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Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets
We address disequilibrum abundances of some simple molecules in the
atmospheres of solar composition brown dwarfs and self-luminous extrasolar
giant planets using a kinetics-based 1D atmospheric chemistry model. Our
approach is to use the full kinetics model to survey the parameter space with
effective temperatures between 500 K and 1100 K. In all of these worlds
equilibrium chemistry favors CH4 over CO in the parts of the atmosphere that
can be seen from Earth, but in most disequilibrium favors CO. The small surface
gravity of a planet strongly discriminates against CH4 when compared to an
otherwise comparable brown dwarf. If vertical mixing is like Jupiter's, the
transition from methane to CO occurs at 500 K in a planet. Sluggish vertical
mixing can raise this to 600 K; but clouds or more vigorous vertical mixing
could lower this to 400 K. The comparable thresholds in brown dwarfs are
K. Ammonia is also sensitive to gravity, but unlike CH4/CO, the
NH3/N2 ratio is insensitive to mixing, which makes NH3 a potential proxy for
gravity. HCN may become interesting in high gravity brown dwarfs with very
strong vertical mixing. Detailed analysis of the CO-CH4 reaction network
reveals that the bottleneck to CO hydrogenation goes through methanol, in
partial agreement with previous work. Simple, easy to use quenching relations
are derived by fitting to the complete chemistry of the full ensemble of
models. These relations are valid for determining CO, CH4, NH3, HCN, and CO2
abundances in the range of self-luminous worlds we have studied but may not
apply if atmospheres are strongly heated at high altitudes by processes not
considered here (e.g., wave breaking).Comment: Astrophysical Journal, in press. Clarity improvements throughout and
one new figure. 17 figures, 20 page
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