The Photoevaporation of Hot Jupiters and Its Observational Consequences

Abstract

Extra-solar planets (exoplanets) exist in a wide variety of environments. The photoionization-driven evaporation of planetary atmospheres is a fundamental process for hot exoplanets, and is likely to have a significant impact on planets in the habitable zones of M-dwarfs. Given its importance, fully 3-D multi-physics simulations are needed in order to compare with observations. Using AstroBEAR, the transfer of ionizing photons into the atmosphere of a hot Jupiter is modeled self-consistently, the launch of the wind is tracked, and its large-scale evolution via tidal and non-inertial forces is examined. Chapter 2 describes the simulation methods used throughout the work. Simulations of anomalous absorption in the WASP-12 system are presented in chapter 3. Simulations for planets of 0.263 and 0.07 M♃ and stellar fluxes of 2 x 1013 and 2 x 1014 photons/cm2/s are presented and the properties of their winds studied in chapter 4. The role of radiation pressure in shaping exoplanet photoevaporation is examined in chapter 5. Radiation pressure from the host star has been proposed as a mechanism to drive the escaping atmosphere into a "cometary" tail and explain the high Doppler-shift velocities observed in the Lyman-α absorption. Using simulations of HD~209458b we demonstrate that, for the Lyman-α flux expected for HD~209458, radiation pressure is unlikely to significantly affect photoevaporative winds or explain their observed high velocities. Charge exchange between the stellar and planetary winds has also been suggested as a method for creating the observed Lyman-α absorption signature, but the results presented in chapter 6 suggest that it creates insufficient absorption to explain the observations. Alternative explanations and future directions for study are suggested in chapter 7