6
within the uncertainties. However the model parameters differ significantly in other ways, and the
ATMO2020++ model provides a superior fit as can be seen in Figure 1. Key differences are:
• Metallicity:ThecolorsofWISE0359indicatethatitisatypicalfieldobject(Section4). Asolar
metallicity would therefore be expected, as we find here, compared to the [m/H] = −0.3 dex
determined by Beiler et al. (2023).
• Surface Gravity: The ATMO2020++ model parameters of T = 450 K and log g = 4.5,
eff
together with evolutionary models (Marley et al. 2021), give an age and mass for WISE 0359
of around 2.5 Gyr and 14 Jupiter masses. This compares to 20 Myr and 1 Jupiter mass for
the lower gravity Beiler et al. (2023) Sonora model. The kinematic age of the local T dwarf
populationisaround3.5Gyr(Hsuetal.2021), andsimulationsofthefieldsubstellarpopulation
show a median age for a 450 K Y dwarf of around 6 Gyr (Kirkpatrick et al. 2021). Hence a
value of log g = 4.5, with an associated age of a few Gyr, is more plausible than a value of log
g = 3.5, with an associated age of a few Myr.
• Opacities: The ATMO2020++ model (blue line) reproduces the spectral features seen in Figure
1 better than the Sonora model (pink line, see the difference spectra in lower panel). In
particular, the near-infrared peaks, the λ ≈ 5.0 µm region where opacities due to CO and H O
2
dominate(Leggettetal.2021,Figure10),andthestrongdoubleNH absorptionatλ ≈ 10.5µm
3
are better matched.
Both the Sonora and ATMO models generate spectra with strong PH absorption features at
3
4.1 ≲ λ µm ≲ 4.3 (see e.g. Figure 10 of Leggett et al. 2021). This is a disequilibrium species,
with PH expected to be the stable form of phosphorus in cool atmospheres (Visscher et al. 2006).
3
However the chemical pathways for phosphorus are more complex (Visscher et al. 2006; Wang et al.
2016) and less certain (Bains et al. 2023). Although the feature is seen in the solar system giant
planets, it may be that the different composition or gravity of the brown dwarf atmosphere results
in phosphorus taking a different form. For this work we generated a small number of ATMO2020++
model atmospheres with no phosphorus. Figure 1 shows that such a model better reproduces the
4 µm flux peak for this Y0 brown dwarf. Removing this opacity does not significantly change the
synthetic JWST F480M magnitude, but will brighten the Spitzer and WISE [4.5] and W2 magnitudes
by ∼ 20%.
4. THE FIRST JWST Y DWARF PHOTOMETRY
JWST photometry has been published for three Y dwarf systems, at the time of writing. We
discuss each system separately below. To facilitate comparison of these new data with previous
ground-based near-infrared, Spitzer, and WISE observations, as well as future JWST observations,
we have calculated color transformations for a selection of filters using our ATMO2020++ grid. The
photometry is available at: 10.5281/zenodo.7931460. Appendix C presents color-color plots for the
transformations, as well as polynomial fits for interpolation.
We have used the transformations in Appendix C to convert existing data (from Leggett et al.
2021) to the JWST F162M, F360M, and F480M systems. These filters were selected because the
differences H− F162M, Spitzer [3.6] − F360M, and Spitzer [4.5] (or WISE W2) − F480M are small
(see Appendix C), and therefore the transformation is less prone to error. Also, these filters are