The Effect of Metallicity on the Nonequilibrium Abundance of Hydrogen-dominated Exoplanet Atmospheres

Abstract

Abstract The atmospheric metallicity greatly influences the composition of exoplanet atmospheres. The effect of metallicity on the thermochemical equilibrium is well studied, though its effect on the disequilibrium abundance is loosely constrained. In this study, we have used the quenching approximation to study the effect of metallicity on the quenched abundance for a range of parameters (temperature: 500–2500 K, pressure: 10−4–103 bar, metallicity: 0.1–1000× solar metallicity). We determine the chemical timescale by finding rate-limiting steps in a reduced chemical network with a network-analysis tool and the thermochemical equilibrium abundance. The equilibrium abundance results are similar to the literature. The CO, H2O, and CO2 abundances increase with metallicity in the parameter range considered. The CH4 abundance increases with metallicity for CO/CH4 < 1 and is unaffected for CO/CH4 > 1. The chemical timescale of CO shows minimal change with metallicity, while the CH4 chemical timescale is inversely proportional to atmospheric metallicity. The quench level of CO shifts into the high-pressure region, and the quench level of CH4 shows complex behavior with metallicity. We benchmarked the quenching approximation with a one-dimensional photochemistry-transport model for two test exoplanets (GJ 1214 b and HD 189733 b) and found it to be in good agreement. We also found that the quenching approximation is a powerful tool to constrain atmospheric parameters. We demonstrated this by constraining the metallicity and transport strength for the test exoplanets HR 8799 b, HD 189733 b, GJ 436 b, and WASP-39 b.