Spot activity of the binary CG Cyg: Observations during maximum spottedness

Abstract

We present the results of new multicolor photometry of the chromospherically active binary CG Cyg acquired in 2005-2009 (136 hours of observations). The light curves for each season reveal rotational brightness modulation due to spots that varies in amplitude and phase from season to season. We have determined the longitudes of spotted areas: for each season, they were located on the primary, close to the line joining the centers of the components. The longitude distribution of spots was analyzed for 44 years of observations of CG Cyg using all data available in the literature. The active longitudes of CG Cyg are not fixed at the quadratures, as was believed earlier: most of the time (1965-2003), the spots were concentrated at two active longitudes at the quadratures, at orbital phases 0.28 ± 0.06 and 0.70 ± 0.08, but, during a shorter time after 2004, they were located along the line joining the component centers, at orbital phases 0.50 ± 0.04 and 0.93 ± 0.03 . We detected as witch of the active longitude by180 ◦ during 1.5 months in 2008, accompanied by an increase in the amplitude of the rotational brightness modulation and only a slight increase in the star's spotted area (by 5%). Our analysis of archive data reveals that switches of the active longitude during time intervals of 1-1.5 months were observed three times during the entire observational history of CG Cyg (in 1991, as well as in our observations of 2003 and 2008). All these switches were accompanied by similar phenomena: an increase in the amplitude of the rotational brightness modulation (by 0.06 m , 0.02 m ,a nd0.04 m ) and an increase in the spotted area (by 79%, 11%, and 5%). We used a zonal spot model to reconstruct the parameters of the spotted regions on CG Cyg. At all our observing epochs, the spots were located at low latitudes, in a region that was symmetric about the equator, 10 ◦ to 14 ◦ wide on either side. The spots are cooler than the surrounding photosphere by 2000 K. The spotted area varied only slightly from season to season, comprising 13%−15% of the surface area of the star, close to the historic spottedness maximum for the CG Cyg system.