Evolution of starspot regions in DM UMa

Abstract

B andV photometry of DM UMa obtained between January, 1980 and June, 1984 is presented. Analysis yields a mean photometric period 7d.478±0d.010, compared to the known oribital period of 7d.492±0d.009. Light curves obtained during any two seasons do not agree in any of the following: shape, amplitude, phases of the light maxima and minima, mean light level, or brightness at the light maxima and minima. From the change inB-V over the photometric period, we concludethat the hemisphere visible during the light minimum is cooler than that seen during light maximum. The mean colorB-V=1m.065±0m.002 is consistent with K1 III or K2 IV. Phases of light minima lie on two well-separated groups with different slopes; the corresponding periods are 7d.471±0d.002 and 7d.481±0d.001, in dicating that both migrate linearly towards decreasing orbital phase. In terms of the starspot model this indicates that two respective centers of activity were situated at different longitudes and latitudes on a differentially rotating star. From circumstantial evidence we infer that the dark region seen from 1979 onwards disintegrated sometime between the 1982 and 1983 observing seasons, leaving behind an area of relatively high surface brightness. We can put a lower limit of about four years on the lifetime of a center of activity.