A Self-Consistent 3D Hydrodynamic Model for Helium Transit Signatures in Evaporating Hot Jupiters

Abstract

The HeI triplet line (1083 nm), together with hydrodynamic models, can be used to characterize atmospheric escape of exoplanets. However, most of the available models cannot capture the three dimensional (3D) physics of escaping atmospheres, such as tidal forces and the interaction with stellar winds. To investigate how 3D effects affect the helium transit signature, we update our 3D atmospheric evaporation model to self-consistently solve the hydrodynamic equations together with the atomic hydrogen and helium populations. We also produce synthetic helium transits. Our atmospheric escape models assume a Hot Jupiter interacting with stellar wind of ranging mass-loss rates and two XUV fluxes, representative of an old and a young star. Models considering an old star show a decrease of helium triplet density with increasing stellar wind strength, which occurs for two reasons. First, stronger winds reduce the volume of the escaping atmosphere, which decreases obscuration of atmospheric transits. Secondly, as a consequence of a less extended atmosphere, optical depth is reduced, impacting both photoionization and heating, which in turn affect the gas temperature of planetary material, reducing the density of helium triplet. The model assuming a younger star shows an extended outflow, with escape rates 25 times higher. For the same stellar wind strength, the helium transit is 3.3 times deeper than when assuming the XUV of an older star. Weaker stellar winds and/or strong XUV flux allow for pre-transit helium absorption, while all scenarios show (different levels of) post-transit absorptions, described by the presence of a comet-like tail.