A Relationship between Mean Rotation Period in Lower Main-Sequence Stars and Its Observed Range

Abstract

Chromospheric Ca II H and K fluxes have been measured in a sample of ~100 stars on or near the 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-significant periods in at least five seasons. We compute the range in the observed period, Delta P, and suggest that it is a measure of, and a lower limit to, the surface differential rotation (SDR). Several physical and selection effects can affect the measured Delta 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 effects (e.g., multiple active regions, latitude bands) are either small, or primarily act to reduce the measured Delta P relative to its true value. Including results for the Sun, we find that Delta P depends on the mean seasonal rotation period , such that Delta P is proportional to to the power of 1.3 +/- 0.1, independent of mass. We briefly discuss this in the context of dynamo models, and other observations of surface differential rotation and active region structure.