The On/Off Nature of Star-Planet Interactions^*

Evidence suggesting an observable magnetic interaction between a star and its hot Jupiter appears as a cyclic variation of stellar activity synchronized to the planet's orbit. In this study we monitored the chromospheric activity using several stellar activity indicators of seven stars with hot Jupiters using new high-resolution echelle spectra collected with ESPaDOnS over a few nights in 2005 and 2006 from the CFHT (Ca II H λ3968, K λ3933, the Ca II infrared triplet [IRT] λ8662 line, Hα λ6563, and He I λ5876). Synchronicity of the Ca II H and K emission of HD 179949 with its planet's orbit is clearly seen in four out of six epochs, while rotational modulation with P_rot = 7 days is apparent in the other two seasons. We observe a similar phenomenon on υ And. This on/off nature of star-planet interaction (SPI) in the two systems is likely a function of the changing stellar magnetic field structure throughout its activity cycle. Variability in the transiting system HD 189733 is likely associated with an active region rotating with the star; however, the flaring in excess of the rotational modulation may be associated with its hot Jupiter. As for HD 179949, the peak variability as measured by the mean absolute deviation (MAD) for both HD 189733 and τ Boo leads the subplanetary longitude by ~70°. The tentative correlation between this activity and the ratio of M_psin i to the planet's rotation period, a quantity proportional to the hot Jupiter's magnetic moment, first presented by Shkolnik and coworkers remains viable. This work furthers the characterization of SPI, improving its potential as a probe of extrasolar planetary magnetic fields.