VULCAN: an Open-Source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres

TL;DR

VULCAN is an open-source, validated chemical kinetics code designed for modeling hot exoplanetary atmospheres, capable of reproducing complex disequilibrium chemistry with a reduced reaction network, and useful for studying atmospheric composition and processes.

Contribution

This paper introduces VULCAN, a validated, open-source chemical kinetics code with a reduced network for exoplanet atmospheres, enabling efficient disequilibrium chemistry modeling and analysis.

Findings

VULCAN accurately reproduces complex disequilibrium models of hot Jupiters.

The quenching approximation is reliable for methane but not for acetylene.

VULCAN computes 1D atmospheric models in minutes, facilitating large parameter studies.

Abstract

We present an open-source and validated chemical kinetics code for studying hot exoplanetary atmospheres, which we name VULCAN. It is constructed for gaseous chemistry from 500 to 2500 K using a reduced C-H-O chemical network with about 300 reactions. It uses eddy diffusion to mimic atmospheric dynamics and excludes photochemistry. We have provided a full description of the rate coefficients and thermodynamic data used. We validate VULCAN by reproducing chemical equilibrium and by comparing its output versus the disequilibrium-chemistry calculations of Moses et al. and Rimmer & Helling. It reproduces the models of HD 189733b and HD 209458b by Moses et al., which employ a network with nearly 1600 reactions. We also use VULCAN to examine the theoretical trends produced when the temperature-pressure profile and carbon-to-oxygen ratio are varied. Assisted by a sensitivity test designed to identify the key reactions responsible for producing a specific molecule, we revisit the quenching approximation and find that it is accurate for methane but breaks down for acetylene, because the disequilibrium abundance of acetylene is not directly determined by transport-induced quenching, but is rather indirectly controlled by the disequilibrium abundance of methane. Therefore, we suggest that the quenching approximation should be used with caution and must always be checked against a chemical kinetics calculation. A one-dimensional model atmosphere with 100 layers, computed using VULCAN, typically takes several minutes to complete. VULCAN is part of the Exoclimes Simulation Platform (ESP; http://www.exoclime.net/) and publicly available at http://github.com/exoclime/VULCAN .