Emission spectroscopy of KELT-2A b with Gemini-North/IGRINS-2: hints of a stellar C/O ratio and sub-stellar metallicity
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 ∼ 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 H2O at a signal-to-noise ratio of {6.57}, confirming the previous result at a higher significance. While ^12CO is identified in the log-likelihood map near the expected planetary velocity, a definitive detection of ^12CO 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} log(H2O) = −3.78+0.27−0.20 {with the free chemistry model}. {The chemical equilibrium model yields} C/O= 0.19+0.12−0.07, [C/H]=−0.81+0.41−0.34, and [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 ∼2σ 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 H2O 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.}
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BibTeX
@misc{hayashi2026emission,
title = {{Emission spectroscopy of KELT-2A b with Gemini-North/IGRINS-2: hints of a stellar C/O ratio and sub-stellar metallicity}},
author = {Yuya Hayashi and Stevanus K. Nugroho and Shota Notsu and Changeat Quentin and Peter C. B. Smith and Krishna Kanumalla and Michael R. Line and Jorge A. Sanchez and Tomohiro C. Yoshida and Kazumasa Ohno and Hideko Nomura and Taiki Kagetani and Norio Narita and Kiyoe Kawauchi and Akihiko Fukui and Enric Palle},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.19377},
eprint = {2610.19377},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.19377},
}
RIS
TY - UNPB
TI - Emission spectroscopy of KELT-2A b with Gemini-North/IGRINS-2: hints of a stellar C/O ratio and sub-stellar metallicity
AU - Yuya Hayashi
AU - Stevanus K. Nugroho
AU - Shota Notsu
AU - Changeat Quentin
AU - Peter C. B. Smith
AU - Krishna Kanumalla
AU - Michael R. Line
AU - Jorge A. Sanchez
AU - Tomohiro C. Yoshida
AU - Kazumasa Ohno
AU - Hideko Nomura
AU - Taiki Kagetani
AU - Norio Narita
AU - Kiyoe Kawauchi
AU - Akihiko Fukui
AU - Enric Palle
PY - 2026
DA - 2026/10/21
PB - arXiv
DO - 10.48550/arxiv.2610.19377
UR - https://arxiv.org/abs/2610.19377
AB - 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_2O at a signal-to-noise ratio of {6.57}, confirming the previous result at a higher significance. While ^12CO is identified in the log-likelihood map near the expected planetary velocity, a definitive detection of ^12CO 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} log(H_2O) = −3.78^+0.27_−0.20 {with the free chemistry model}. {The chemical equilibrium model yields} C/O= 0.19^+0.12_−0.07, [C/H]=−0.81^+0.41_−0.34, and [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 ∼2σ 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 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.}
ER -