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

Abstract

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}$$-5.08^{+0.43}_{-0.43}$, as well as CO with 2.3$σ$ confidence and an abundance of log$_{10}$$-4.21^{+0.48}_{-0.81}$ 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 $μm$, 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 $0.91^{+0.08}_{-0.17}$ 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 hotspot offset. Further phase-resolved observations will be required to explore the underlying atmospheric dynamics in more detail.