Rotation and magnetic activity go hand-in-hand in stars with convective envelopes: as single stars spin down, their magnetic activity diminishes. STARS will supply data on the internal and surface rotation rates and on stellar age, and this provides an empirical test of theoretical ideas on the coupling between the convective envelope and the radiative interior. Measuring radial rotational shear is crucial to test models of boundary-layer dynamos in which a shear layer between the interior and the envelope is assumed to drive the dynamo.
The F-type main-sequence stars form an interesting class of stars: their magnetic activity appears to be markedly suppressed so that magnetic braking is weak (Schrijver 1993). This is consistent with their rapid rotation in comparison with cooler dwarf stars. These stars are also very interesting since they should exhibit relatively strong oscillations, including perhaps g modes, which will permit a good determination of their internal structure and rotation.
Giant stars with masses between 1.5 and are of interest because magnetic braking begins only when they acquire a convective envelope during their evolution towards the giant branch (e.g. Rutten and Pylyser 1988). Heavier stars evolve fast, and the magnetic brake is therefore ineffectual (cf. Endal and Sofia, 1979). Less massive stars evolve slowly with strong magnetic braking, but core-envelope (near)synchronization may be enforced throughout the evolution, as in the present Sun. Between these extremes, however, there is a mass interval in which there may exist a strong radial rotational shear that STARS can detect.