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A.J. Friedson, E.H. Wilson (JPL/CalTech), J.I. Moses (LPI)
We are investigating the photochemistry of the close-in extrasolar giant planet HD209458b, including photoionization and associated ion-molecule chemistry. Our model includes the major species formed by the elements H, C, and O and their ions, as well as atomic Na, Mg, and Fe. Water, the most abundant species after hydrogen and helium, plays a fundamental role in two ways. First, water molecules tend to pick up a positive charge through charge transfer reactions with other molecular ions and subsequently dissociate through recombination. Second, absorption by water at wavelengths less than ~170 nm effectively shields heavier molecules, as well as atomic Mg and Fe, from ionizing radiation. Sodium is the most abundant species that can be ionized at wavelengths longer than 170 nm, and may be the principal ion at low altitudes [1]. The water abundance decreases rapidly with height above the ~10-microbar level due to photodissociation and diffusive separation. Above this altitude, the ionosphere may be more reminiscent of that of Jupiter, with the principal ions being H+ and H3+ [2]. C+, which has been observed in the escaping envelope of HD209458b [3], also forms. Our results have important implications for the detection of species escaping from HD209458b and for the degree of ionization of its atmosphere
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.