5
The cryogenic portion of the Spitzer mission ended in 2009. The Spitzer infrared spectrograph
(Houck et al. 2004) provided exquisite mid-infrared spectra of T dwarfs as cool as T ≈ 600 K (e.g.
eff
Cushing et al. 2006; Leggett et al. 2009). Since that date, only limited spectra have been obtained
at wavelengths longer than λ ≈ 2.5 µm, wavelengths where cool brown dwarfs emit significant flux.
While progress has been made using the mid-infrared photometric data from Spitzer and WISE, and
some λ ≈ 4 µm data from the ground, the community has been anxiously awaiting the first JWST
data for Y dwarfs (Marley & Leggett 2009).
Beileretal.(2023)publishedthefirstJWSTspectrumofaYdwarf,togetherwithlongerwavelength
photometry. The object is WISE J035934.06-540154.6 (herafter WISE 0359), which is classified as
a Y0 dwarf (Kirkpatrick et al. 2021) and appears to be typical of its type (see Section 4). Figure 1
shows the observational data, with the best fit Sonora spectrum from Beiler et al., and a fit using
the ATMO 2020++ models. A fit with a disequilibrium metal-poor Sonora model reproduces the
observations quite well (Figure 1) however the required low surface gravity implies an unlikely very
young age and low mass (Beiler et al. 2023).
We find that an “off-the-shelf” ATMO 2020++ model with T = 450 K, log g = 4.5, and [m/H]
eff
= 0 produces an excellent fit to the data, as shown in Figure 1. An increase or decrease in T by
eff
50 K changes the luminosity significantly, and the flux levels at 4.5 µm and 12.0 µm would increase
or decrease by 40%, meaning the 400 K and 500 K models do not match the observations. Figure 6 of
Leggett et al. (2021) illustrates the dependency of the SED on the other atmospheric parameters. For
this observed spectrum, we found that the relative heights of the 4.5 µm and 10 µm flux peaks, the
height and shape of the 8 – 9 µm shoulder, and the depths of the CO and NH absorption features,
3
allowed us to constrain log g and [m/H], and to select the best fit from our available grid by eye.
The near-infrared spectral region was not considered in the fitting process, because very little
flux emerges there (5% of the total flux). However Figure 1 shows that the ATMO2020++ model
produces an excellent match to the near-infrared observations. For reference, Appendix B presents a
comparison of the JWST spectrum to a 450 K BT-Settl model spectrum (Allard 2014, 2016) as the
BT-Settl grid is commonly used for warmer brown dwarfs. The BT-Settl model produces an inferior
fit compared to Sonora or ATMO2020++, at these temperatures. Appendix B also compares the
spectrumtoa450Kdisequilibriumcloud-freespectrumfromLacy&Burrows(2023). Theagreement
is again inferior, although we find in the next Section that the Lacy & Burrows (2023) disequilibrium
spectra which include water clouds may agree better with the observations of Y dwarfs cooler than
350 K (Figure 2).
Beiler et al. (2023) determine F = 6.89±0.16×10−17 W m−2 for WISE 0359, by integrating over
bol
theobserved1–12µmspectrum,applyingalinearinterpolationoverthe12–21µmphotometricdata
points, and adopting a Rayleigh-Jean tail for the longest wavelengths. Our adopted ATMO2020++
model integrates to 7.41 × 10−17 W m−2, and is within ≈ 3 σ of the measured value. The over-
luminosity seen in Figure 1 for our model at 14 ≲ λ µm ≲ 21 likely contributes to the difference,
however we note that, at this early stage in the JWST pipeline, small errors in the MIRI flux
calibration cannot be excluded and these may contribute to the apparent over-luminosity. The
models indicates that a Rayleigh-Jean tail is a good approximation for λ > 21 µm.
Beiler et al. (2023) use the bolometric luminosity and evolutionary models to determine T =
eff
467+16 K. The T for both the Sonora and ATMO2020++ model fits agree with this value,
−18 eff