Molecular formation along the atmospheric mass loss of HD 209458 b and similar Hot Jupiters

TL;DR

This study models the chemical processes in the atmospheric mass loss of Hot Jupiters, revealing the formation of simple molecules like OH+ and H2O+ within the exosphere, challenging the notion of purely atomic atmospheres.

Contribution

Developed a 1D chemical and photochemical model including 56 species and 566 reactions to explore molecular formation in Hot Jupiter atmospheres under mass loss conditions.

Findings

Formation of molecules like OH+, H2O+, and H3O+ within 10^7 km of the planet.

Estimated high column density of OH+ suggests potential for observational detection.

Molecular formation may be more prominent in less irradiated Hot Jupiters.

Abstract

The chemistry along the mass loss of Hot Jupiters is generally considered to be simple, consisting mainly of atoms, prevented from forming more complex species by the intense radiation field from their host stars. In order to probe the region where the temperature is low (T < 2000 K), we developed a 1D chemical and photochemical reaction model of the atmospheric mass loss of HD 209458 b, involving 56 species, including carbon chain and oxygen bearing ones, interacting through 566 reactions. The simulation results indicate that simple molecules like OH+, H2O+ and H3O+ are formed inside the region, considering that residual H2 survives in the exosphere, a possibility indicated by recent observational work. The molecules are formed and destroyed within a radial distance of less than 10^7 km, but the estimated integrated column density of OH+, a potential tracer of H2, is high enough to allow detection, which, once achieved, would indicate a revision of chemical models of the upper atmosphere of Hot Jupiters. For low density Hot Jupiters receiving less intense XUV radiation from their host stars than HD 209458 b, molecular species could conceivably be formed with a higher total column density.