DISEQUILIBRIUM CARBON, OXYGEN, AND NITROGEN CHEMISTRY IN THE ATMOSPHERES OF HD 189733b AND HD 209458b

Abstract

We have developed 1-D photochemical and thermochemical kinetics and diffusion\nmodels for the transiting exoplanets HD 189733b and HD 209458b to study the\neffects of disequilibrium chemistry on the atmospheric composition of "hot\nJupiters." Here we investigate the coupled chemistry of neutral carbon,\nhydrogen, oxygen, and nitrogen species, and we compare the model results with\nexisting transit and eclipse observations. We find that the vertical profiles\nof molecular constituents are significantly affected by transport-induced\nquenching and photochemistry, particularly on cooler HD 189733b; however, the\nwarmer stratospheric temperatures on HD 209458b can help maintain\nthermochemical equilibrium and reduce the effects of disequilibrium chemistry.\nFor both planets, the methane and ammonia mole fractions are found to be\nenhanced over their equilibrium values at pressures of a few bar to less than a\nmbar due to transport-induced quenching, but CH4 and NH3 are photochemically\nremoved at higher altitudes. Atomic species, unsaturated hydrocarbons\n(particularly C2H2), some nitriles (particularly HCN), and radicals like OH,\nCH3, and NH2 are enhanced overequilibrium predictions because of quenching and\nphotochemistry. In contrast, CO, H2O, N2, and CO2 more closely follow their\nequilibrium profiles, except at pressures < 1 microbar, where CO, H2O, and N2\nare photochemically destroyed and CO2 is produced before its eventual\nhigh-altitude destruction. The enhanced abundances of HCN, CH4, and NH3 in\nparticular are expected to affect the spectral signatures and thermal profiles\nHD 189733b and other, relatively cool, close-in transiting exoplanets. We\nexamine the sensitivity of our results to the assumed temperature structure and\neddy diffusion coefficientss and discuss further observational consequences of\nthese models.\n