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Description
The hot-Jupiter WASP-12b is a heavily irradiated exoplanet in a short-period orbit around a G0-star with twice the metallicity of the Sun. A recent thermochemical equilibrium analysis based on Spitzer and ground-based infrared observations suggests that the presence of CH_4_ in its atmosphere and the lack of H_2_O features can only be explained if the carbon-to-oxygen ratio in the planet's atmosphere is much greater than the solar ratio ([C]/[O]=0.54). Here, we use a one-dimensional photochemical model to study the effect of disequilibrium chemistry on the observed abundances of H_2_O, CO, CO_2_, and CH_4_ in the WASP-12b atmosphere. We consider two cases: one with solar [C]/[O] and another with [C]/[O]=1.08. The solar case predicts that H_2_ O and CO are more abundant than CO_2_ and CH_4_, as expected, whereas the high [C]/[O] model shows that CO, C_2_H_2_, and HCN are more abundant. This indicates that the extra carbon from the high [C]/[O] model is in hydrocarbon species. H_2_ O photolysis is the dominant disequilibrium mechanism that alters the chemistry at higher altitudes in the solar [C]/[O] case, whereas photodissociation of C_2_H_2_ and HCN is significant in the super-solar case. Furthermore, our analysis indicates that C_2_H_2_ is the major absorber in the atmosphere of WASP-12b and the absorption features detected near 1.6 and 8{mu}m may be arising from C_2_H_2_ rather than CH_4_. The Hubble Space Telescope's WFC3 can resolve this discrepancy, as C_2_H_2_ has absorption between 1.51 and 1.54{mu}m, while CH_4_ does not.
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