The mystery of water in the atmosphere of $\ddot{I}„$ Boötis b continues: insights from revisiting archival CRIRES observations

Abstract

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 $\ddot{I}„$ 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 $\hat{I}¼$m. 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 ${\log }_{10}(\mathrm{CO})$ = -3.44${}_{-0.85}^{+1.63}$ VMR. We find a marginal evidence for H${}_2$O with ${\log }_{10}({\mathrm{H}}_2\mathrm{O})$ = -5.13${}_{-6.37}^{+1.22}$ VMR. This translates to super solar C/O (0.95${}_{-0.31}^{+0.06}$), marginally sub-solar metallicity (-0.21 ${}_{-0.87}^{+1.66}$). Due to the relatively large uncertainty on H${}_2$O abundance, we cannot confidently resolve the tension on the presence of H${}_2$O and the super-solar atmospheric metallicity of $\ddot{I}„$ Boötis b. We recommend further observations of $\ddot{I}„$ Boötis b in the wavelength ranges simultaneously covering CO and ${\mathrm{H}}_2\mathrm{O}$ to confirm the peculiar case of the planet's super-solar C/O and metallicity.

16 pages, 13 figures; Accepted for publication in MNRAS