A population view of transiting hot giant exoplanets: Tracing Fe and Ti chemistry with ESPRESSO and MAROON-X

Vollständiger Abstract

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Hot and ultra-hot Jupiters offer a unique laboratory for studying atmospheric chemistry at the population level using ground-based high-resolution spectroscopy. Iron (Fe) and titanium (Ti) are key tracers of thermal and chemical structures, yet they exhibit markedly different observational trends across the population. We present a homogeneous re-analysis of high-resolution transmission spectra for ten hot and ultra-hot Jupiters observed with ESPRESSO on ESO’s Very Large Telescope and MAROON-X on Gemini-North. We searched for neutral Fe and Ti absorption and performed injection–recovery tests using forward models spanning a range of Ti-depletion levels and temperature–pressure profiles. To enable direct comparison across planets observed with different instruments and signal-to-noise ratios, we introduced the relative cross-correlation metric, Δ Ti-Fe . We detect Fe in seven planets and Ti in four planets at significances above 5 σ . Across the population, Δ Ti-Fe decreases sharply towards lower equilibrium temperatures. Under the assumption of equal Ti depletion across planets, isothermal models fail to reproduce this trend, instead requiring a temperature-dependent depletion of Ti that increases towards cooler planets, consistent with cold-trapping processes in cooler atmospheres. Models with inverted temperature–pressure profiles naturally reproduce the decline without invoking such temperature-dependent depletion. In these atmospheres, Ti is converted into TiO in deeper, cooler layers and subsequently removed from the gas phase through condensation into Ti-bearing species, leading to a strong suppression of the observable atomic Ti signal while Fe remains largely atomic. Nevertheless, even in these gradient models, an additional overall depletion of Ti relative to Fe is required to match the hottest planets. Our results demonstrate that the observable refractory chemistry is governed by the interplay of molecular partitioning, ionisation, condensation, and cold-trapping processes, as well as the vertical structure of ultra-hot Jupiter atmospheres. Although these results reveal clear population-level trends, additional observations will be necessary to distinguish between temperature-dependent cold-trapping and overall depletion scenarios. Expanding homogeneous high-resolution surveys to larger and more diverse samples, targeting both emission and transmission observations, will refine these constraints and provide critical insights into the chemistry of strongly irradiated giant planets.