The mystery of water in the atmosphere of τ Boötis b continues: Insights from revisiting archival CRIRES observations

The chemical abundances of gas-giant exoplanet atmospheres hold clues to the formation and evolution pathways that sculpt the exoplanet population. Recent ground-based high-resolution spectroscopic observations of the non-transiting hot Jupiter Boötis b from different instruments have resulted in a tension on the presence of water vapour in the planet's atmosphere, which impact the planet's inferred C/O and metallicity. To investigate this, we revisit the archival CRIRES observations of the planet's day-side in the wavelength range 2.28-2.33 m. We re-analyse them using the latest methods for correcting stellar and telluric systematics, and free- and equilibrium-chemistry Bayesian atmospheric retrieval. We find that a spurious detection of CH can arise from inadequate telluric correction. We confirm the detection of CO and constrain its abundance to be near solar = -3.44 volume mixing ratios (VMR). We find a marginal evidence for HO with = -5.13 VMR. This translates to super solar C/O (0.95), marginally sub-solar metallicity (-0.21). Due to the relatively large uncertainty on HO abundance, we cannot confidently resolve the tension on the presence of HO and the super-solar atmospheric metallicity of Boötis b. We recommend further observations of Boötis b in the wavelength ranges simultaneously covering CO and to confirm the peculiar case of the planet's super-solar C/O and metallicity.