2023ApJ...957L..36G

Isotope ratios have recently been measured in the atmospheres of directly imaged and transiting exoplanets from ground-based observations. The arrival of JWST allows us to characterize exoplanetary atmospheres in further detail and opens up wavelengths inaccessible from the ground. In this work we constrain the carbon and oxygen isotopes ¹³C, ¹⁸O, and ¹⁷O from CO in the atmosphere of the directly imaged companion VHS 1256 b through retrievals of the ∼4.1-5.3 μm NIRSpec G395H/F290LP observations from the early-release science program (ERS 1386). We detect and constrain ¹³C¹⁶O, ¹²C¹⁸O, and ¹²C¹⁷O at 32σ, 16σ, and 10σ confidence respectively, thanks to the very high signal-to-noise observations. We find the ratio of abundances are more precisely constrained than their absolute values, with ${}^{12}{\rm{C}}{/}^{13}{\rm{C}}={62}_{-2}^{+2}$, in between previous measurements for companions (∼30) and isolated brown dwarfs (∼100). The oxygen isotope ratios are ${}^{16}{\rm{O}}{/}^{18}{\rm{O}}={425}_{-28}^{+33}$ and ${}^{16}{\rm{O}}{/}^{17}{\rm{O}}={1010}_{-100}^{+120}$. All of the ratios are lower than the local interstellar medium and solar system, suggesting that abundances of the more minor isotopes are enhanced compared to the primary. This could be driven by isotope fractionation in protoplanetary disks, which can potentially alter the carbon and oxygen ratios through isotope selective photodissociation, gas/ice partitioning, and isotopic exchange reactions. In addition to CO, we constrain ¹H${}_{2}^{16}{\rm{O}}$ and ¹²C¹⁶O₂ (the primary isotopologues of both species), but find only upper limits on ¹²C¹H₄ and ¹⁴N¹H₃. This work highlights the power of JWST to constrain isotopes in exoplanet atmospheres, with great promise in determining formation histories in the future.