Extrasolar planets, stellar winds and chromospheric hotspots

Recent observations have shown what is believed to be planet-induced chromospheric activity on stars with hot Jupiters. We present a model of the magnetic interaction between a planet and a star with a dipolar magnetic field. Reconnection between the fields of the star and planet accelerates electrons along the field lines that connect the star and planet. By determining the locations at which these field lines connect to the stellar surface, we can model the surface pattern of the chromospheric response to the planet-star interaction. We find that the inclination of the rotation axis of the star determines the magnitude of the emission, while a misalignment of the magnetic and rotation axes produces a phase shift between the peak of the observed chromospheric emission and the phase where the planet is directly in front of the star. This phase shift increases as the extent of the stellar corona increases. This model reproduces the cyclic signature of chromospheric enhancement seen in stars with hot Jupiters. It can reproduce the 65 degrees phase lag reported for HD 179949 if the closed corona of the star extends out to the orbital radius of the planet at 8.5R(star). It cannot, however, reproduce the phase lag of 169 degrees reported for nu And.