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 (C0-C6 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 C0-C2 chemical scheme (species with up to two carbon atoms) and with the C0-C6 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 C2H2 and HCN as possible tracers of warm C-rich atmospheres.

[Conclusions] The use of this new chemical scheme (instead of the C0-C2) 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.