Astronomers report the first spectral results from the Dyn-Atmo survey, a James Webb Space Telescope (JWST) program that targets directly imaged substellar companions with independently measured masses. The team observed HD 13724 B, a T4-type brown dwarf orbiting a Sun-like star at about 0.2 arc seconds on the sky (roughly 14 astronomical units). They combined new JWST spectra with an updated dynamical mass of 38.3 ± 0.6 Jupiter masses to constrain the companion’s atmosphere.

The observations used JWST’s NIRSpec/G395H integral field unit (IFU) to collect moderate-resolution spectra (resolution R ≈ 2700) across 3–5 micrometers. An IFU takes many spectra across a small region, so it can separate the faint companion light from the much brighter star. The authors fit the continuum-subtracted spectrum with several self-consistent atmospheric model grids (BT-SETTL, Sonora Diamondback, Sonora Flame Skimmer, NEWERA-PHOENIX) and also performed flexible atmospheric “retrievals” using the petitRADTRANS tool. A retrieval treats gas abundances and temperature as parameters to be inferred directly from the spectrum, while self-consistent grids impose internal chemistry and physics.

Using a Bayesian average across the models, they report bulk atmospheric values of effective temperature Teff = 1150 ± 195 K, surface gravity log g = 5.1 ± 0.3 (cgs units), metallicity [M/H] = 0.4 ± 0.4, vertical mixing parameter log10(Kzz) = 6.33^{+2.59}_{-2.22}, and a carbon-to-oxygen ratio C/O = 0.50 ± 0.13. The retrievals prefer an atmosphere close to chemical equilibrium, with enhanced overall metallicity and a roughly solar C/O ratio. Molecular signatures seen in the analysis include methane (CH4), carbon dioxide (CO2), 12CO and water (H2O); the authors did not find robust evidence for other trace gases or for strong disequilibrium chemistry.