Using a Quench Level Approximation to Estimate the Effect of Metallicity on the Abundances of N-bearing Species in H2-dominated Atmospheres

Abstract

Variations in atmospheric elemental nitrogen can considerably affect the abundance of major nitrogen-bearing species such as NH _3 and HCN. Also, due to vertical mixing and photochemistry, their abundance deviates from thermochemical equilibrium. The goal of this study is to understand the effect of atmospheric metallicity on the composition of NH _3 , N _2 , and HCN over a large parameter space in the presence of vertical mixing, which when combined with the work on CHO-bearing species in Soni & Acharyya can provide a comprehensive understanding of the effect of atmospheric metallicity. We used quenching approximations and a full chemical kinetics model for the calculations, and a comparison between these two methods was made. To generate thermal profiles, the petitRADTRANS code was used. Chemical timescales of NH _3 and N _2 are found to be complex functions of metallicity, while HCN is inversely proportional. Using quenched abundances of NH _3 and CO, the quenched abundance of HCN can be constrained since it remains in equilibrium with NH _3 , CO, and H _2 O. Quenched NH _3 increases with increasing K _zz until a particular point, after which it becomes independent of vertical mixing. There is a sweet spot in the K _zz parameter space to maximize the quenched HCN for a given T _int and T _equi ; the parameter space moves toward a lower equilibrium temperature, and the abundance of HCN increases with metallicity. Finally, we used a data set of quenched abundances to provide a list of potential candidates in which the observation of HCN is possible.

Keywords

• Exoplanet atmospheres
• Metallicity
• Atmospheric composition