Emission Spectroscopy of KELT-2A b with Gemini-North/IGRINS-2: Hints of a Stellar C/O Ratio and Substellar 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 ∼ 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 H 2 O at a signal-to-noise ratio of 6.57, confirming the previous result at a higher significance. While 12 CO is identified in the log-likelihood map near the expected planetary velocity, a definitive detection of 12 CO remains elusive given the current data quality. We perform atmospheric retrieval both with IGRINS-2 data alone and jointly with archival Spitzer/IRAC photometric data points. The retrieved temperature–pressure profile is noninverted, consistent with prior observations. From the joint fit we constrain the water abundance to log ( H 2 O ) = − 3.7 8 − 0.20 + 0.27 with the free chemistry model. The chemical equilibrium model yields C / O = 0.1 9 − 0.07 + 0.12 , [ C / H ] = − 0.8 1 − 0.34 + 0.41 , and [ O / H ] = − 0.3 7 − 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 ∼2 σ level. If confirmed, this combination of a stellar C/O ratio with a substellar metallicity is difficult to reconcile with gas accretion between the H 2 O and CO snowlines, which predicts a superstellar 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.