Spectropolarimetric observations of the transiting planetary system of the K dwarf HD 189733^*

¹ Laboratoire d'Astrophysique de Marseille, CNRS UMR 6110 Traverse du Siphon, 376 Marseille, France e-mail: Claire.Moutou@oamp.fr
² Laboratoire d'Astrophysique Toulouse-Tarbes, Observatoire Midi Pyrénées, 14 Av. E. Belin, 31400 Toulouse, France
³ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
⁴ LESIA, Observatoire de Paris Meudon, Place J. Janssen, 92195 Meudon Cedex, France
⁵ Dept. of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, K7K 7B4, Canada
⁶ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis Bd Arago, 75014 Paris, France
⁷ Observatoire de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland

Received: 4 May 2007
Accepted: 12 July 2007

Abstract

Context.With a Jupiter-mass planet orbiting at a distance of only 0.031 AU, the active K2 dwarf HD 189733 is a potential candidate in which to study the magnetospheric interactions of a cool star with its recently-discovered close-orbiting giant planet.

Aims.We decided to explore the strength and topology of the large-scale magnetosphere of HD 189733, as a future benchmark for quantitative studies for models of the star/planet magnetic interactions.

Methods.To this end, we used ESPaDOnS, the new generation spectropolarimeter at the Canada-France-Hawaii 3.6 m telescope, to look for Zeeman circular polarisation signatures in the line profiles of HD 189733 in 2006 June and August.

Results.Zeeman signatures in the line profiles of HD 189733 are clearly detected in all spectra, demonstrating that a field is indeed present at the surface of the star. The Zeeman signatures are not modulated with the planet's orbital period but apparently vary with the stellar rotation cycle. The reconstructed large-scale magnetic field, whose strength reaches a few tens of G, is significantly more complex than that of the Sun; it involves in particular a significant toroidal component and contributions from magnetic multipoles of order up to 5. The Caii H & K lines clearly feature core emission, whose intensity is apparently varying mostly with rotation phase. Our data suggest that the photosphere and magnetic field of HD 189733 are sheared by a significant amount of differential rotation.

Conclusions.Our initial study confirms that HD 189733 is an optimal target for investigating activity enhancements induced by closely orbiting planets. More data are needed, densely covering both the orbital and rotation cycles, to investigate whether and how much the planet contributes to the overall activity level of HD 189733.