PHOTOSPHERIC ACTIVITY AMONG STARS IN OLD OPEN CLUSTERS: NGC 7789

We present initial results of the search for photospheric stellar activity in the open cluster NGC 7789 based on seven seasons of broadband V observations and one season of broadband BVR observations. We have achieved a precision as good as 0.9 mmag for the nightly average V differential magnitudes of the best stars and a precision of 0.4 mmag for the annual average V magnitudes. The brightness fluctuations of the photometrically chosen main-sequence stars show a distinct positive correlation between the variations in V and those in B and R on the night-to-night timescale; the amplitude of the fluctuations is also larger for the longer wavelengths. The V-band light curves of the annual mean differential magnitudes of the main-sequence stars are similar to both the fluctuations observed in nearby bright stars and to spacecraft solar irradiance data. The standard deviations of the variability increase with B - V color for the main-sequence stars. From these results, we conclude that the variations are caused by phenomena associated with stellar activity.