H-K Project

Long-term monitoring of stellar rotation by the HK Project has revealed the presence of surface differential rotation. The Sun's surface doesn't rotate as a solid body - different latitudes rotate at different angular velocities. The mean latitude of active regions also varies in step with the sunspot cycle. This is easily seen in Maunder's ``Butterfly Diagran'' (at right) showing the latitudinal position of sunspots with time.

Because we can't resolve the stellar disk, we can't measure the latitude of active regions. However, we can monitor the change in rotation period with time. The diagram below shows the change in rotation period for two stars with similar mass and age (and therefore rotation period). However, the two stars show completely opposite behavior. The star on the left, HD 160346, resembles the Sun in that rotation period decrases over the course of an activity cycle. The star on the right, HD 10476 (109 Piscium) has rotation period increasing over the cycle. At present no theory explains why two oetherwise similar stars have completely different behaviors. The difference could be that active region migrate equatorward for one and poleward for the other, or that HD 10476 has a poleward acceleration.