A vigorous activity cycle mimicking a planetary system in HD 200466^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: elena.carolo@oapd.inaf.it
² Dipartimento di Astronomia – Universitá di Padova, Vicolo dell’Osservatorio 2, Padova, Italy
³ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain
⁴ Dipartimento di Fisica – Universitá di Padova, via Marzolo 8, Padova, Italy
⁵ McDonald Observatory, The University of Texas at Austin, Austin TX 78712, USA
⁶ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, Catania, Italy

Received: 21 November 2013
Accepted: 20 May 2014

Abstract

Stellar activity can be a source of radial velocity (RV) noise and can reproduce periodic RV variations similar to those produced by an exoplanet. We present the vigorous activity cycle in the primary of the visual binary HD 200466, a system made of two almost identical solar-type stars with an apparent separation of 4.6 arcsec at a distance of 44 ± 2 pc. High precision RV over more than a decade, adaptive optics (AO) images, and abundances have been obtained for both components. A linear trend in the RV is found for the secondary. We assumed that it is due to the binary orbit and once coupled with the astrometric data, it strongly constrains the orbital solution of the binary at high eccentricities (e ~ 0.85) and quite small periastron of ~21 AU. If this orbital motion is subtracted from the primary radial velocity curve, a highly significant (false alarm probability <0.1%) period of about 1300 d is obtained, suggesting in a first analysis the presence of a giant planet, but it turned out to be due to the stellar activity cycle. Since our spectra do not include the Ca II resonance lines, we measured a chromospheric activity indicator based on the H_α line to study the correlation between activity cycles and long-term activity variations. While the bisector analysis of the line profile does not show a clear indication of activity, the correlation between the H_α line indicator and the RV measurements identify the presence of a strong activity cycle.