Hot Jupiters and Hot Spots: The Short‐ and Long‐Term Chromospheric Activity on Stars with Giant Planets

We monitored the chromospheric activity in the Ca II H & K lines of 13solar-type stars (including the Sun); 8 of them over three years at the CFHTand 5 in a single run at the VLT. Ten of the 13 targets have close planetarycompanions. All of the stars observed at the CFHT show long-term (months toyears) changes in H & K intensity levels. Four stars display short-term (days)cyclical activity. For two, HD 73256 and kappa^1 Ceti, the activity is likelyassociated with an active region rotating with the star, however, the flaringin excess of the rotational modulation may be associated with a hot jupiter. Aplanetary companion remains a possibility for kappa^1 Ceti. For the other two,HD 179949 and upsilon And, the cyclic variation is synchronized to the hotjupiter's orbit. For both stars this synchronicity with the orbit is clearlyseen in two out of three epochs. The effect is only marginal in the third epochat which the seasonal level of chromospheric activity had changed for bothstars. Short-term chromospheric activity appears weakly dependent on the meanK-line reversal intensities for the sample of 13 stars. Also, a suggestivecorrelation exists between this activity and the M_p sin(i) of the star's hotjupiter. Because of their small separation (<= 0.1 AU), many of the hotjupiters lie within the Alfv\'en radius of their host stars which allows adirect magnetic interaction with the stellar surface. We discuss the conditionsunder which a planet's magnetic field might induce activity on the stellarsurface and why no such effect was seen for the prime candidate, tau Boo. Thiswork opens up the possibility of characterizing planet-star interactions, withimplications for extrasolar planet magnetic fields and the energy contributionto stellar atmospheres.Comment: Accepted to the Astrophysical Journal; 39 pages including 17 figure