/mantle boundary. The induced small-scale turbulent energy transport reduces the temperature gradient in the atmosphere, explaining the observed increase in near-infrared J-H and J-K colors of L dwarfs and hot EGPs, while a warming up of the deep atmosphere along the L to T transition, as the CO/CH4 instability vanishes, naturally solves the two aforementioned puzzles, and provides a physical explanation of the L to T transition. This new picture leads to a drastic revision of our understanding of BD and EGP atmospheres and their evolution.
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Submitted on : Thursday, June 30, 2022 - 5:40:35 PM
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P. Tremblin, D. S. Amundsen, G. Chabrier, I. Baraffe, B. Drummond, et al.. Cloudless Atmospheres for L/T Dwarfs and Extrasolar Giant Planets. The Astrophysical Journal Letters, 2016, 817, ⟨10.3847/2041-8205/817/2/L19⟩. ⟨insu-03710589⟩

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