Emission spectroscopy of KELT-2A b with Gemini-North/IGRINS-2: hints of a stellar C/O ratio and sub-stellar metallicity

We present the results of high-resolution emission spectroscopy of the hot Jupiter KELT-2A~b using IGRINS-2 on the Gemini-North telescope, covering the $H$- and $K$-bands at a spectral resolution of $R\sim \mathrm{45,000}$. Observations were carried out over five nights, covering both pre- and post-eclipse orbital phases. Stellar and telluric contaminations are removed via singular value decomposition. We detect ${\mathrm{H}}_2\mathrm{O}$ at a signal-to-noise ratio of {6.57}, confirming the previous result at a higher significance. While ${}^{12}\mathrm{C}\mathrm{O}$ is identified in the log-likelihood map near the expected planetary velocity, a definitive detection of ${}^{12}\mathrm{C}\mathrm{O}$ remains elusive given the current data quality. We perform atmospheric retrieval both with IGRINS-2 data alone and jointly with archival $\mathit{\text{Spitzer}}$/IRAC photometric data points. The retrieved temperature–pressure profile is non-inverted, consistent with prior observations. {From the joint fit we constrain the water abundance to} $\mathrm{l}\mathrm{o}\mathrm{g}({\mathrm{H}}_2\mathrm{O})=-3.78_{-0.20}^{+0.27}$ {with the free chemistry model}. {The chemical equilibrium model yields} $\mathrm{C}\mathrm{/}\mathrm{O}=0.19_{-0.07}^{+0.12}$, $[\mathrm{C}\mathrm{/}\mathrm{H}]=-0.81_{-0.34}^{+0.41}$, and $[\mathrm{O}\mathrm{/}\mathrm{H}]=-0.37_{-0.17}^{+0.27}${: the C/O ratio agrees with the stellar value, while the carbon and oxygen abundances are moderately depleted relative to the host star, though still consistent at the $\sim 2\sigma$ level.} If confirmed, {this combination of a stellar C/O ratio with a sub-stellar metallicity is difficult to reconcile with gas accretion between the ${\mathrm{H}}_2\mathrm{O}$ and CO snowlines, which predicts a super-stellar C/O. It instead points to the accretion of metal-depleted gas outside the CO snowline with only limited planetesimal enrichment, and hence to a disk-free, high-eccentricity migration history.}