  Recent observations suggest that stellar magnetic activity may be influenced
by the presence of a close-by giant planet. Specifically, chromospheric hot
spots rotating in phase with the planet orbital motion have been observed
during some seasons in a few stars harbouring hot Jupiters. The spot leads the
subplanetary point by a typical amount of about 60-70 degrees, with the extreme
case of upsilon And where the angle is about 170 degrees. The interaction
between the star and the planet is described considering the reconnection
between the stellar coronal field and the magnetic field of the planet.
Reconnection events produce energetic particles that moving along magnetic
field lines impact onto the stellar chromosphere giving rise to a localized hot
spot. A simple magnetohydrostatic model is introduced to describe the coronal
magnetic field of the star connecting its surface to the orbiting planet. The
field is assumed to be axisymmetric around the rotation axis of the star and
its configuration is more general than a linear force-free field. With a
suitable choice of the free parameters, the model can explain the phase
differences between the hot spots and the planets observed in HD 179949,
upsilon And, HD 189733, and tau Bootis, as well as their visibility modulation
on the orbital period and seasonal time scales. The possible presence of cool
spots associated with the planets in tau Boo and HD 192263 cannot be explained
by the present model. However, we speculate about the possibility that
reconnection events in the corona may influence subphotospheric dynamo action
in those stars producing localized photospheric (and chromospheric) activity
migrating in phase with their planets.
