Disentangling disc and atmospheric signatures of young brown dwarfs with JWST/NIRSpec

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 12 CO, H 2 O, CO 2 , SiO, and several hydrides. The CO fundamental band at 4.6 μm enables detections of 13 CO and C 18 O. We report isotope ratios of carbon: 12 C/ 13 C = 79 −11 +14 (TWA 27A) and 75 −2 +2 (TWA 28), and oxygen: 16 O/ 18 O = 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.