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Magnetic cycles of the planet-hosting star τ Bootis – II. A second magnetic polarity reversal |
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| 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 |
| Institution: | LATT-UMR 5572, CNRS &Univ. P. Sabatier, 14 Av. E. Belin, F-31400 Toulouse, France;LAM-UMR 6110, CNRS &Univ. de Provence, 38 rue Fréderic Juliot-Curie, F-13013 Marseille, France;HIA/NRC, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada;LESIA–UMR 8109, CNRS &Univ. Paris VII, 5 Place Janssen, F-92195 Meudon Cedex, France;Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015-130, USA;School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS;1234 Hewlett Place, Victoria, BC V8S 497, Canada |
| Abstract: | In this paper, we present new spectropolarimetric observations of the planet-hosting star τ Bootis, using ESPaDOnS and Narval spectropolarimeters at Canada–France–Hawaii Telescope and Telescope Bernard Lyot, respectively. We detected the magnetic field of the star at three epochs in 2008. It has 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 2007 June. This is the second such reversal observed in 2 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 a 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 the planet may be strong enough to force at least the thin convective envelope into corotation. τ Boo shows variability in the Ca ii H & K and Hα throughout the night and on a night-to-night time-scale. We do not detect enhancement in the activity of the star that may be related to the conjunction of the planet. Further data are needed to conclude about the activity enhancement due to the planet. |
| Keywords: | stars: activity stars: individual: τ Boo stars: magnetic fields planetary systems |