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

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 .

Accepted for publication in ApJS http://www.exoclime.net/

References in corpus (12)

• The NumPy array: a structure for efficient numerical computation
• On the radiative equilibrium of irradiated planetary atmospheres
• Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b
• Atmospheric Sulfur Photochemistry on Hot Jupiters
• A chemical model for the atmosphere of hot Jupiters
• Dynamics and Disequilibrium Carbon Chemistry in Hot Jupiter Atmospheres, With Application to HD 209458b
• Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets
• Atmospheric Dynamics of Hot Exoplanets
• The Effects of Consistent Chemical Kinetics Calculations on the Pressure-Temperature Profiles and Emission Spectra of Hot Jupiters
• Analytical Models of Exoplanetary Atmospheres. II. Radiative Transfer via the Two-stream Approximation
• New chemical scheme for studying carbon-rich exoplanet atmospheres
• THOR: A New and Flexible Global Circulation Model to Explore Planetary Atmospheres

Cited by in corpus (3)

• Chemical enrichment of giant planets and discs due to pebble drift
• Theoretical transmission spectra of exoplanet atmospheres with hydrocarbon haze: Effect of creation, growth, and settling of haze particles. I. Model description and first results
• Retrieval Analysis of the Emission Spectrum of WASP-12b: Sensitivity of Outcomes to Prior Assumptions and Implications for Formation History