JWST Measurements of ¹³C, ¹⁸O, and ¹⁷O in the Atmosphere of Super-Jupiter VHS 1256 b

Abstract

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 C / 13 C = 62 − 2 + 2 , in between previous measurements for companions (∼30) and isolated brown dwarfs (∼100). The oxygen isotope ratios are 16 O / 18 O = 425 − 28 + 33 and 16 O / 17 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 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.

High-precision Atmospheric Characterization of a Y Dwarf with JWST NIRSpec G395H Spectroscopy: Isotopologue, C/O Ratio, Metallicity, and the Abundances of Six Molecular Species

“…4.5. Implication of 12 CO/ 13 CO Ratio Recent 13 CO detections in young giant planets and brown dwarfs (Zhang et al 2021a(Zhang et al , 2021bEsparza-Borges et al 2023;Gandhi et al 2023) present an exciting opportunity to study the potential value of isotopologues in connecting the atmospheric properties to planet and star formation pathways. A brown dwarf formed through gravitational collapse formation pathways should share similar isotopologues with the interstellar medium (ISM) values, which are set through processes including ion-exchange reactions (Langer et al 1984), isotope-selective photodissociation (Bally & Langer 1982;van Dishoeck & Black 1988;Visser et al 2009), and gas-to-ice isotopologue partitioning (Acharyya et al 2007;Smith et al 2015).…”

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. V. Do Self-consistent Atmospheric Models Represent JWST Spectra? A Showcase with VHS 1256–1257 b

“…Finally, the recent identification of isotopologue ratios as tracers of formation motivates the development of models that can explore their potential different values. Recently, isotopologues ( 13 C, 18 O, and 17 O) were identified in the atmosphere of VHS 1256 b through comparisons of JWST data with synthetic spectra generated within a retrieval framework (Gandhi et al 2023). It is important to investigate the capability of self-consistent models in detecting these chemical elements, estimating their abundance, and studying the impact of systematic errors in the models on these values.…”