13
T ≲ 325 K. It would not be surprising that the structure of the atmosphere changes for the coldest
eff
Y dwarfs, where water clouds may form within the photosphere (Lacy & Burrows 2023, and Figure
2). There may also be dynamical changes — Leggett et al. (2021) find that the 260 K WISE 0855 is
undergoing vigorous mixing with K = 8.7, and that the deviation from a standard adiabat occurs
zz
deeper in the atmosphere at P = 50 bar, at temperatures where the nitrogen chemistry is changing
and the chlorides and sulfides are condensing.
We look forward to additional JWST data for the coldest Y dwarfs, which will enable exploration
of this next frontier in the story of cold planetary-mass brown dwarfs.