The coming of age of exoplanetary atmospheres astronomy has inspired recent work in theory and simulation.  In the current talk, I focus on three lines of work on atmospheric chemistry that has been done in my group.  First, I discuss accurate analytical formulae for hydrogen-dominated, C-H-O-N systems in chemical equilibrium that yield the relative abundances of 9 molecules (Heng & Tsai 2016).  It is demonstrated that these formulae perform at ~1% accuracy (or better) compared to calculations done using a Gibbs free energy minimization code.  Second, I discuss a chemical kinetics solver we built from scratch named VULCAN, which solves a large set of coupled mass conservation equations with chemical source and sink terms (Tsai et al. 2017).  We demonstrate that VULCAN produces essentially identical results compared to the work of Julie Moses and Paul Rimmer for the benchmark HD 189733b and HD 209458b case studies, despite having a reduced chemical network of only about 300 reactions.  Ongoing work aims to add nitrogen and photochemistry to VULCAN.  Third, I discuss and revisit the technique of chemical relaxation, which attempts to replace the large set of chemical source and sink terms with a single source term for each molecular species that is described by a chemical timescale.  The details of how to compute this chemical timescale are discussed.  We show that this method is accurate to about a factor of several in terms of the molecular abundances, but the enhanced efficiency makes it suitable for implementation in three-dimensional general circulation models (GCMs).  We discuss how GCMs with built-in chemical relaxation modules may be used to explore disequilibrium chemistry and generate realistic simulations of transmission spectra, which generally probe regions of the exoplanetary atmosphere that have complex geometry and chemical conditions.  Next-generation transmission and emission spectra from the James Webb Space Telescope (JWST) should be constraining enough to confront these models in detail.

Atmospheric chemistry in (currently observable) exoplanets: Review of a suite of techniques

254th National Meeting (2017)

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