Photospheric activity and rotation of the planet-hosting star CoRoT-Exo-4a

📝 Abstract

The space experiment CoRoT has recently detected a transiting hot Jupiter in orbit around a moderately active F-type main-sequence star (CoRoT-Exo-4a). This planetary system is of particular interest because it has an orbital period of 9.202 days, the second longest one among the transiting planets known to date. We study the surface rotation and the activity of the host star during an uninterrupted sequence of optical observations of 58 days. Our approach is based on a maximum entropy spot modelling technique extensively tested by modelling the variation of the total solar irradiance. It assumes that stellar active regions consist of cool spots and bright faculae, analogous to sunspots and solar photospheric faculae, whose visibility is modulated by stellar rotation. The modelling of the light curve of CoRoT-Exo-4a reveals three main active longitudes with lifetimes between about 30 and 60 days that rotate quasi-synchronously with the orbital motion of the planet. The different rotation rates of the active longitudes are interpreted in terms of surface differential rotation and a lower limit of 0.057 \pm 0.015 is derived for its relative amplitude. The enhancement of activity observed close to the subplanetary longitude suggests a magnetic star-planet interaction, although the short duration of the time series prevents us from drawing definite conclusions.

💡 Analysis

The space experiment CoRoT has recently detected a transiting hot Jupiter in orbit around a moderately active F-type main-sequence star (CoRoT-Exo-4a). This planetary system is of particular interest because it has an orbital period of 9.202 days, the second longest one among the transiting planets known to date. We study the surface rotation and the activity of the host star during an uninterrupted sequence of optical observations of 58 days. Our approach is based on a maximum entropy spot modelling technique extensively tested by modelling the variation of the total solar irradiance. It assumes that stellar active regions consist of cool spots and bright faculae, analogous to sunspots and solar photospheric faculae, whose visibility is modulated by stellar rotation. The modelling of the light curve of CoRoT-Exo-4a reveals three main active longitudes with lifetimes between about 30 and 60 days that rotate quasi-synchronously with the orbital motion of the planet. The different rotation rates of the active longitudes are interpreted in terms of surface differential rotation and a lower limit of 0.057 \pm 0.015 is derived for its relative amplitude. The enhancement of activity observed close to the subplanetary longitude suggests a magnetic star-planet interaction, although the short duration of the time series prevents us from drawing definite conclusions.

📄 Content

arXiv:0901.4618v1 [astro-ph.SR] 29 Jan 2009 Astronomy & Astrophysics manuscript no. ms11487 c⃝ESO 2021 September 13, 2021 Photospheric activity and rotation of the planet-hosting star CoRoT-Exo-4a⋆ A. F. Lanza1, S. Aigrain2, S. Messina1, G. Leto1, I. Pagano1, M. Auvergne3, A. Baglin3, P. Barge4, A. S. Bonomo1,4,5, A. Collier Cameron6, G. Cutispoto1, M. Deleuil4, J. R. De Medeiros7, B. Foing8, and C. Moutou4 1 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, United Kingdom 3 LESIA, CNRS UMR 8109, Observatoire de Paris, 5 place J. Janssen, 92195 Meudon, France 4 Laboratoire d’Astrophysique de Marseille (UMR 6110), Technopole de Chˆateau-Gombert, 38 rue Fr´ed´eric Joliot-Curie, 13388 Marseille cedex 13, France 5 Dipartimento di Fisica e Astronomia, Universit`a degli Studi di Catania, Via S. Sofia, 78, 95123 Catania, Italy 6 School of Physics and Astronomy, University of St. Andrews, North Haugh, St Andrews, Fife Scotland KY16 9SS 7 Departamento de F´ısica, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, RN, Brazil 8 ESA/ESTEC/SRE-S, Postbus 299, 2200 AG Noordwijk, The Netherlands Received ; accepted ABSTRACT Context. The space experiment CoRoT (Convection, Rotation and Transits) has recently detected a transiting hot Jupiter in orbit around a moderately active F-type main-sequence star (CoRoT-Exo-4a). This planetary system is of particular interest because it has an orbital period of 9.202 days, the second longest one among the transiting planets known to date. Aims. We study the surface rotation and the activity of the host star during an uninterrupted sequence of optical observations of 58 days. Methods. Our approach is based on a maximum entropy spot modelling technique extensively tested by modelling the variation of the total solar irradiance. Recently, it has been successfully applied to model the light curve of another active star with a transiting planet observed by CoRoT, i.e., CoRoT-Exo-2a. It assumes that stellar active regions consist of cool spots and bright faculae, analogous to sunspots and solar photospheric faculae, whose visibility is modulated by stellar rotation. Results. The modelling of the light curve of CoRoT-Exo-4a reveals three main active longitudes with lifetimes between ∼30 and ∼60 days that rotate quasi-synchronously with the orbital motion of the planet. The different rotation rates of the active longitudes are interpreted in terms of surface differential rotation and a lower limit of 0.057 ± 0.015 is derived for its relative amplitude. The enhancement of activity observed close to the subplanetary longitude suggests a magnetic star-planet interaction, although the short duration of the time series prevents us from drawing definite conclusions. Conclusions. The present work confirms the quasi-synchronicity between stellar rotation and planetary orbital motion in the CoRoT- Exo-4 system and provides for the first time a lower limit for the surface differential rotation of the star. This information can be important in trying to understand the formation and evolution of this highly interesting planetary system. Moreover, there is an indication for a possible star-planet magnetic interaction that needs to be confirmed by future studies. Key words. stars: magnetic fields – stars: late-type – stars: activity – stars: rotation – planetary systems – stars: individual (CoRoT- Exo-4a)

1. Introduction CoRoT is a photometric space experiment devoted to asteroseis- mology and the search for extrasolar planets by the method of transits (Baglin et al. 2006). It has recently discovered CoRoT- Exo-4b, a Jupiter-sized planet transiting across the disc of an F-type main-sequence star with an orbital period of 9.202 days (Aigrain et al. 2008). This is the second longest period among the transiting planetary systems known to date, putting CoRoT- Exo-4b in a region of the mass-period parameter space that was previously empty (cf. Moutou et al. 2008). Moreover, the out- of-transit light curve shows a modulation with an amplitude of a few 0.001 mag that can be attributed to photospheric brightness ⋆Based on observations obtained with CoRoT, a space project op- erated by the French Space Agency, CNES, with partecipation of the Science Programme of ESA, ESTEC/RSSD, Austria, Belgium, Brazil, Germany, and Spain. inhomogeneities carried into and out of view by the rotation of the star. Given its late spectral type, those inhomogeneities can be considered analogous to cool spots and bright faculae ob- served in the Sun, owing their existence to photospheric mag- netic fields. The active regions of CoRoT-Exo-4a are sufficiently stable to allow an estimate of its rotation period through an autocorre- lation analysis, yielding a period of 8.87 ± 1.12 days. This re- sult indicates that the stellar rotation and the orbital motion of the planet are quasi-synchronized, which

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