Spot activity in the RS CVn binary DM Ursae Majoris

Abstract

We present extensive UBVRI photometry of the active RS CVn binary DM UMa obtained at two different observatories over 1988-2008. We find the light curve of the star to be highly variable. The long-term photometry shows no evidence of any cyclic spot activity. On one occasion, the amplitude of modulation in V band was 0.30 mag, a rather high value for an active star in a binary seen at a comparatively low orbital inclination. The mean V magnitude of DM UMa during 1993-94 was brighter than that over 1979-85 by about 0.35 mag. We interpret the monotonic increase in mean brightness from 1984 onwards as due to the steady disappearance of star-spots in the near-polar latitudes of the star. The V-band data clearly show evidence for the presence of two distinct regions of enhanced spot activity on the visible primary, which are fixed in the orbital frame of reference, one facing the companion star and the other away from it. It appears that the nearby companion star suppresses the natural tendency of the active star for differential rotation and modifies the physical processes leading to the formation of spots, besides spinning it up by locking its rotation with the orbital motion. The variations in (V - R) and (V - I) colours with the V magnitudes clearly indicate that star-spots about 800 K cooler than the unspotted photosphere are responsible for the rotational modulation of light in DM UMa. The spot-filling factors derived from TiO-band strengths, which are available in the literature, are found to show the expected anticorrelation with the V magnitudes of the star. Apparently, there is excess flux in U and B bands, which increases as the star becomes fainter, partly compensating for the reduction of flux in those bands due to spot activity. The excess flux, probably, originates from plages or facular regions associated with the spot activity, and indicates that the simple two-component spot model with spotted and unspotted photospheric regions is not adequate to represent the star-spot activity in DM UMa.