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Context. Young brown dwarfs serve as analogues of giant planets and provide benchmarks for atmospheric and formation models. JWST has enabled access to near-infrared spectra of brown dwarfs with unprecedented sensitivity.
Aims. We aim to constrain the chemical compositions, temperature structures, isotopic ratios, properties of the continuum and line emission from their circumstellar discs.
Methods. We performed atmospheric retrievals and disc modelling on JWST/NIRSpec medium-resolution (R ∼ 2700) spectra covering 0.97-5.27 μm. Our approach combines radiative transfer, line-by-line opacities, parametrised temperature profiles, and flexible equilibrium chemistry for the atmospheres. We also included a ring component from the disc, with blackbody continuum and optically thin CO slab emission.
Results. We detected and constrained more than twenty molecular and atomic species in the atmospheres, including ¹²CO, H₂O, CO₂, SiO, and several hydrides. The CO fundamental band at 4.6 μm enables detections of ¹³CO and C¹⁸O. We report isotope ratios of carbon: ¹²C/¹³C = 79_–11⁺¹⁴ (TWA 27A) and 75_–2⁺² (TWA 28), and oxygen: ¹⁶O/¹⁸O = 645_–70⁺⁸⁰ (TWA 27A) and 681_–50⁺⁵³ (TWA 28) based on water isotopologues. Both objects show significant excess infrared emission, which we modelled as warm (≈650 K) blackbody rings. We identified optically thin CO emission from hot gas (≥1600 K) in the discs, necessary to reproduce the redder part of the spectra. The atmospheric carbon-to-oxygen ratios are 0.54±0.02 (TWA 27A) and 0.59±0.02 (TWA 28), consistent with solar values.
Conclusions. We characterised the atmospheres and discs of two young brown dwarfs through simultaneous constraints on temperature, composition, isotope ratios, and disc properties. These observations demonstrate the ability of JWST/NIRSpec to study young objects, enabling future studies of circumplanetary discs.