Small hydrocarbon molecules in cloud-forming Brown Dwarf and giant gas planet atmospheres

TL;DR

This study investigates complex hydrocarbon molecules in the atmospheres of Brown Dwarfs and giant gas planets, revealing their potential abundance and the factors influencing their presence, which impacts future atmospheric and kinetic modeling.

Contribution

It provides new insights into hydrocarbon chemistry in dust-forming atmospheres using Drift-phoenix simulations, highlighting the role of element depletion and physical parameters.

Findings

Hydrocarbon molecules like C2H6 and C2H2 could be abundant in upper atmospheres.

Carbon is distributed among molecules such as CO, CH4, HCN, and C4H, depending on metallicity.

Transport-induced quenching affects hydrocarbon presence, with quenching heights varying by data source.

Abstract

We study the abundances of complex carbon-bearing molecules in the oxygen-rich dust- forming atmospheres of Brown Dwarfs and giant gas planets. The inner atmospheric re- gions that form the inner boundary for thermochemical gas-phase models are investigated. Results from Drift-phoenix atmosphere simulations, which include the feedback of phase- non-equilibrium dust cloud formation on the atmospheric structure and the gas-phase abun- dances, are utilised. The resulting element depletion leads to a shift in the carbon-to-oxygen ratio such that several hydrocarbon molecules and cyanopolycyanopolyynene molecules can be present. An increase in surface gravity and/or a decrease in metallicity support the increase in the partial pressures of these species. CO, CO2, CH4, and HCN contain the largest fraction of carbon. In the upper atmosphere of low-metallicity objects, more carbon is contained in C4H than in CO, and also CH3 and C2H2 play an increasingly important role as carbon-sink. We determine chemical relaxation time-scales to evaluate if hydrocarbon molecules can be affected by transport-induced quenching. Our results suggest that a considerable amount of C2H6 and C2H2 could be expected in the upper atmospheres not only of giant gas planets, but also of Brown Dwarfs. However, the exact quenching height strongly depends on the data source used. These results will have an impact on future thermo-kinetic studies, as they change the inner boundary condition for those simulations.