Tracing bulk elemental ratios in exoplanetary atmospheres with TiO chemistry

TL;DR

This study demonstrates that TiO features in the optical transmission spectra of hot Jupiters can reveal their bulk chemical compositions, especially C/O ratios, aiding in understanding planetary formation.

Contribution

It introduces a new method using TiO and H2O spectral features to constrain exoplanetary atmospheric bulk elemental ratios, accounting for non-equilibrium chemistry and temperature-pressure variations.

Findings

TiO features are sensitive to C/O ratios in hot Jupiter atmospheres.

The proposed metric can differentiate between different chemical models.

Good agreement with observations for some exoplanets like WASP-121b.

Abstract

Knowing the bulk elemental abundances of an exoplanetary atmosphere is not an easy task, but it is crucial to understand the formation history of planets. The purpose of this work is to show that the observability of TiO features at optical wavelengths in the transmission spectra of hot Jupiter atmospheres is sensitive to the bulk chemical properties of the atmosphere. For that, we run a grid of chemical models which include TiO formation and destruction, for the ultra-hot Jupiter WASP-19b and an ultra-hot version of HD~209458b. We take into account non-equilibrium chemistry and changes in the temperature and pressure structure of these atmospheres caused by different C/O ratios. We calculate synthetic transmission spectra for these models, and study the relative strengths of TiO and \ce{H2O} features quantitatively. To compare with observations, we use a model independent metric for molecular abundances, $\Delta Z_{\rm TiO-H_2O}/H_{\rm eq}$ that has been previously used in observational studies of exoplanetary atmospheres. We find that with this metric we can differentiate between different chemical models and place constraints on the atmosphere's bulk carbon and oxygen abundance. From chemical considerations we expected that the TiO abundance would depend on the bulk nitrogen, however found that any change to N/H did not result in changes to the resulting TiO. We apply our method to a set of known exoplanets that have been observed in the relevant optical wavelengths and find good agreement between low resolution observations and our model for WASP-121b, marginally good agreement with WASP-79b, WASP-76b, and WASP-19b, and poorer agreement with HD 209458b. Our method can be particularly helpful for indirect studies of the bulk abundances of carbon and oxygen.