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A Lindblad 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 portions of the disk inward or outward from the high-density portion. The resonance affects the orbits of stars in disk galaxies. It also as affects orbits within protoplanetary disks, potentially contributing to planet formation and planetary migration. The resonance is named for Swedish astronomer Bertil Lindblad, who uncovered such resonances modeling spiral galaxies.
Lindblad resonances are an analog of 2:1 orbital resonances 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 the 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 radius beyond that of this high-density portion, the natural orbital speed leads to an orbital period twice the time it takes the wave of high density to complete a circuit (an outer Lindblad resonance or OLR) and similarly, at a radius within that of the high density region, the natural orbital speed produces an orbital period of half the period in which it takes the high density to complete a circuit (an inner Lindblad resonance or ILR). The close encounters with the high density region affect the balance of inertia and the gravitational forces on the affected region, giving it a less-circular orbit and the resonance is at a radius where such affects on an orbit cycle such that the orbit repeats identically. The balance of inertia and gravitational forces causing this are termed a Lindblad 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 corotation resonance is another resonance effect of such high-density portions of a disk, roughly the analog of L4 and L5 Lagrangian points.