Tracing bulk elemental ratios in exoplanetary atmospheres with TiO chemistry

¹ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
e-mail: cridland@strw.leidenuniv.nl
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 17 April 2020
Accepted: 1 July 2020

Abstract

Deciphering the bulk elemental abundances of exoplanetary atmospheres is not an easy task, yet it is crucial to understanding 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. To this end, we ran 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 calculated synthetic transmission spectra for these models, and studied the relative strengths of TiO and H₂O features quantitatively. To compare with observations, we used a model-independent metric for molecular abundances, ΔZ_TiO−H2O/H_eq, which has previously been 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 bulk carbon and oxygen abundances of the atmosphere. From chemical considerations, we expected the TiO abundance to depend on the bulk nitrogen. However, we find that changes in N/H do not result in changes in the resulting TiO. We applied 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 will be particularly helpful for indirect studies of the bulk abundances of carbon and oxygen.