Long-term starspot evolution, activity cycle and orbital period variation of SZ Piscium

Abstract

A sequence of V -band light curves of the active close binary SZ Piscium (F8 V-IV+ K1 IV), extending from 1957 to 1998, is presented and analysed to derive the spot distribution and evolution on the component stars. In our modelling approach the Roche geometry and Kurucz's atmospheric models were adopted. The resulting maps of the spot surface distribution were regularized by means of the Maximum Entropy and Tikhonov criteria to take full advantage of the increased geometrical resolution during eclipses. By comparing the maps obtained with these two criteria, it was possible to discriminate between surface features actually required by the data and artifacts introduced by the regularization process. Satisfactory ts were obtained assuming spots on both components and the unspotted V -band luminosity ratio: L1=L2 =0 :35 0:035. The derived yearly spot distributions indicate that the spot patterns consist of two components, one uniformly and the other non-uniformly distributed in longitude, the latter suggesting the presence of preferential longitudes. Starspots at latitudes higher than75 were not required to reproduce the photometric modulation. For the spot pattern on the less luminous F8 V primary component there are only some hints of the possible presence of spots around the substellar point facing the K1 IV secondary component. On the more luminous and larger secondary component there is clear evidence for the presence of three active longitudes. A stable active longitude around the substellar point facing the primary star appears to be quite compact with an extension of40 on several maps. The other two active longitudes display irregular changes of the spot area and location. The variation of the total spotted area on the secondary component shows sizeable fluctuations and a possible short-term cycle of about 13 yr, which is more clearly apparent in the variation of the spotted area unevenly distributed in longitude. A longer-term cycle of about 30 yr with a markedly non-sinusoidal variation might also be present. The spatial association between photospheric spots and chromospheric plages (as detected in the UV lines) appears signicant for the active region around the substellar point in the K1 IV component. A possible connection between magnetic activity and orbital period variation is suggested by comparing the variation of the total spotted area with the O C diagram of the primary eclipse times. The time span covered by the available data is, however, not long enough for us to draw any denite conclusions.