Detection of Carbon Monoxide's 4.6 Micron Fundamental Band Structure in WASP-39b's Atmosphere with JWST NIRSpec G395H

Grant, David; Lothringer, Joshua D.; Wakeford, Hannah R.; Alam, Munazza K.; Alderson, Lili; Bean, Jacob L.; Benneke, Björn; Désert, Jean-Michel; Daylan, Tansu; Flagg, Laura; Hu, Renyu; Inglis, Julie; Kirk, James; Kreidberg, Laura; López-Morales, Mercedes; Mancini, Luigi; Mikal-Evans, Thomas; Molaverdikhani, Karan; Palle, Enric; Rackham, Benjamin V.; Redfield, Seth; Stevenson, Kevin B.; Valenti, Jeff A.; Wallack, Nicole L.; Aggarwal, Keshav; Ahrer, Eva-Maria; Crossfield, Ian J. M.; Crouzet, Nicolas; Iro, Nicolas; Nikolov, Nikolay K.; Wheatley, Peter J.; JWST Transiting Exoplanet Community ERS Team
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The Astrophysical Journal

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Carbon monoxide (CO) is predicted to be the dominant carbon-bearing molecule in giant planet atmospheres and, along with water, is important for discerning the oxygen and therefore carbon-to-oxygen ratio of these planets. The fundamental absorption mode of CO has a broad, double-branched structure composed of many individual absorption lines from 4.3 to 5.1 μm, which can now be spectroscopically measured with JWST. Here we present a technique for detecting the rotational sub-band structure of CO at medium resolution with the NIRSpec G395H instrument. We use a single transit observation of the hot Jupiter WASP-39b from the JWST Transiting Exoplanet Community Early Release Science (JTEC ERS) program at the native resolution of the instrument (R ~ 2700) to resolve the CO absorption structure. We robustly detect absorption by CO, with an increase in transit depth of 264 ± 68 ppm, in agreement with the predicted CO contribution from the best-fit model at low resolution. This detection confirms our theoretical expectations that CO is the dominant carbon-bearing molecule in WASP-39b's atmosphere and further supports the conclusions of low C/O and supersolar metallicities presented in the JTEC ERS papers for WASP-39b.