2024A&A...690A..63B

Context. HD 209458 b is one of the most studied exoplanets to date. Despite this, atmospheric characterisation studies yielded inconsistent species detections and abundances. Values reported for the C/O ratio range from ≈0.1 to 1.0. Of particular interest is the simultaneous detection of H₂O and HCN reported by some studies using high-resolution ground-based observations, which would require the atmospheric C/O ratio to be fine-tuned to a narrow interval around 1. HCN has however not been detected from recent space-based observations.
Aims. We aim to provide an independent study of HD 209458 b's atmosphere with high-resolution observations, in order to infer the presence of several species, including H₂O and HCN.
Methods. We observed four primary transits of HD 209458 b at a high resolution (ℛ ≈ 92000) with CRIRES+ in the near infrared (band H, 1.431243-1.837253 μm). After reducing the data with pycrires, we prepared the data using the SysRem algorithm and performed a cross-correlation (CCF) analysis of the transmission spectra. We also compared the results with those obtained from simulated datasets constructed by combining the Exo-REM self-consistent model with the petitRADTRANS package.
Results. Combining the four transits, we detect H₂O with a signal-to-noise CCF metric of 8.7σ. This corresponds to a signal emitted at K_p = 151.3_–23.4^+31.1 km s^–1 and blueshifted by -6_–2⁺¹ km s^–1, consistent with what is expected for HD 209458 b. We do not detect any other species among C₂H₂, CH₄, CO, CO₂, H₂S, HCN, and NH₃. Comparing this with our simulated datasets, this result is consistent with a C/O ratio of 0.1 and an opaque cloud top pressure of 50 Pa, at a 3 times solar metallicity. This would also be consistent with recent JWST observations. However, none of the simulated results obtained with a bulk C/O ratio of 0.8, a value suggested by previous studies using GIANO-B and CRIRES, are consistent with our observations.