The Rotation Boundary parallels the Granulation Boundary and separates the rapid rotators on the hot side from the slow rotators on the cool side. Magnetic braking is almost certainly the cause of the slow rotation, along with changes in moment of inertia as stars evolve. As stars evolve from the main sequence across the Rotation Boundary, the loss of angular momentum in at least the convective envelope is very abrupt. On the cool side of the boundary, rotation depends only on position in the H-R diagram, a result thought to result from the control of rotation by the parameters of the convection through the Rotostat Mechanism. The Coronal Boundary maps the loss of x-ray emission which comes from the coronae of stars.
Rotation dies away from magnetic braking and the increase in moment of inertia.
The existence of the Coronal Boundary has been challenged recently. 411, Convection as a Regulator of Dynamos .