A Relationship Between Mean Rotation Period in Lower Main - Sequence Starsand Its Observed Range

Chromospheric Ca II H and K uxes have been measured in a sample of 100 stars on or near the lower main sequence at Mount Wilson Observatory. Observations were made several times a week and span more than ten years. Within an observing season, many stars show periodic variations due to rotation. Thirty-six of the stars have highly-signiicant periods in at least ve seasons. We compute the range in the observed period, P, and suggest that it is a measure of, and a lower limit to, the surface diierential rotation (SDR). Several physical and selection eeects can aaect the measured P value. An analysis of the cumulative variance distribution at various time scales, however, demonstrates that Ca II variations due to active region growth and decay are of longer period and smaller amplitude than those due to rotation. We argue that other eeects (e.g., multiple active regions, latitude bands) are either small, or primarily act to reduce the measured P relative to its true value. Including results for the Sun, we nd that P depends on the mean seasonal rotation period hPi, such that P / hPi 1:30:1 , independent of mass. We brieey discuss this in the context of dynamo models, and other observations of surface diierential rotation and active region structure.