Unraveling the Mystery of the Peculiar and Young Hot Jupiter CoRoT-2b. I. H ₂ O and CO Detection from Dayside Observations with Gemini-S/IGRINS

We present ground-based high-resolution spectroscopic pre-eclipse observations of the hot Jupiter CoRoT-2b obtained with the IGRINS spectrograph on Gemini South. Using cross-correlation analysis, we detect the Doppler-shifted signature of the planet’s thermal emission with a signal-to-noise ratio of 4.32. Our independent analyses confirm the presence of H 2 O with a confidence level of 2.6 σ and an abundance of log 10 − 5.0 8 − 0.43 + 0.43 , as well as CO with 2.3 σ confidence and an abundance of log 10 − 4.2 1 − 0.81 + 0.48 in CoRoT-2b’s atmosphere, using two fully independent data reduction and retrieval pipelines. No significant detections of CH 4 , CO 2 , TiO, or VO are reported. While our cross-correlation analysis tentatively suggests the presence of HCN and OH, retrieval analysis does not confirm these molecules. The detected H 2 O and CO features indicate that CoRoT-2b’s dayside spectrum is not featureless, as previously inferred from lower-resolution observations, but instead reveals a complex atmospheric structure. Interestingly, we find a lack of significant molecular features at wavelengths shorter than 1.7 μ m, potentially due to high-altitude absorbers such as H − , clouds, or observational systematics. From our retrieved abundances of CO and H 2 O, we constrain a supersolar C/O ratio of 0.9 1 − 0.17 + 0.08 and a subsolar metallicity. This study provides the first high-resolution constraints on the atmospheric composition of CoRoT-2b and serves as the foundation for future investigations into its peculiar westward hot spot offset. Further phase-resolved observations will be required to explore the underlying atmospheric dynamics in more detail.