A Spectroscopic Thermometer: Individual Vibrational Band Spectroscopy with the Example of OH in the Atmosphere of WASP-33b

Abstract

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.

Evidence of Water Vapor in the Atmosphere of a Metal-rich Hot Saturn with High-resolution Transmission Spectroscopy

“…Since then, the role of high-resolution spectroscopy in exoplanet atmospheric science has undergone significant advancements and remains a powerful technique even in the era of the James Webb Space Telescope (JWST). There are many hot Jupiters whose atmospheres have been characterized using this method, including the detection of atomic (e.g., Hoeijmakers et al 2019;Borsa et al 2021;Kesseli et al 2022;Stangret et al 2022) and molecular species such as H 2 O (e.g., Alonso-Floriano et al 2019;Sánchez-López et al 2019;Giacobbe et al 2021;Webb et al 2022), HCN (e.g., Cabot et al 2018;Giacobbe et al 2021;Sánchez-López et al 2022), CH 4 (e.g., Guilluy et al 2019;Giacobbe et al 2021), CO (e.g., Snellen et al 2010;Giacobbe et al 2021;Ramkumar et al 2023), OH (Landman et al 2021;Nugroho et al 2021;Brogi et al 2023;Wright et al 2023), and even TiO and VO (Nugroho et al 2017;Cont et al 2021;Prinoth et al 2022;Pelletier et al 2023).…”