Study of FK Comae Berenices

Abstract

Doppler imaging techniques have earlier been used to study the starspots and their evolution over a four year period in a single, late-type star FK Com. In the present work we publish new photometric observations of FK Com for the year 2001 and analyse them together with the previously published photometry obtained since 1966. These observations enable us to study the spot configuration on the stellar surface over much longer time period than the Doppler imaging alone permits, and so to look for possible activity cycles. The longitudinal spot configuration is recovered from the spot filling factor maps obtained with light curve inversion method. From the maps it is clear that the shape of the light curve is usually caused by one active region, which is often extended, and only occasionally by two regions. The spots tend to occur at two active longitudes which are 180° apart. These active longitudes are periodically active, i.e. the dominant part of the spot activity abruptly changes the longitude after about 3 years, indicating the flip-flop event. The full activity cycle is estimated to be 6.4 years. There is also clear evidence for migration of the active longitudes with at least three different rates. These rates correspond to the rotational periods of 2 d .40038′0 d .00009 (for the years 1979-1993), 2 d .4030′0 d .0003 (1994-1997) and 2 d .3960′0 d .0004 (1997-2001). These periods are confirmed by using a more traditional time series analysis. The different migration rates of the active longitudes can be explained by weak solar-type differential rotation.