Magnetic cycles of the planet-hosting star τ Bootis

We have obtained new spectropolarimetric observations of the planet-hosting star tau Bootis, using the ESPaDOnS and NARVAL spectropolarimeters at the Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL). With this data set, we are able to confirm the presence of a magnetic field at the surface of tau Boo and map its large-scale structure over the whole star. The large-scale magnetic field is found to be fairly complex, with a strength of up to 10 G; it features a dominant poloidal field and a small toroidal component, the poloidal component being significantly more complex than a dipole. The overall polarity of the magnetic field has reversed with respect to our previous observation (obtained a year before), strongly suggesting that tau Boo is undergoing magnetic cycles similar to those of the Sun. This is the first time that a global magnetic polarity switch is observed in a star other than the Sun; given the infrequent occurrence of such events in the Sun, we speculate that the magnetic cycle period of tau Boo is much shorter than that of the Sun. Our new data also allow us to confirm the presence of differential rotation, both from the shape of the line profiles and the latitudinal shearing that the magnetic structure is undergoing. The differential rotation surface shear that tau Boo experiences is found to be 6 to 10 times larger than that of the Sun, in good agreement with recent claims that differential rotation is strongest in stars with shallow convective zones. We propose that the short-magnetic cycle period is due to the strong level of differential rotation. With a rotation period of 3.0 and 3.9 d at the equator and pole, respectively, tau Boo appears as the first planet-hosting star whose rotation (at intermediate latitudes) is synchronized with the orbital motion of its giant planet (period 3.3 d). Assuming that this synchronization is not coincidental, it suggests that the tidal effects induced by the giant planet can be strong enough to force the thin convective envelope (though not the whole star) into corotation. We also detect time-dependent activity fluctuations on tau Boo, but cannot unambiguously determine whether they are intrinsic to the star or induced by the planet; more observations of similar type are needed to determine the role of the close-in giant planet orbiting tau Boo on both the activity enhancements and the magnetic cycle of the host star.