Exploration of the Near-infrared Colors of Cold Y Dwarfs from the Ground and Space

The James Webb Space Telescope (JWST) has provided critical mid-infrared data for cold brown dwarfs. It has also provided low-resolution near-infrared spectra, and for faint sources these are the first spectra at these wavelengths. We use these data and other literature sources to synthesize near-infrared photometry on the Mauna Kea Observatories (MKO) system for 19 T and Y dwarfs, on the Euclid system for 44 T and Y dwarfs, and on the Roman systems for 48 T and Y dwarfs. We also synthesize Euclid I E magnitudes for 15 T and Y dwarfs. Using the S. A. Beiler et al. (2024) observational effective temperatures ( T eff ), together with ATMO 2020++ model colors, we show that the absolute 4.6 μ m magnitude can be used as a proxy for T eff . We present a polynomial fit to the M W2 : T eff relationship for cool dwarfs with 250 ≲ T eff K ≲ 1000. We select five Y dwarfs with 275 ≲ T eff K ≲ 400 that have a range in near- to mid-infrared colors. Comparison of the JWST spectral energy distribution to ATMO 2020++ models indicate that Y dwarfs that are bluer in J − W2 are lower gravity or more metal rich than their redder counterparts, with stronger CO and CO 2 absorption at 4.2 ≲ λ μ m ≲ 4.9. The near-infrared color diagrams show significant scatter, with complex dependencies on T eff , metallicity, and gravity. In order to disentangle these effects, opacity sources for cool model atmospheres need to be more complete at λ < 1 μ m.