From Cold to Hot Irradiated Gaseous Exoplanets: Fingerprints of Chemical Disequilibrium in Atmospheric Spectra

Abstract

Almost all planetary atmospheres are affected by disequilibrium chemical processes. In this paper we introduce our recently developed Chemical Kinetic Model (ChemKM). We show that the results of our HD 189733b model are in good agreement with previously published results, except at µbar regime, where molecular diffusion and photochemistry are the dominant processes. We thus recommend careful consideration of these processes when abundances at the top of the atmosphere are desired. We also propose a new metric for a quantitative measure of quenching levels. By applying this metric, we find that quenching pressure decreases with the effective temperature of planets, but it also varies significantly with other atmospheric parameters such as [Fe/H], log(g), and C/O. In addition, we find that the "Methane Valley", a region between 800 and 1500 K where above a certain C/O threshold value a greater chance of CH 4 detection is expected, still exists after including the vertical mixing. The first robust CH 4 detection on an irradiated planet (HD 102195b) places this object within this region; supporting our prediction. We also investigate the detectability of disequilibrium spectral fingerprints by JWST, and suggest focusing on the targets with T eff between 1000 and 1800 K, orbiting around M-dwarfs, having low surface gravity but high metallicity and a C/O ratio value around unity. Finally, constructing Spitzer color-maps suggests that the main two color-populations are largely insensitive to the vertical mixing. Therefore any deviation of observational points from these populations are likely due to the presence of clouds and not disequilibrium processes. However, some cold planets (T eff <900 K) with very low C/O ratios (<0.25) show significant deviations; making these planets interesting cases for further investigation.

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

“…We follow Molaverdikhani et al (2019b) in suggesting the use of the coefficient of variation (CV) to estimate the deviation from chemical equilibrium. This quantity can be calculated as the ratio of the standard deviation of a given species abundance, s, to its temporal mean abundance at any given pressure level.…”

“…the differences and suggests that the photochemistry plays a negligible role. This is expected because the photochemistry mostly affects the mbar regime and higher altitudes (see, e.g., Molaverdikhani et al 2019b and references therein) and the HAT-P-7b dayside quenching points are located at deeper levels. The two exceptions are the evening terminator and a portion of the morning terminator, where the quenching pressures can be lower than mbar, as discussed in Sect.…”

“…A chemical network is essential to quantitatively trace the chemical abundances resulting from interactions between major and minor species at different pressures and temperatures. For this, we used ChemKM, a photochemical kinetic model (Molaverdikhani et al 2019b), to perform a detailed study of gasphase chemical kinetics and quenching levels on HAT-P-7b for A31, page 3 of 25 A&A 635, A31 (2020) Fig. 2.…”

“…In addition, under different circumstances and for different species, the abundance above the quenched level can deviate from a constant value in a chemical kinetic context (see, e.g., Molaverdikhani et al 2019b). If significant, this deviation might pose a problem in the retrieved abundances, as shown by Changeat et al (2019), for example.…”

Discovery of a hot, transiting, Earth-sized planet and a second temperate, non-transiting planet around the M4 dwarf GJ 3473 (TOI-488)

“…Depending on the exact temperature structure and methane abundance profile, vertical mixing could lead to methane quenching (e.g. Molaverdikhani et al 2019b). Hydrocarbon haze (soot) production could act as a carbon-sink in the atmosphere, 0DVV>0 @ 5DGLXV>5 @ )H )H0J6L23 0J6L23 +220J6L23 +22 (DUWKOLNHGHQVLW\ Fig.…”