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Update 5/11/16

Started catching up a bit from the end of the semester. Read Schneiter et al. 2016 and Christie et al. 2016:

Schneiter paper makes synthetic observations of Lyman-alpha absorption in tails created by interacting solar and planetary winds, with photoionization included. They have nineteen models of varying stellar UV flux (photoionization rate), stellar wind conditions, and mass-loss rate of the planet. Both the stellar and planetary winds are isotropic, and radiation pressure from the star is approximated by reducing stellar gravity. They find that by including photoionization, a smaller neutral tail is formed; they also find a lower time to a stationary state than in previous models without photoionization. The most absorption is found in the blue-shifted side, between -130 and -40 km/s, the extent of which is dependent on the mass loss rate of the planet and on the ionizing flux. By comparing their models to observation, the heat efficiency of HD 209458b can be predicted to be less than 50%. In addition, it can be seen that the observed Lyman-alpha absorption does not necessarily require charge exchange to accelerate the neutral hydrogen sufficiently.

2.5D spherical simulations of planetary and stellar wind interactions, including charge exchange, were performed. Density was fixed at the base of the planetary wind and an inflow boundary condition on one half of the simulation served to emulate the stellar wind. In addition to charge exchange, advection, photoionization and recombination, and collisional ionization were included. The escape parameter lambda was used to categorize the models; it was found that there were two distinct regimes, with a transition region between. With lambda ⇐ 4 (high planetary temp), the planetary wind becomes transonic before colliding with the stellar wind, creating a large tail that takes a significant amount of time to mix. With lambda ≥ 6 (low planetary temp), the planetary wind has no chance to become transonic before it encounters the stellar wind, and the winds interact turbulently rather than collide, resulting in a well-mixed, barely evident tail. The transition region between these is also shown clearly in the calculated mass-loss rates of the simulations.