Surface Differential Rotation in a Sample of Cool Dwarf Stars

A study of rotation in lower main-sequence stars has been progressing at Mount Wilson Observatory since 1980. The rotation program is part of a long-term study of stellar activity cycles, akin to the 11-year sunspot cycle. This dissertation uses the available time series of Ca II H and K chromospheric data in an attempt to uncover information on surface differential rotation in a sample of stars (including the Sun) based upon frequent and reliable measurements of rotation. A method is developed to diagnose the potential contribution of active region growth and decay within an observing season. The existence of surface differential rotation is perceived and analyzed for 37 stars. For some stars, patterns of rotation versus level of mean activity are apparent, and are interpreted to indicate the existence of migrating dynamo waves. The evolution of surface differential rotation is suggested from results based on stars of different ages. Specific results include: (1) There is a power-law relationship between rotation and fractional differential rotation with exponent 3/4. (2) There appears to be an abrupt change in differential rotation morphology and dynamo number which occurs at P_rot ~ 10 days. One of these morphologies coincides with stars bordering the Vaughan -Preston Gap. (3) For fast-rotating stars, active regions appear to prefer high-latitude belts, as inferred from the shape of their seasonal time series. (4) Some slowly -rotating stars do show the same equatorward migration pattern over the activity cycle as the Sun. However, an equal number of slowly-rotating stars show poleward (anti-solar) migration. It remains to be seen whether this is caused by some physical difference between these stars or is time -dependent. A model for time series fitting is examined, and its limitations are discussed. The problem of determining the chromospheric contrast is outlined using high-resolution spectroscopy of the Ca II K line of the Sun and stars.