Emission spectroscopy of KELT-2A b with Gemini-North/IGRINS-2: hints of a stellar C/O ratio and sub-stellar metallicity
Year: 2026
Type: preprint
Abstract: 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 $R \sim 45{,}000$. 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 $\rm H_2O$ at a signal-to-noise ratio of {6.57}, confirming the previous result at a higher significance. While $\rm ^{12}CO$ is identified in the log-likelihood map near the expected planetary velocity, a definitive detection of $\rm ^{12}CO$ remains elusive given the current data quality. We perform atmospheric retrieval both with IGRINS-2 data alone and jointly with archival \textit{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} $\rm log(H_2O) = {-3.78^{+0.27}_{-0.20}}$ {with the free chemistry model}. {The chemical equilibrium model yields} $\rm C/O={ 0.19^{+0.12}_{-0.07}}$, $\rm [C/H]=-0.81^{+0.41}_{-0.34}$, and $\rm [O/H] = -0.37^{+0.27}_{-0.17}${: 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 $\sim2σ$ 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 $\rm{H_2O}$ 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.}
Source: arXiv (Cornell University)
Institutions: RIKEN, RIKEN Pioneering Research Institute, The University of Tokyo, Arizona State University, University of Milan, National Astronomical Observatory of Japan, Ritsumeikan University, Instituto de Astrofísica de Canarias, Komaba Institute for Science, The University of Tokyo, Universidad de La Laguna
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Science Foundation, European Commission, Korea Astronomy and Space Science Institute, Ministerio de Ciencia e Innovación, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo, Japan Society for the Promotion of Science, European Regional Development Fund, Agencia Estatal de Investigación
OpenAlex ID: W7222244068