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 τ 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 dayside in the wavelength range 2.28 to 2.33 μm. We reanalyse them using the latest methods for correcting stellar and telluric systematics, and free-chemistry Bayesian atmospheric retrieval. We find that a spurious detection of CH_4 can arise from inadequate telluric correction. We confirm the detection of CO and constrain its abundance to be near solar log_10(CO) = -3.44^+1.63_-0.85 VMR. We find a marginal evidence for H_2O with log_10(H_2O) = -5.13^+1.22_-6.37 VMR. This translates to super solar C/O (0.95^+0.06_-0.31), marginally sub-solar metallicity (-0.21 ^+1.66_-0.87). Due to the relatively large uncertainty on H_2O abundance, we cannot confidently resolve the tension on the presence of H_2O and the super-solar atmospheric metallicity of τ Boötis b. We recommend further observations of τ Boötis b in the wavelength ranges simultaneously covering CO and H_2O to confirm the peculiar case of the planet's super-solar C/O and metallicity.