Photoionization of planetary winds: case study HD 209458b

Abstract

Close-in hot Jupiters are exposed to a tremendous photon flux that ionizes the neutral escaping material from the planet leaving an observable imprint that makes them an interesting laboratory for testing theoretical models. In this work, we present 3D hydrodynamic simulations with radiation transfer calculations of a close-in exoplanet in a blow-off state. We calculate the Ly α absorption and compare it with observations of HD 209458b and previous simplified model results. Our results show that the hydrodynamic interaction together with a proper calculation of the photoionization process are able to reproduce the main features of the observed Ly α absorption, in particular at the blue-shifted wings of the line. We found that the ionizing stellar flux produce an almost linear effect on the amount of absorption in the wake. Varying the planetary mass-loss rate and the radiation flux, we were able to reproduce the 10 per cent absorption observed at −100 km s−1.