The First Y Dwarf Data From JWST Show That Dynamic and Diabatic Processes Regulate Cold Brown Dwarf Atmospheres

TL;DR

JWST observations of Y dwarfs reveal that their atmospheres are governed by dynamic and diabatic processes, deviating from standard adiabatic models, with implications for understanding their atmospheric chemistry and temperature profiles.

Contribution

This paper demonstrates that non-adiabatic, disequilibrium models better fit JWST spectra of Y dwarfs, introducing a new approach for analyzing ultra-cool brown dwarf atmospheres.

Findings

Spectra fit better with non-adiabatic models than standard models.

Absence of expected PH_3 feature suggests different atmospheric chemistry.

Identification of a new 295 K Y dwarf in a luminosity gap.

Abstract

The James Webb Space Telescope (JWST) is now observing Y dwarfs, the coldest known brown dwarfs, with effective temperatures T_eff <= 475 K. The first published observations provide important information: not only is the atmospheric chemistry out of equilibrium, as previously known, but the pressure-temperature profile is not in the standard adiabatic form. The rapid rotation of these Jupiter-size, isolated, brown dwarfs dominates the atmospheric dynamics, and thermal and compositional changes disrupt convection. These processes produce a colder lower atmosphere, and a warmer upper atmosphere, compared to a standard adiabatic profile. Leggett et al. (2021) presented empirical models where the pressure-temperature profile was adjusted so that synthetic spectra reproduced the 1 <= lambda um <= 20 spectral energy distributions of brown dwarfs with 260 <= T_eff K <= 540. We show that spectra generated by these models fit the first JWST Y dwarf spectrum better than standard-adiabat models. Unexpectedly, there is no 4.3 um PH_3 feature in the JWST spectrum and atmospheres without phosphorus better reproduce the 4 um flux peak. Our analysis of new JWST photometry indicates that the recently discovered faint secondary of the WISE J033605.05-014350AB system (Calissendorff et al. 2023) has T_eff = 295 K, making it the first dwarf in the significant luminosity gap between the 260 K WISE J085510.83-071442.5, and all other known Y dwarfs. The adiabat-adjusted disequilibrium-chemistry models are recommended for analyses of all brown dwarfs cooler than 600 K, and a grid is publicly available. Photometric color transformations are provided in an Appendix.