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

Abstract

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. We aim to constrain their chemical compositions, temperature structures, isotopic ratios, and disc emission. We perform retrievals and disc modelling on JWST/NIRSpec medium-resolution ($R\approx 2700$) spectra spanning 0.97--5.27 $\mu$m, combining radiative transfer, line-by-line opacities, parameterised temperature profiles, and flexible equilibrium chemistry. We include a disc ring with blackbody continuum and optically thin CO emission. We detect over twenty species, including ${}^{12}$CO, H${}_2$O, CO${}_2$, SiO, and hydrides. The CO band at 4.6 $\mu$m reveals ${}^{13}$CO and C${}^{18}$O. Carbon isotope ratios are ${}^{12}$C/${}^{13}$C = $79_{-11}^{+14}$ (TWA 27A) and $75_{-2}^{+2}$ (TWA 28); oxygen ratios are ${}^{16}$O/${}^{18}$O = $645_{-70}^{+80}$ and $681_{-50}^{+53}$. Both objects show excess infrared emission, consistent with warm ($\approx 650$ K) blackbody rings, and optically thin CO from hot gas ($\ge 1600$ K) needed to match the red spectra. The atmospheric C/O ratios are $0.54\pm 0.02$ (TWA 27A) and $0.59\pm 0.02$ (TWA 28), consistent with solar values. We characterise the atmospheres and discs of two young brown dwarfs through joint constraints on temperature, composition, isotopes, and discs, demonstrating JWST/NIRSpec's ability to probe young objects and circumplanetary discs.

22 pages, 13 figures. Accepted for publication in A&A

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