Magnetic cycles of the planet-hosting star τ Bootis: II. a second magnetic polarity reversal

• Main Authors: R. Fares, J. -f. Donati, C. Moutou, D. Bohlender, C. Catala, M. Deleuil, E. Shkolnik, A. C. Cameron, M. M. Jardine, G. A. H. Walker
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Canada; narval
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.380.7422; http://www.leif.org/EOS/0906-4515v1.pdf

In this paper, we present new spectropolarimetric observations of the planethosting star τ Bootis, using ESPaDOnS and Narval spectropolarimeters at Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL), respectively. We detected the magnetic field of the star at three epochs in 2008. It is a weak magnetic field of only a few Gauss, oscillating between a predominant toroidal component in January and a dominant poloidal component in June and July. A magnetic polarity reversal was observed relative to the magnetic topology in June 2007. This is the second such reversal observed in two years on this star, suggesting that τ Boo has a magnetic cycle of about 2 years. This is the first detection of a magnetic cycle for a star other than the Sun. The role of the close-in massive planet in the short activity cycle of the star is questioned. τ Boo has strong differential rotation, a common trend for stars with shallow convective envelope. At latitude 40 ◦ , the surface layer of the star rotates in 3.31 d, equal to the orbital period. Synchronization suggests that the tidal effects induced by