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A corotation resonance is a type of orbital resonance created by to a region of higher density within an astrophysical disk such as a galactic disk or circumstellar disk, affecting some nearby portions of the disk. The resonance affects the orbits of stars in disk galaxies. It also affects orbits in protoplanetary disks, with potential effects on planet formation and planetary migration.
Corotation resonances are an analog of the Lagrangian points L4 and L5 of solid bodies (planets or moons), consisting of a portion of the disk orbiting in such a resonance with any portion that has a higher-than-average density within the disk. The movement of high-density portion is not necessarily that of the orbit of its material: it is likely to be a wave of high density traveling faster or slower than that part of the disk's orbital speed. At some distance not far from this high-density portion, disk material orbits with cycles somewhat in the manner of the stable halo orbits around L4 or L5 (the distance to these resonances are not the same as distances to L4 and L5). The balance of inertia and gravitational forces causing this are termed the corotation torque.
These phenomena are used to describe the overall affects of high-density regions of spiral galaxies (e.g., bars and spiral arms) on the orbits of individual stars, and of those of protoplanetary disks (e.g., from spiral density waves) on planet formation.
A Lindblad resonance is another type of resonance with such high-density portions of a such a disk, roughly the analog of a 2:1 orbital resonance.