Spot activity on HD 89546 (FG UMa) from long-term photometry

We present the analysis of 20 years of time-series BV photometry of the SB1 RS CVn binary HD 89546. The system's yearly mean V brightness, the B - V color index, the photometric period, and the light curve amplitude all show clear cyclic variability with an approximate to 9-year time scale. We also find some evidence for brightness variability on a time scale longer than the 20-year time span of our observations, perhaps indicating a longer cycle analogous to the solar Gleissberg cycle. We estimate the unspotted V magnitude of HD 89546 to be 7.(m)154, which is approximate to 0.(m)2 brighter than the observed maximum brightness. Spot modelling of the system shows that spot temperature variations affect the observed B - V color as well as the V brightness. Two active longitudes are observed, centered around 180 degrees and 360 degrees longitude on the G9 III primary, each covering a longitude range of 120 degrees. Furthermore, two inactive longitude zones are seen spanning only 60 degrees between the two active longitudes. The longitudinal distribution of the spots exhibits no strong cyclic variability but does show rapid jumps of 120 degrees that look like the flip-flop phenomenon. We estimate the differential rotation coefficient of the star as k = 0.086 by considering the range of observed photometric period variations and assumed latitudinal spot variations over 45 degrees. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim