A Comprehensive Atmospheric Retrieval Analysis of 22 James Webb Space Telescope Spectral Energy Distributions of Cool Brown Dwarfs

We present a uniform atmospheric retrieval analysis of 22 late-T and Y-type brown dwarfs within 20 pc, observed with the James Webb Space Telescope NIRSpec PRISM and MIRI low-resolution spectrometer. This dataset provides the first continuous ∼0.95–12 μ m spectroscopic coverage of late-T and Y-type brown dwarfs, which in turn enables precise constraints on their thermal structures and volume mixing ratios (VMRs) of H 2 O, CH 4 , CO, CO 2 , NH 3 , H 2 S, K, Na, and PH 3 . We find positive correlations between the VMR of H 2 O and CH 4 , and CO and CO 2 , consistent with thermochemical equilibrium chemistry. Using the VMRs, we derive atmospheric metallicity, which is positively correlated with H 2 O and CH 4 , showing H 2 O and CH 4 trace oxygen and carbon content, respectively, allowing us to effectively measure (O/H) bulk and (C/H) bulk . We also report tentative PH 3 detections in roughly half the sample, suggesting potential vertical mixing or nonequilibrium chemistry. Apart from chemical properties, we retrieve masses and radii spanning ∼6–77 M Jup N and ∼0.66–1.53 R Jup N , respectively. We compare the derived log 10 ( g ) (∼4–5.5 (cm s −2 )) and T eff (∼350–1100 K) with Sonora Bobcat evolutionary models and find an age range of 0.4 to 10 Gyr amongst the sample. Comparing our retrieved thermal profiles with the Elf-Owl forward model thermal profiles, we find a systematic difference between the two, likely arising due to the difference in chemistry treatment.