An Etymological Dictionary of Astronomy and Astrophysics

A neutral region at the boundary of a → molecular cloud created by the penetration of → far ultraviolet (FUV) radiation from associated stars. The FUV radiation (6 eV ≤ hν ≤ 13.6 eV) dissociates the molecules and heats the gas and dust. A warm, atomic → H I region is thus created and the chemistry and thermal balance of the region are determined by the penetrating FUV photons. The progressive absorption of FUV photons leads to the occurrence of transitions between atomic and molecular phases, such as H I/H₂ and C II/C I/CO transitions. By extension, any neutral region where the physics is controlled by FUV photons can be called a PDR, as it is the case for → diffuse interstellar clouds or the edge of → circumstellar disks.
The PDR concept was first studied by A. G. G. M. Tielens and D. Hollenbach (1985, ApJ 291, 722).

See also: → photodissociation + → region.