The Effect of Metallicity on the Non-Equilibrium Abundance of Hydrogen Dominated Exoplanet Atmosphere

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$^{-4}$-10$^3$ bar, metallicity: 0.1-1000 $\times$ 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, H$_2$O, and CO$_2$ abundances increase with metallicity in the parameter range considered. The CH$_4$ abundance increases with metallicity for CO/CH$_4$ $<$ 1 and is unaffected for CO/CH$_4$ $>$ 1. The chemical timescale of CO shows minimal change with the metallicity, while the CH$_4$ chemical timescale is inversely proportional to atmospheric metallicity. The quench level of CO shifts into the high-pressure region, and the quench level of CH$_4$ shows complex behavior with metallicity. We benchmarked the quenching approximation with the 1D 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.