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

May 2026 • 2026ApJ..1002..164K

Authors • Kothari, Harshil • Cushing, Michael C. • Beiler, Samuel A. • Visscher, Channon • Marley, Mark S. • Burningham, Ben • Schneider, Adam C. • Kirkpatrick, J. Davy

Abstract • 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 μm 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 H₂O, CH₄, CO, CO₂, NH₃, H₂S, K, Na, and PH₃. We find positive correlations between the VMR of H₂O and CH₄, and CO and CO₂, consistent with thermochemical equilibrium chemistry. Using the VMRs, we derive atmospheric metallicity, which is positively correlated with H₂O and CH₄, showing H₂O and CH₄ trace oxygen and carbon content, respectively, allowing us to effectively measure (O/H)_bulk and (C/H)_bulk. We also report tentative PH₃ 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 MJupN and ∼0.66─1.53 RJupN , respectively. We compare the derived log10(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.

Links

• ELECTR https://doi.org/10.3847/1538-4357/ae5bb7
• PREPRINT http://arxiv.org/abs/2604.05104