New chemical scheme for studying carbon-rich exoplanet atmospheres

Abstract

Context. While the existence of more than 1800 exoplanets have been confirmed, there is evidence of a wide variety of elemental chemical composition, that is to say different metallicities and C/N/O/H ratios. Atmospheres with a high C/O ratio (above 1) are expected to contain a high quantity of hydrocarbons, including heavy molecules (with more than two carbon atoms). To correctly study these C-rich atmospheres, a chemical scheme adapted to this composition is necessary. Aims. We have implemented a chemical scheme that can describe the kinetics of species with up to six carbon atoms (C 0 -C 6 scheme). This chemical scheme has been developed with combustion specialists and validated by experiments that were conducted on a wide range of temperatures (300−2500 K) and pressures (0.01−100 bar). Methods. To determine for which type of studies this enhanced chemical scheme is mandatory, we created a grid of 12 models to explore different thermal profiles and C/O ratios. For each of them, we compared the chemical composition determined with a C 0 -C 2 chemical scheme (species with up to two carbon atoms) and with the C 0 -C 6 scheme. We also computed synthetic spectra corresponding to these 12 models. Results. We found no difference in the results obtained with the two schemes when photolyses were excluded from the model, regardless of the temperature of the atmosphere. In contrast, differences can appear in the upper atmosphere (P >∼ 1−10 mbar) when there is photochemistry. These differences are found for all the tested pressure-temperature profiles if the C/O ratio is above 1. When the C/O ratio of the atmosphere is solar, differences are only found at temperatures lower than 1000 K. The differences linked to the use of different chemical schemes have no strong influence on the synthetic spectra. However, with this study, we have confirmed C 2 H 2 and HCN as possible tracers of warm C-rich atmospheres. Conclusions. The use of this new chemical scheme (instead of the C 0 -C 2 ) is mandatory for modelling atmospheres with a high C/O ratio and, in particular, for studying the photochemistry in detail. If the focus is on the synthetic spectra, a smaller scheme may be sufficient, because it will be faster in terms of computation time.

VULCAN: An Open-source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres

“…However, previous studies of the atmospheric circulation of hot Jupiters, in three dimensions, have demonstrated that it takes the form of equator-to-pole circulation cells that extend over several orders of magnitude in vertical/radial pressure (Heng et al 2011;Perna et al 2012;Parmentier et al 2013;Mendonca et al 2016), which renders the eddy-diffusion approximation suspect. Nevertheless, it has become entrenched within the atmospheric chemistry community to use such an approximation (Allen et al 1981;Line et al 2011;Moses et al 2011Moses et al , 2013aMoses et al , 2013bHu et al 2012Hu et al , 2015Kopparapu et al 2012;Madhusudhan 2012;Agúndez et al 2014;Venot et al 2015;Rimmer & Helling 2016), and we will do the same for the purpose of comparison to previous work in the literature. We consider the use of eddy diffusion to be a "necessary evil" for 1D calculations.…”

“…Unlike for the Earth and Solar System bodies, the bulk of the focus is on currently observable exoplanetary atmospheres, which fall into the temperature range of 500-2500 K (for reviews, see Seager & Deming 2010;Madhusudhan et al 2014;Heng & Showman 2015). There exists a diverse body of work on the atmospheric chemistry of exoplanets, and the published models fall into two basic groups: chemical equilibrium (Burrows & Sharp 1999;Lodders & Fegley 2002;Madhusudhan 2012;Blecic et al 2016) and photochemical kinetics (Line et al 2011;Moses et al 2011Moses et al , 2013aMoses et al , 2013bHu et al 2012Hu et al , 2015Kopparapu et al 2012;Venot et al 2012Venot et al , 2015Line & Yung 2013;Agúndez et al 2014;Zahnle & Marley 2014;Rimmer & Helling 2016). Some of this work traces its roots back to the study of brown dwarfs and low-mass stars (Burrows & Sharp 1999;Lodders & Fegley 2002).…”

Atmospheric Chemistry for Astrophysicists: A Self-Consistent Formalism and Analytical Solutions for Arbitrary C/O

“…Despite its somewhat late start in the study of exoplanets (e.g., Burrows & Sharp 1999;Zahnle et al 2009;Moses et al 2011Moses et al , 2013aMoses et al , 2013bHu et al 2012Hu et al , 2013Madhusudhan 2012;Line & Yung 2013;Blecic et al 2015;Venot et al 2015), atmospheric chemistry has a long and rich history in the Earth and planetary sciences and the study of brown dwarfs (e.g., Prinn & Barshay 1977;Barshay & Lewis 1978;Allen & Yung 1981;Fegley & Lodders 1996;Lodders & Fegley 2002;Ciesla & Charnley 2006, pp. 209-230).…”

“…Furthermore, Venot et al (2015) have shown using calculations of chemical kinetics that acetylene and hydrogen cyanide are the dominant hydrocarbons in carbon-rich atmospheres. In reality, both reactions are net reactions that consist of large networks of individual reactions, some of which produce transient species en route to the products.…”

“…2. When the atmosphere is carbon-rich, methane is the dominant molecule but has to compete with acetylene in some circumstances (Madhusudhan 2012;Moses et al 2013a;Venot et al 2015). Water is the dominant oxygen carrier only at low temperatures, superceded by carbon monoxide at high temperatures (Madhusudhan 2012;Moses et al 2013aMoses et al , 2013b.…”