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5. CONCLUSIONS
Since the discovery of what is now recognized as the first T dwarf, Gliese 229B, in 1995 (Nakajima
etal.1995),ithasbeenrecognizedthatverticaltransportofgasinturbulentbrowndwarfatmospheres
results in disequilibrium chemistry (e.g. Fegley & Lodders 1996; Griffith & Yelle 1999; Lodders 1999).
The Y dwarfs are now demonstrating that, as well as having atmospheres which are out of chemical
equilibrium, the pressure-temperature (P − T) structure does not follow the standard radiative-
convective adiabatic form (Leggett et al. 2021). This is not surprising, as these turbulent and fast
rotating atmospheres are subject to dynamical, thermal, and chemical changes which disrupt the
convective transport of heat from the lower to upper atmosphere (Showman & Kaspi 2013; Tremblin
et al. 2015; Tan & Showman 2017; Showman et al. 2019; Tremblin et al. 2019; Tan & Showman 2021).
All standard-adiabat atmospheric models for brown dwarfs cooler than 600 K generate SEDs which
are too faint at wavelengths of 2 µm to 4 µm, and around 12 µm (e.g. Leggett et al. 2021; Lacy
& Burrows 2023). Leggett et al. (2021) showed that a modified P − T profile with a cooler lower
atmosphere (where the near-infrared flux originates) and warmer upper atmosphere (where the mid-
infrared flux originates) improved the fit to observations substantially. The discrepancy at λ ≈
3.6 µm, for example, was reduced by a factor of ≈ 5 in the adiabat-adjusted models. Simulations of
rapidly rotating cloud-free atmospheres (as generally applicable to Y dwarfs) calculate such a change
in the P −T profile — a cooler lower atmosphere and warmer upper atmosphere (Tan & Showman
2021, their Figure 4).
The first Y dwarf JWST observations to be published support the need for a non-adiabatic P −T
profile. We fit the first spectrum, of the Y0 dwarf WISE 0359 (Beiler et al. 2023), with synthetic
spectra generated by the ATMO2020++ adiabat-adjusted disequilibrium chemistry models. We find
that these produce a superior fit, compared to standard-adiabat models. The shape of the absorption
features and the flux peaks are better reproduced. Furthermore the model generates a good fit
without the need to use an atypical metallicity or age (surface gravity) for a field dwarf. We strongly
recommend that the ATMO2020++ models (Phillips et al. 2020; Leggett et al. 2021; Meisner et al.
2023) are used to analyse observations of brown dwarfs cooler than 600 K. Synthetic photometry and
spectroscopy is available at 10.5281/zenodo.7931460, and the ERC-ATMO Opendata page.
The JWST observation shows that the strong absorption by PH expected at λ ≈ 4.3 µm is
3
not present in the Y dwarf spectrum. Spectra calculated by ATMO2020++ atmospheres with no
phosphorus show a much improved agreement with the observations at 4.1 ≤ λ µm ≤ 4.3. Pathways
for phosphorus chemistry in turbulent brown dwarf atmospheres with disequilibrium chemistry need
to be reexamined.
We also explore the colors of four Y dwarfs observed with JWST, and recently published. We find
that the two Y0 dwarfs, WISE 0359 and WISE 0336A, have colors typical of the field, and color-
inferred temperatures which are consistent with earlier results. The recently discovered WISE 0336B
(Calissendorff et al. 2023) is the first object to lie in the gap between the extreme WISE 0855 and
all other Y dwarfs; we estimate that T ≈ 295 K for this object. The fourth Y dwarf, WISE 1828 is
eff
known to be peculiar, and its JWST colors continue to indicate that it is an extremely metal poor
binary system, although no companion has been found at separations greater than 5 AU (De Furio
et al. 2023).
The locations of WISE 0336B and WISE 0855 in color-magnitude diagrams suggest that the
ATMO2020++ models underestimate the flux at F162M and at F360M, for brown dwarfs with