The rotation of cool main-sequence stars

The monotonic decrease in rotation with advancing spectral type is one of the interesting characteristics discovered early in the study of stellar rotation. One of the objectives of the present investigation is to delineate the run of main sequence rotation with spectral type for cool stars. A second is to propose a conceptual parallel between the giant star changes and those seen for main sequence stars, and to use the parallel as a framework for understanding the run of lower main sequence rotation with spectral type. It is found that the idea of main sequence stars evolving through a phase of rapid rotational braking early in their lives leads immediately to an explanation of the observed decrease in main sequence rotation rates with advancing spectral type. There are at least two likely physical causes for the cessation of the rapid braking.