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

TL;DR

This study uses JWST spectra to analyze 22 cool brown dwarfs, deriving their atmospheric compositions, physical properties, and comparing thermal profiles with models, revealing chemical correlations and potential non-equilibrium chemistry.

Contribution

First comprehensive atmospheric retrieval of 22 late-T and Y brown dwarfs with continuous 0.95-12 um spectra from JWST, constraining their chemistry, physical properties, and thermal structures.

Findings

H2O and CH4 VMRs positively correlated, indicating equilibrium chemistry.

Derived metallicities correlate with H2O and CH4, tracing oxygen and carbon.

Tentative PH3 detections suggest vertical mixing or non-equilibrium processes.

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 LRS. This dataset provides the first continuous 0.95-12 um 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 non-equilibrium chemistry. Apart from chemical properties, we retrieve masses and radii spanning approximately 6-77 M_Jup and 0.66-1.53 R_Jup, respectively. We compare the derived log10(g) values of about 4-5.5 cm s^-2 and Teff values of about 350-1100 K with Sonora Bobcat evolutionary models and find an age range of 0.4 to 10 Gyr across 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 from differences in chemistry treatment.