Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b

K Molaverdikhani,P Woitke,V Parmentier,D Samra,N Iro,B W P Lew,R J Macdonald,Ch Helling

doi:10.1051/0004-6361/201937044

Abstract

Aims. The atmospheres of ultra-hot Jupiters (UHJs) are commonly considered to be at thermochemical equilibrium. We aim to provide disequilibrium chemistry maps for a global understanding of the chemistry in the atmosphere of HAT-P-7b and assess the importance of disequilibrium chemistry on UHJs. Methods. We applied a hierarchical modeling approach using 97 1D atmospheric profiles from a 3D general circulation model of HAT-P-7b. For each atmospheric 1D profile, we evaluated our kinetic cloud formation model consistently with the local gas-phase composition in chemical equilibrium. This served as input to study the quenching of dominating CHNO-binding molecules. We evaluated quenching results from a zeroth-order approximation in comparison to a kinetic gas-phase approach. Results. We find that the zeroth-order approach of estimating quenching points agrees well with the full gas-kinetic modeling results. However, it underestimates the quenching levels by about one order of magnitude at high temperatures. Chemical disequilibrium has the greatest effect on the nightside and morning abundance of species such as H, H2O, CH4, CO2, HCN, and all CnHm molecules; heavier CnHm molecules are more affected by disequilibrium processes. The CO abundance, however, is affected only marginally. While dayside abundances also notably change, those around the evening terminator of HAT-P-7b are the least affected by disequilibrium processes. The latter finding may partially explain the consistency of observed transmission spectra of UHJs with atmospheres in thermochemical equilibrium. Photochemistry only negligibly affects molecular abundances and quenching levels. Conclusions. In general, the quenching points of the atmosphere of HAT-P-7b are at much lower pressures than in the cooler hot-jupiters. We propose several avenues to determining the effect of disequilibrium processes on UHJs that are in general based on abundance and opacity measurements at different local times. It remains a challenge to completely disentangle this from the chemical effects of clouds and that of a primordial nonsolar abundance.

Highlights

Results of chemical kinetic modeling Temporal variations of the H2O, CO2, and CH4 vertical abundances at the equator and longitude φ = 225◦ in Figs. 5a, d, and g to demonstrate how disequilibrium processes change the composition of the upper atmosphere
The temporal evolution of the molecular abundances of H2O, CO2, and CH4 are color-coded by time, where blue to red shows the progress in time from thermochemical equilibrium to their disequilibrium steady states
We examine the importance of disequilibrium abundance changes by computing the percent difference between the initial and final p(τx(λ) = 1) surfaces for the molecules that are anticipated to be most prominent in spectra of HAT-P-7b

Summary

Introduction

Ultra-hot Jupiters (UHJs) are highly irradiated, tidally locked planets with extreme day-to-night temperature differences (e.g., Delrez et al 2017; Parmentier et al 2018; Bell & Cowan 2018; Kreidberg et al 2018; Komacek & Tan 2018; Lothringer et al 2018; Lothringer & Barman 2019; Molaverdikhani et al 2019a; Tan & Komacek 2019; Arcangeli et al 2019; Mansfield et al 2020; Wong et al 2019). While previous studies have typically been conducted on colder planets, Kitzmann et al (2018) reported that the chemical state of the atmosphere of KELT-9b deviates from thermochemical equilibrium at pressures lower than 1 mbar in a 1D setup According to their simulations, disequilibrium processes drive some of the atomic and molecular abundances away from their chemical equilibrium states at pressures even higher than 1 mbar. Relaxation schemes are commonly applied to omit the long computational times required to ensure independence of the solution from the initial conditions (see, e.g., Cooper & Showman 2006; Drummond et al 2018a,b; Mendonça et al 2018) In such schemes, it is assumed that if the timescale for hydrodynamic transport processes becomes longer than the chemical timescale, the local atmosphere will achieve chemical equilibrium (wherein both forward and backward reactions occur with a similar rate).

Quenching levels: a zeroth-order approximation

Effect of photochemistry on the quenching levels

CO and CH4 abundance in the equatorial and terminator regions

Observability of disequilibrium chemistry in the atmosphere of HAT-P-7b

Discussion