Reduced chemical scheme for modelling warm to hot hydrogen-dominated atmospheres

¹ Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583, Universités Paris Est Créteil (UPEC) et Paris Diderot (UPD), Créteil, France
e-mail: olivia.venot@lisa.u-pec.fr
² Laboratoire Réactions et Génie des Procédés, LRGP UMP 7274 CNRS, Université de Lorraine, 1 rue Grandville, BP 20401, 54001 Nancy, France
³ Laboratoire d’Astrophysique de Bordeaux, Université Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, Pessac 33615, France
⁴ Astrophysics Group, University of Exeter, EX4 4QL Exeter, UK
⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁶ Maison de la Simulation, CEA, CNRS, Université Paris-Sud, UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, France

Received: 14 December 2018
Accepted: 11 February 2019

Abstract

Context. Three-dimensional models that account for chemistry are useful tools to predict the chemical composition of (exo)planet and brown dwarf atmospheres and interpret observations of future telescopes, such as James Webb Space Telescope (JWST) and Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL). Recent Juno observations of the NH₃ tropospheric distribution in Jupiter also indicate that 3D chemical modelling may be necessary to constrain the deep composition of the giant planets of the solar system. However, due to the high computational cost of chemistry calculations, 3D chemical modelling has so far been limited.

Aims. Our goal is to develop a reduced chemical scheme from the full chemical scheme of Venot et al. 2012 (A&A, 546, A43) able to reproduce accurately the vertical profiles of the observable species (H₂O, CH₄, CO, CO₂, NH₃, and HCN). This reduced scheme should have a size compatible with three-dimensional models and be usable across a large parameter space (e.g. temperature, pressure, elemental abundance). The absence of C₂H₂ from our reduced chemical scheme prevents its use to study hot C-rich atmospheres.

Methods. We used a mechanism-processing utility program designed for use with Chemkin-Pro to reduce a full detailed mechanism. The ANSYS^© Chemkin-Pro Reaction Workbench allows the reduction of a reaction mechanism for a given list of target species and a specified level of accuracy. We took a warm giant exoplanet with solar abundances, GJ 436b, as a template to perform the scheme reduction. To assess the validity of our reduced scheme, we took the uncertainties on the reaction rates into account in Monte Carlo runs with the full scheme, and compared the resulting vertical profiles with the reduced scheme. We explored the range of validity of the reduced scheme even further by applying our new reduced scheme to GJ 436b’s atmosphere with different elemental abundances, to three other exoplanet atmospheres (GJ 1214b, HD 209458b, HD 189733b), a brown dwarf atmosphere (SD 1110), and to the troposphere of two giant planets of the solar system (Uranus and Neptune).

Results. For all cases except one, the abundances predicted by the reduced scheme remain within the error bars of the model with the full scheme. Expectedly, we found important differences that cannot be neglected only for the C-rich hot atmosphere. The reduced chemical scheme allows more rapid runs than the full scheme from which it is derived (~30× faster).

Conclusions. We have developed a reduced scheme containing 30 species and 181 reversible reactions. This scheme has a large range of validity and can be used to study all kinds of warm atmospheres, except hot C-rich ones that contain a high amount of C₂H₂. It can be used in 1D models, for fast computations, but also in 3D models for hot giant (exo)planet and brown dwarf atmospheres.