Four HD 209458 b transits through CRIRES+: Detection of H_2O and non-detections of C_2H_2, CH_4, and HCN

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 approx 0.1 to 1.0. Of particular interest is the simultaneous detection of H$_2$O 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. 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 H$_2$O and HCN. We observed four primary transits of HD 209458 b at a high resolution ($ R 92\,000$) with CRIRES+ in the near infrared (band H, 1.431243--1.837253 mu m). After reducing the data with pycrires we prepared the data using the SysRem algorithm and performed a cross-correlation (CCF) analysis of the transmission spectra. We also compared the results with those obtained from simulated datasets constructed by combining the Exo-REM self-consistent model with the petitRADTRANS package. Combining the four transits, we detect H$_2$O with a signal-to-noise CCF metric of $8.7 This corresponds to a signal emitted at $K_p = $ kmcdot s$^ $ and blueshifted by $-6^ $ kmcdot s$^ $, consistent with what is expected for HD 209458 b. We do not detect any other species among C$_2$H$_2$, CH$_4$, CO, CO$_2$, H$_2$S, HCN, and NH$_3$. 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.