corotate

Corotate is a term used in astronomy, astrophysics, and related fields to describe a state in which an object or region shares the same angular velocity as another reference frame or pattern. When something corotates, it effectively rotates in step with the reference, so in a rotating frame tied to the reference, the corotating material appears stationary.

In orbital and tidal contexts, corotation often relates to synchronization between rotation and orbital motion. A familiar example is tidal locking, where a body's rotation period equals its orbital period around a companion, causing the same face to point toward the partner. Corotation can also refer to more general cases where rotation is synchronized with a specific pattern speed rather than with the orbital motion alone.

In disk dynamics, corotation is used to describe a special radius. The corotation radius is the distance from a central object where the local angular velocity of the disk equals the pattern speed of a perturbation (such as a planet’s gravitational influence or a rotating magnetic field). At r_cor, the corotation torque arises from the exchange of angular momentum between the disk and the perturbation, influencing processes such as planet migration and disk evolution.

In galactic dynamics, corotation denotes the radius at which the orbital angular velocity of stars in a disk matches the pattern speed of the spiral arm structure. Within corotation, stars orbit faster than the pattern; outside, they orbit more slowly. This concept helps describe resonant zones and the angular momentum exchange that shapes spiral structure.

Etymology: corotate comes from com- “together” and rotate, reflecting shared rotation.