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

Abstract

The star CoRoT102781750 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. 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. 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 $P_{r}ot=8.8$ hours $(f_0=2.735$ c d${}^{-1})$ mean rotation period. The evolution of the active regions helped to derive a period change of $dP/dt=1.6\cdot 10^{-6}$ (18 s over the run) and a differential rotation of $\hat{I}\pm =\hat{I}”\hat{I}\text{c◯}/\hat{I}\text{c◯}=0.0074$. The $0\fm 015$ linear decrease and a local $0\fm 005$ 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 $f_1=7.172$ c d${}^{-1}$ pulsation connected to a $14\fd 83$ periodicity via an equidistant triplet structure. The best explanation for our observation is a ${\hat{I}}^2$ Cep star with a corotating dust disk.

12 pages, 10 figures, abstract truncated. (For better availability.)