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K.A. Janes, M.N. Hayes-Gehrke (Boston University)
White-light fluctuations resulting from photospheric activity (starspots) have long been observed in stars of some young clusters, such as the Hyades and Pleiades. Chromospheric activity is observed in older nearby (therefore bright) field stars. Observations with the SOHO satellite (Frolich, et al, Solar Physics, 170, 1, 1997) show that the Sun fluctuates in white light with an amplitude of a few hundred parts per million as sunspots and plages come and go. If these variations could be detected in open cluster stars of various ages, it would be possible to characterize directly the relationship between stellar age and mass (location on the main sequence) with the level of stellar activity. We have been exploring the prospects for detecting microvariability among stars in several old open clusters with broad-band CCD photometry. As the Frolich, et al results show on the Sun, the variations are larger in blue light than in the red. So by observing alternately in two or three colors, and by taking advantage of differential photometry among many cluster stars, we can detect variations at the level of a one or two millimagnitudes between observations separated by weeks or more. Although some stars show indications of variability at this level, some further improvement in precision is needed. We are exploring ways to achieve this goal.