A spectroscopic thermometer: individual vibrational band spectroscopy with the example of OH in the atmosphere of WASP-33b

Sam O. M. Wright*, Stevanus K. Nugroho, Matteo Brogi, Neale P. Gibson, Ernst J. W. de Mooij, Ingo Waldmann, Jonathan Tennyson, Hajime Kawahara, Masayuki Kuzuhara, Teruyuki Hirano, Takayuki Kotani, Yui Kawashima, Kento Masuda, Jayne L. Birkby, Chris A. Watson, Motohide Tamura, Konstanze Zwintz, Hiroki Harakawa, Tomoyuki Kudo, Klaus HodappShane Jacobson, Mihoko Konishi, Takashi Kurokawa, Jun Nishikawa, Masashi Omiya, Takuma Serizawa, Akitoshi Ueda, Sébastien Vievard, Sergei N. Yurchenko

*Corresponding author for this work

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Individual vibrational band spectroscopy presents an opportunity to examine exoplanet atmospheres in detail, by distinguishing where the vibrational state populations of molecules differ from the current assumption of a Boltzmann distribution. Here, retrieving vibrational bands of OH in exoplanet atmospheres is explored using the hot Jupiter WASP-33b as an example. We simulate low-resolution spectroscopic data for observations with the JWST's NIRSpec instrument and use high-resolution observational data obtained from the Subaru InfraRed Doppler instrument (IRD). Vibrational band–specific OH cross-section sets are constructed and used in retrievals on the (simulated) low- and (real) high-resolution data. Low-resolution observations are simulated for two WASP-33b emission scenarios: under the assumption of local thermal equilibrium (LTE) and with a toy non-LTE model for vibrational excitation of selected bands. We show that mixing ratios for individual bands can be retrieved with sufficient precision to allow the vibrational population distributions of the forward models to be reconstructed. A fit for the Boltzmann distribution in the LTE case shows that the vibrational temperature is recoverable in this manner. For high-resolution, cross-correlation applications, we apply the individual vibrational band analysis to an IRD spectrum of WASP-33b, applying an “unpeeling” technique. Individual detection significances for the two strongest bands are shown to be in line with Boltzmann-distributed vibrational state populations, consistent with the effective temperature of the WASP-33b atmosphere reported previously. We show the viability of this approach for analyzing the individual vibrational state populations behind observed and simulated spectra, including reconstructing state population distributions.
Original languageEnglish
Article number41
JournalAstronomical Journal
Issue number2
Early online date04 Jul 2023
Publication statusPublished - 01 Aug 2023


  • Exoplanet atmospheric composition
  • Exoplanet atmospheres
  • Hot Jupiters
  • Astronomy data modeling
  • High resolution spectroscopy
  • Near infrared astronomy


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