2019MNRAS.482.1485P

Transmission spectroscopy of exoplanets has the potential to provide precise measurements of atmospheric chemical abundances, in particular of hot Jupiters whose large sizes and high temperatures make them conducive to such observations. To date, several transmission spectra of hot Jupiters have revealed low amplitude features of water vapour compared to expectations from cloud-free atmospheres of solar metallicity. The low spectral amplitudes in such atmospheres could either be due to the presence of aerosols that obscure part of the atmosphere or due to inherently low abundances of H₂O in the atmospheres. A recent survey of transmission spectra of ten hot Jupiters used empirical metrics to suggest atmospheres with a range of cloud/haze properties but with no evidence for H₂O depletion. Here, we conduct a detailed and homogeneous atmospheric retrieval analysis of the entire sample and report the H₂O abundances, cloud properties, terminator temperature profiles, and detection significances of the chemical species. This study finds that the majority of hot Jupiters have atmospheres consistent with subsolar H₂O abundances at their day-night terminators. The best constrained abundances range from log(H₂O) of -5.04^+0.46_–0.30 to -3.16^+0.66_–0.69, which compared to expectations from solar-abundance equilibrium chemistry correspond to 0.018^+0.035_–0.009×solar to 1.40^+4.97_–1.11×solar. Besides H₂O we report statistical constraints on other chemical species and cloud/haze properties, including cloud/haze coverage fractions which range from 0.18^+0.26_–0.12 to 0.76^+0.13_–0.15. The retrieved H₂O abundances suggest subsolar oxygen and/or supersolar C/O ratios, and can provide important constraints on the formation and migration pathways of hot giant exoplanets.