2
their emitted spectral energy distributions (SEDs). Processes such as condensation of the refractory
elements and grain sedimentation, and disequilibrium chemistry driven by vertical transport of gas,
are now known to be critical, changing the abundance of dominant species by orders of magnitude
(e.g. Fegley & Lodders 1996; Griffith & Yelle 1999; Lodders 1999; Ackerman & Marley 2001; Allard
et al. 2001; Saumon et al. 2006; Leggett et al. 2016). The SEDs of cool stars and brown dwarfs –
objects with masses too low for sustained fusion (e.g. Burrows & Liebert 1993; Saumon & Marley
2008; Phillips et al. 2020) – can now be well reproduced, at least for T > 600 K (e.g. Stephens et al.
eff
2009; Leggett et al. 2017).
During the last three years new models of cold dwarf atmospheres have become available. Karalidi
et al. (2021) and Marley et al. (2021) present Sonora brown dwarf atmospheric and evolutionary
modelsforcloud-freeatmosphereswitharangeofmetallicity, bothinandoutofchemicalequilibrium.
Lacy & Burrows (2023) present models of Y dwarfs with water clouds and disequilibrium chemistry,
with a range of metallicity. The latter models calculate spectra which closely agree with the Sonora
spectra, although the pressure-temperature relationships in the upper atmosphere differ (Figures 20
and 22 of Lacy & Burrows 2023). We note that the primary photospheric region of a Y dwarf is
expected to be cloud free, as condensation curves of alkalis, sulfides, and chlorides show that those
grains would exist deep in the atmosphere, and similarly water clouds would only exist high in the
atmosphere (e.g. Morley et al. 2014; Leggett et al. 2021).
Researchers have also approached the analysis of atmospheres using the retrieval method, where
observations guide the generation of the model structure. Zalesky et al. (2019, 2022) and Hood
et al. (2023) adopt this approach in analyses of T and Y dwarfs. To date these analyses have been
limited to the near-infrared spectral region only, meaning that a limited region of the photosphere is
probed. Generally the retrieval analyses determine atmospheric parameters that are consistent with
the forward model grid analyses, however in some cases non-physical values emerge from the retrieval
analysis, suggesting an incomplete understanding of the atmosphere (e.g. Hood et al. 2023).
For Y dwarfs, most model spectra significantly underestimate the flux at wavelengths between 2 µm
and 4 µm. For example, the synthetic Sonora-Cholla colors presented by Karalidi et al. (2021) have
K magnitudes which are about a magnitude too faint, unless the surface gravity is reduced to an
unlikely, very low, value (Figure 18 of Karalidi et al. 2021). Similarly the synthetic spectra generated
by the models of Lacy & Burrows (2023) are a factor of two to three too faint between 2 µm and
3.8 µm (Figure 17 of Lacy & Burrows 2023). Leggett et al. (2021) demonstrate that in order to
reproduce both the near-infrared and mid-infrared spectra of Y dwarfs, the lower atmosphere must
be cooler and the upper atmosphere warmer, compared to a standard pressure-temperature adiabatic
relationship. Deviations from the standard adiabat are a natural outcome of rapid rotation (Tan &
Showman 2021) and thermal and compositional changes in the atmosphere (Tremblin et al. 2019).
In Section 2 we briefly describe the ATMO 2020 models (Phillips et al. 2020) which were empirically
adjusted for Leggett et al. (2021) and extended into a larger grid for Meisner et al. (2023). We refer to
the latter model grid set as the ATMO2020++ models. In Section 3 we compare the first published
JWST spectrum of a Y dwarf, and its best-fit Sonora model SED, to an ATMO2020++ SED. In
Section 4 we present an analysis of color-color diagrams using recently published JWST photometry
for four Y dwarfs. Section 5 presents our conclusions.
The Appendices supply supporting material. Appendix A adds more detailed information about
the ATMO2020++ atmospheres. Appendix B presents a comparison of the Y dwarf spectrum to