Using a quench level approximation to estimate the effect of metallicity on N-bearing species abundances 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 the 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 and Acharyya (2023) 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. For generating thermal profiles, petitRADTRANS code is used. Chemical timescales of NH$_3$ and N$_2$ are found to be complex functions of metallicity, while HCN is inversely proportional. Using NH$_3$ and CO quenched abundances, the HCN quenched abundance can be constrained since it remains in equilibrium with NH$_3$, CO, and H$_2$O. Quenched NH$_3$ increases with increasing K$_{zz}$ untill 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 towards the lower equilibrium temperature, and HCN abundance increases with metallicity. Finally, we used the dataset of quenched abundances to provide a list of potential candidates in which HCN observation can be possible.