8
expected to be commonly used for brown dwarf studies, and are included in current JWST programs
to study Y dwarfs (e.g. GTO 1189). We note that there is no JWST filter with a bandpass similar
to the ground-based 1.25 µm J-band. The 1.0 — 1.3 µm F115W filter bandpass includes strong
absorption bands of H O, CH , and NH for Y dwarfs (e.g. Lacy & Burrows 2023), and the filter
2 4 3
transformations indicate that the J− F115W color is very sensitive to metallicity. Historically the
J filter has been the default for ground-based Y dwarf followup, however the H filter may now be a
better choice.
To estimate F162M from the photometry sample, the H magnitude was used, unless H was not
available in which case J was used, together with the color H (or J) − [4.5] (or W2). To estimate
F360M, the [3.6] magnitude was used, unless that was not available, in which case WISE W1 was
used, together with the color [3.6] − [4.5] (or W1 − W2). Similarly, to estimate F480M, the Spitzer
[4.5] magnitude was used, unless that was not available, in which case WISE W2 was used, together
with the color [3.6] − [4.5] (or W1 − W2).
Figure 2 shows color-color diagrams where the green symbols are the new JWST data and smaller
grey dots are the ground-based near-infrared, Spitzer, and WISE observations transformed on to the
JWST system. Known extreme metal-poor T dwarfs (e.g. Meisner et al. 2023) have been excluded
from the Figure because those color transformations are more uncertain due to the dependency on
metallicity (see Appendix C). In Figure 2 we show sequences for three ATMO2020++ models: solar
metallicity log g = 4.0 and log g = 4.5 models, and a metal-poor log g = 4.5 model. The log g = 4.0
model approximately corresponds to an age of 0.8 Gyr for the Y dwarfs, and the log g = 4.5 to
∼ 6 Gyr (Marley et al. 2021). Figure 3 and Table 1 give relationships between the F162M − F480M
and F360M − F480M colors and T , and the absolute F480M magnitude and T , for these three
eff eff
models. Table 2 gives the F162M, F360M, and F480M magnitudes for the three systems discussed
below, as well as the T values inferred from the color relationships.
eff
F162M − F480M:M sequences from the disequilibrium chemistry models by Lacy & Burrows
F480M
(2023) are also shown in Figure 2 for 250 ≤ T K ≤ 350. The onset of water clouds reduces the
eff
emergent flux at F480M, and increases the flux at F162M (Lacy & Burrows 2023). We discuss the
implications of this for the two coolest objects, WISE 0336B and WISE 0855 below.
4.1. WISE J033605.05-014350.4
Calissendorff et al. (2023) present JWST imaging observations of the Y0 dwarf WISE J033605.05-
014350.4 (hereafter WISE 0336) in the F150W and F480M filters. The target was resolved into a
binary system with a 0′.′09 separation, where the secondary is 2 – 3 magnitudes fainter than the
primary. This is a very low-mass system — assuming an age of 1 – 5 Gyr, evolutionary models
indicate that the primary has a mass of 7.5 – 20 Jupiter masses, while the secondary has a mass of
4 – 12.5 Jupiter masses (Calissendorff et al. 2023).
We converted the Calissendorffet al. (2023) WISE 0336A,B F150W magnitudes to F162M adopting
the F150W − F162M trend for Y dwarfs shown in Appendix C. For F360M we adopted the [3.6]
magnitude of the unresolved system as that of the primary, given the large δmag between primary
and secondary, and estimated F360M from that value.