paper

A photochemical model for the carbon-rich planet WASP-12b

arXiv:1110.2793 · doi:10.1088/0004-637X/745/1/77

Abstract

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 in its atmosphere and the lack of $\h2o$ features can only be explained if the carbon-to-oxygen ratio in the planet's atmosphere is much greater than the solar ratio ($\ctoo = 0.54$). Here, we use a 1-D photochemical model to study the effect of disequilibrium chemistry on the observed abundances of $\h2o, \com, \co2$ and in the WASP-12b atmosphere. We consider two cases: one with solar $\ctoo$ and another with $\ctoo = 1.08$. The solar case predicts that $\h2o$ and $\com$ are more abundant than $\co2$ and , as expected, whereas the high $\ctoo$ model shows that $\com$, CH and HCN are more abundant. This indicates that the extra carbon from the high $\ctoo$ model is in hydrocarbon species. $\h2o$ photolysis is the dominant disequilibrium mechanism that alters the chemistry at higher altitudes in the solar $\ctoo$ case, whereas photodissociation of CH and HCN is significant in the super-solar case. Furthermore, our analysis indicates that is the major absorber in the atmosphere of WASP-12b and the absorption features detected near 1.6 and 8 micron may be arising from CH rather than . The Hubble Space Telescope's WFC3 can resolve this discrepancy, as has absorption between microns, while does not.

Accepted to Astrophysical Journal. Reaction list available with the journal version of the paper

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