Draft version September 27, 2023
Typeset using LATEX preprint style in AASTeX631
The First Y Dwarf Data From JWST Show That Dynamic and Diabatic Processes
Regulate Cold Brown Dwarf Atmospheres
S. K. Leggett1 and Pascal Tremblin2
1Gemini Observatory/NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA
2Universite Paris-Saclay, UVSQ, CNRS, CEA, Maison de la Simulation, 91191, Gif-sur-Yvette, France
ABSTRACT
The James Webb Space Telescope (JWST) is now observing Y dwarfs, the coldest
known brown dwarfs, with effective temperatures T ≲ 475 K. The first published
eff
observations provide important information: not only is the atmospheric chemistry
out of equilibrium, as previously known, but the pressure-temperature profile is not in
the standard adiabatic form. The rapid rotation of these Jupiter-size, isolated, brown
dwarfs dominates the atmospheric dynamics, and thermal and compositional changes
disrupt convection. These processes produce a colder lower atmosphere, and a warmer
upper atmosphere, compared to a standard adiabatic profile. Leggett et al. (2021) pre-
sented empirical models where the pressure-temperature profile was adjusted so that
synthetic spectra reproduced the 1 ≲ λ µm ≲ 20 spectral energy distributions of brown
dwarfs with 260 ≤ T K ≤ 540. We show that spectra generated by these models
eff
fit the first JWST Y dwarf spectrum better than standard-adiabat models. Unexpect-
edly, there is no 4.3 µm PH feature in the JWST spectrum and atmospheres without
3
phosphorus better reproduce the 4 µm flux peak. Our analysis of new JWST photom-
etry indicates that the recently discovered faint secondary of the WISE J033605.05-
014350AB system (Calissendorff et al. 2023) has T ≈ 295 K, making it the first dwarf
eff
in the significant luminosity gap between the 260 K WISE J085510.83-071442.5, and
all other known Y dwarfs. The adiabat-adjusted disequilibrium-chemistry models are
recommended for analyses of all brown dwarfs cooler than 600 K, and a grid is publicly
available. Photometric color transformations are provided in an Appendix.
Keywords: atmospheric dynamics — brown dwarfs — infrared observations
1. INTRODUCTION
One hundred years ago, the 5 pc region of space with our Sun at its center was known to include
three M dwarfs with luminosities around 10−3L — Barnard’s star, Proxima Centauri, and Wolf
⊙
359 (Barnard 1916; Vouˆte 1917; Wolf 1919). Improvements in infrared detector technology in the
1980s enabled the detection of colder sources which are fainter by 5 orders of magnitude. Currently,
we know that same 5 pc region also contains an L, a T and a Y dwarf (Luhman 2013, 2014). The
effective temperatures (T ) of the known L, T, and Y dwarfs are approximately 2000 – 1200 K, 1200
eff
– 475 K, and 475 – 250 K respectively (e.g. Stephens et al. 2009; Kirkpatrick et al. 2021; Leggett et al.
2021). Many advances have been made in modelling the atmospheres of these objects, and hence
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