James Webb Space Telescope Spectra of Cold Brown Dwarfs are Well-reproduced by Phosphine-free, Diabatic, ATMO2020++ Models

Abstract

James Webb Space Telescope (JWST) mid-infrared spectra of cold brown dwarfs (BDs) are becoming available. In Leggett & Tremblin we demonstrated that an ATMO2020++ synthetic spectrum provided an excellent fit to the first published spectrum. Here we show that these models can reproduce the 1–14 μm spectra of BDs with a range of effective temperature: 350 ≤ T eff K ≤ 500. A grid of these models, which include disequilibrium chemistry and non-adiabatic pressure–temperature profiles, is available. In addition, a number of models without phosphine were calculated, as PH3 is absent in the JWST data. Two of the six BDs studied are particularly interesting; one appears to be a young low-mass BD, the other is best fit as a binary. More complete analyses will be possible with an extended model grid, and with additional JWST data. The ATMO2020++ models are recommended for analyses of all BD data.