The magnetic activity cycle of II Pegasi: results from twenty-five years of wide-band photometry

We present an analysis of a sequence of light curves of the RS CVn-type binary II Pegasi extending from 1974 to 1998. The distribution of the spotted area versus longitude is derived by Maximum Entropy and Tikhonov regularized maps, assuming a constant spot temperature (Lanza et al. 1998a). The spot pattern on the active K2 IV star can be subdivided into a component uniformly distributed in longitude and a second unevenly distributed component, which is responsible for the observed photometric modulation. The uniformly distributed component appears to be possibly modulated with an activity cycle of ∼ 13.5 yr. The unevenly distributed component is mainly concentrated around three major active longitudes. The spot activity appears practically permanent at one longitude, but the spot area changes with a cycle of ∼ 9.5 yr. On the contrary, the spot activity is discontinuous at the other two longitudes, and it switches back and forth between them with a cycle of ∼ 6.8 yr. However, before each switching is completed, a transition phase of ∼ 1.05 yr, during which both longitudes are active, occurs. After this transient phase, spot activity remains localized at one of the two longitudes for ∼ 4.7 yr untill another switching event occurs, which re-establishes spot activity at the other longitude. The longitude separation between the permanent and the switching active longitudes is closest during the switching phases and it varies along the ∼ 6.8 yr cycle. Different time scales characterize the activity at the permanent longitude and at the switching longitudes: a period of ∼ 9.5 yr is related to the activity cycle at the permanent longitude, and a period of ∼ 4.3 yr characterizes the spot life time at the switching longitudes in between switching events. The photometric period of the active star changes from season to season with a relative amplitude of 1.5% and a period of ∼ 4.7 yr. Such a variation of the photometric period may be likely associated with the phase shift of the light curves produced by the switching of spot activity from one active longitude to the other. The permanently active longitude shows a steady migration towards decreasing orbital phases, with an oscillating migration rate along the 9.5 yr cycle period and nearly in phase with the variation of its spotted area. The amplitude of the differential rotation derived from such a behaviour is of the order of ∼ 0.023%, about one order of magnitude smaller than estiSend offprint requests to: S.Messina ([email protected]) mated by Henry et al. (1995). The other two active longitudes migrates also towards decreasing orbital phase, but at a discontinuous rate. There appears to be no correlation between the location of the active longitudes with respect to the line joining the two components of the system and their activity level.