A&A 493, 193-200 (2009)
DOI: 10.1051/0004-6361:200810591

Magnetic activity in the photosphere of CoRoT-Exo-2a

Active longitudes and short-term spot cycle in a young Sun-like star

A. F. Lanza¹, I. Pagano¹, G. Leto¹, S. Messina¹, S. Aigrain², R. Alonso³, M. Auvergne⁴, A. Baglin⁴, P. Barge³, A. S. Bonomo^{1, 3, 5}, P. Boumier⁶, A. Collier Cameron⁷, M. Comparato^{1, 5}, G. Cutispoto¹, J. R. De Medeiros⁸, B. Foing⁹, A. Kaiser¹⁰, C. Moutou³, P. S. Parihar¹¹, A. Silva-Valio¹², and W. W. Weiss<sup>10

1</sup>  INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
    e-mail: nuccio.lanza@oact.inaf.it
²  School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
³  Laboratoire d'Astrophysique de Marseille (UMR 6110), Technopole de Château-Gombert, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁴  LESIA, CNRS UMR 8109, Observatoire de Paris, 5 place J. Janssen, 92195 Meudon, France
⁵  Dipartimento di Fisica e Astronomia, Università degli Studi di Catania, via S. Sofia 78, 95123 Catania, Italy
⁶  Institut d'Astrophysique Spatiale (IAS), Batiment 121, 91405 Orsay, France; Université Paris-Sud 11 and CNRS (UMR 8617), France
⁷  School of Physics and Astronomy, University of St. Andrews, North Haugh, St Andrews, Fife Scotland KY16 9SS, UK
⁸  Departamento de Física, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, RN, Brazil
⁹  ESA/ESTEC/SRE-S, Postbus 299, 2200 AG Noordwijk, The Netherlands
¹⁰  Institut für Astronomie, Universität Wien, Türkenschanzstraße 17, 1180 Vienna, Austria
¹¹  Indian Institute of Astrophysics, Block II, Koramangala, Bangalore 560034, India
¹²  CRAAM, Universidade Presbiteriana Mackenzie, Rua da Consolação 896, São Paulo, SP 01302, Brazil

Received 14 July 2008 / Accepted 4 November 2008

Abstract
Context. The space experiment CoRoT has recently detected transits by a hot Jupiter across the disc of an active G7V star (CoRoT-Exo-2a) that can be considered as a good proxy for the Sun at an age of approximately 0.5 Gyr.
Aims. We present a spot modelling of the optical variability of the star during 142 days of uninterrupted observations performed by CoRoT with unprecedented photometric precision.
Methods. We apply spot modelling approaches previously tested in the case of the Sun by modelling total solar irradiance variations, a good proxy for the optical flux variations of the Sun as a star. The best results in terms of mapping of the surface brightness inhomogeneities are obtained by means of maximum entropy regularized models. To model the light curve of CoRoT-Exo-2a, we take into account the photometric effects of both cool spots and solar-like faculae, adopting solar analogy.
Results. Two active longitudes initially on opposite hemispheres are found on the photosphere of CoRoT-Exo-2a with a rotation period of 4.522  0.024 days. Their separation changes by 80° during the time span of the observations. From this variation, a relative amplitude of the surface differential rotation lower than ~1 percent is estimated. Individual spots form within the active longitudes and show an angular velocity ~1 percent lower than that of the longitude pattern. The total spotted area shows a cyclic oscillation with a period of 28.9  4.3 days, which is close to 10 times the synodic period of the planet as seen by the rotating active longitudes. We discuss the effects of solar-like faculae on our models, finding indications of a facular contribution to the optical flux variations of CoRoT-Exo-2a being significantly smaller than in the present Sun.
Conclusions. The implications of such results for the internal rotation of CoRoT-Exo-2a are discussed, based on solar analogy. A possible magnetic star-planet interaction is suggested by the cyclic variation of the spotted area. Alternatively, the 28.9-d cycle may be related to Rossby-type waves propagating in the subphotospheric layers of the star.

Key words: stars: magnetic fields -- stars: late-type -- stars: activity -- stars: rotation -- stars: individual: CoRoT-Exo-2a -- planetary systems

© ESO 2008

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