Periodicity search as a tool for disentangling the contaminated colour light curve of CoRoT 102781750^⋆

¹ Konkoly Observatory of the Hungarian Academy of Sciences, Konkoly-Thege M. u 15-17, 1121 Budapest, Hungary
e-mail: paparo@konkoly.hu
² Observatoire de la Côte d’Azur, Université Nice Sophia-Antipolis, UMR 6525, Parc Valrose, 06108 Nice Cedex 02, France
³ Institute of Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁴ LESIA, Université Pierre et Marie Curie, Université Denis Diderot, Observatoire de Paris, 92195 Meudon Cedex, France

Received: 25 January 2011
Accepted: 10 May 2011

Abstract

Context. The CoRoT space mission (COnvection, ROtation and planetary Transits) launched in December 2006, aims at finding transiting exoplanets and investigating stellar oscillation in adjacent stellar fields, called exo- and seismofields, respectively. Besides the seismofields, CoRoT has a strong potential for seismological research on the exofields. Up to now, only a limited number of RR Lyrae stars have been classified among the CoRoT targets. Knowing the astrophysical importance of the RR Lyrae stars, we attempted to get useful information even from a contaminated light curve of a possible RR Lyrae pulsator.

Aims. The star CoRoT 102781750 reveals a puzzle, showing a very complex and altering variation in different “CoRoT colours”. We established without doubt that more than a single star was situated within the CoRoT mask. Using a search for periodicity as a tool, our aim is to disentangle the composite light curve and identify the type of sources behind the variability.

Methods. Both flux and magnitude light curves were used. Conversion was applied after a jump- and trend-filtering algorithm. We applied different types of period-finding techniques including MuFrAn and Period04.

Results. The amplitude and phase peculiarities obtained from the independent analysis of CoRoT r, g, and b colours and ground-based follow-up photometric observations ruled out the possibility of either a background monoperiodic or a Blazhko type RR Lyrae star being in the mask. The main target, an active star, shows at least two spotted areas that reveal a P_rot = 8.8 h (f₀ = 2.735 c d^-1) mean rotation period. The evolution of the active regions helped to derive a period change of dP/dt = 1.6    ×    10^-6 (18 s over the run) and a differential rotation of α = ΔΩ/Ω = 0.0074. The 0ṃ015 linear decrease and a local 0ṃ005 increase in the dominant period’s amplitude are interpreted as a decay of the old spotted region and an appearance of a new one, respectively. A star that is detected only in the CoRoT b domain shows a f₁ = 7.172 c d^-1 pulsation connected to a 14ḍ83 periodicity via an equidistant triplet structure. The best explanation for our observation is a β Cep star with a corotating dust disk.