Tracing bulk elemental ratios in exoplanetary atmospheres with TiO chemistry

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, ΔZTiO−H2O/Heq, 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.

How drifting and evaporating pebbles shape giant planets

“…For nitrogen we use N 2 and NH 3 , where most of the nitrogen should be in the form of N 2 (e.g., Bosman et al 2019). Even though Ti, Al, K, Na, and V are not very abundant and thus do not contribute significantly to the planetary accretion rates, we include these elements in our model because they can be observed in the atmospheres of hot Jupiters and could also play a crucial role for the chemical evolution inside the atmospheres (Ramírez et al 2020).…”

Recent Updates of Gas-phase Chemical Reactions and Molecular Lines of SiS in CLOUDY

“…It is not included in the UMIST Database for Astrochemistry (UDfA) (McElroy et al 2013) or in Kinetic Database for Astrochemistry (KIDA) (Wakelam et al 2012). We incorporate available reactions (Tsai et al 2021;Churchwell et al 1980;Ramírez et al 2020), but these are small in number. We adopted Si-based chemistry as a proxy for the missing Ti chemistry.…”

“…For example, in the reference column Si5209 refers a copied reaction from Si reaction of UDfA2012 with the reaction number 5209. Ref 1, 2, and 3 refer to Churchwell et al (1980), Ramírez et al (2020), andTsai et al (2021), respectively. The rate coefficient for the reaction, Ti + + H − → Ti + H, is taken from Dalgarno & McCray (1973).…”

Nitrogen as a Tracer of Giant Planet Formation. I. A Universal Deep Adiabatic Profile and Semianalytical Predictions of Disequilibrium Ammonia Abundances in Warm Exoplanetary Atmospheres

“…Hobbs et al (2019) used a photochemical kinetic model to show that the abundances of C-and O-bearing species, such as H 2 O and CO, are insensitive to N/H ratio in hot Jupiters like HD 209458b. Ramírez et al (2020) also investigated the impact of N/H ratio on TiO abundances in ultrahot Jupiters and found that the TiO abundance is nearly independent of N/H. Since the abundances of C-and O-bearing species are insensitive to bulk nitrogen abundance in substellar atmospheres, it appears that the only route to diagnose the bulk nitrogen abundance of a giant planet is from NH 3 and/or HCN.…”