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

• Published in: The Astronomical journal 2024-05, Vol.167 (5), p.237
• Main Authors: Lew, Ben W. P., Roellig, Thomas, Batalha, Natasha E., Line, Michael, Greene, Thomas, Murkherjee, Sagnick, Freedman, Richard, Meyer, Michael, Beichman, Charles, Alves de Oliveira, Catarina, De Furio, Matthew, Johnstone, Doug, Greenbaum, Alexandra Z., Marley, Mark, Fortney, Jonathan J., Young, Erick T., Leisenring, Jarron, Boyer, Martha, Hodapp, Klaus, Misselt, Karl, Stansberry, John, Rieke, Marcia
• Format: Article
• Language: English
• Subjects: Abundance; Ammonia; Astrochemistry; Brown dwarfs; Carbon dioxide; Exoplanet atmospheres; Hydrogen sulfide; Infrared spectra; James Webb Space Telescope; Metallicity; Mixing ratio; Near infrared radiation; Oxygen; Planetary atmospheres; Retrieval; Space telescopes; Spectral resolution; Spectroscopy; Sulfur; Sulphur; Y dwarfs
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/ad3425

The launch of the James Webb Space Telescope (JWST) marks a pivotal moment for precise atmospheric characterization of Y dwarfs, the coldest brown dwarf spectral type. In this study, we leverage moderate spectral resolution observations ( R ∼ 2700) with the G395H grating of the Near-Infrared Spectrograph (NIRSpec) on board JWST to characterize the nearby (9.9 pc) Y dwarf WISEPA J182831.08+265037.8. With the NIRSpec G395H 2.88–5.12 μ m spectrum, we measure the abundances of CO, CO 2 , CH 4 , H 2 S, NH 3 , and H 2 O, which are the major carbon-, nitrogen-, oxygen-, and sulfur-bearing species in the atmosphere. Based on the retrieved volume mixing ratios with the atmospheric retrieval framework CHIMERA, we report that the C/O ratio is 0.45 ± 0.01, close to the solar C/O value of 0.458, and the metallicity is +0.30 ± 0.02 dex. Comparison between the retrieval results and the forward modeling results suggests that the model bias for C/O and metallicity could be as high as 0.03 and 0.97 dex, respectively. We also report a lower limit of the 12 CO/ 13 CO ratio of >40, being consistent with the nominal solar value of 90. Our results highlight the potential for JWST to measure the C/O ratios down to percent-level precision and characterize isotopologues of cold planetary atmospheres similar to WISE 1828.