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C/O ratio as a Dimension for Characterizing Exoplanetary Atmospheres

Until recently, infrared observations of exoplanetary atmospheres have typically been interpreted using models that assumed solar elemental abundances. With the chemical composition fixed, attempts have been made to classify hot Jupiter atmospheres on the basis of stellar irradiation. However, recent observations have revealed deviations from predictions based on such classification schemes, and chemical compositions retrieved from some datasets have also indicated non-solar abundances. In hot hydrogen-dominated atmospheres, the C/O ratio critically influences the relative concentrations of several spectroscopically dominant molecules. Between a C/O of 0.5 (solar value) and 2, the H2O and CH4 abundances can vary by several orders of magnitude in the observable atmosphere, and new hydrocarbon species such as HCN and C2H2 become prominent for C/O >= 1, while the CO abundance remains almost unchanged. Furthermore, a C/O >= 1 can preclude a strong thermal inversion due to TiO and VO in a hot Jupiter atmosphere, since TiO and VO are naturally under-abundant for C/O >= 1. We, therefore, suggest a new two-dimensional classification scheme for hydrogen-dominated exoplanetary atmospheres with irradiation (or temperature) and C/O ratio as the two dimensions. We define four classes in this 2-D space (O1, O2, C1, and C2) with distinct chemical, thermal and spectral properties. We characterize the thermal structures and C/O ratios of six hot Jupiters (XO-1b, CoRoT-2b, WASP-14b, WASP-19b, WASP-33b, and WASP-12b) in the framework of our proposed 2D classification scheme. While the data for several systems in our sample are consistent with carbon-rich atmospheres (i.e. C/O >= 1), new observations are required to conclusively constrain the C/O ratios in their atmospheres. We discuss how observations using existing and forthcoming facilities can constrain C/O ratios in exoplanetary atmospheres.