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In astrophysics, photodissociation regions (or photon-dominated regions, PDRs) are predominantly neutral regions of the interstellar medium in which far ultraviolet photons strongly influence the gas chemistry and act as the most important source of heat.[1] They occur in any region of interstellar gas that is dense and cold enough to remain neutral, but that has too low a column density to prevent the penetration of far-UV photons from distant, massive stars. A typical and well-studied example is the gas at the boundary of a giant molecular cloud.[1] PDRs are also associated with HII regions, reflection nebulae, active galactic nuclei, and Planetary nebulae.[2] All the atomic gas and most of the molecular gas in the galaxy is found in PDRs.[3]
The closest PDRs to the Sun are IC 59 and IC 63, near the bright Be star Gamma Cassiopeiae.[4]
The study of photodissociation regions began from early observations of the star-forming regions Orion A and M17 which showed neutral areas bright in infrared radiation lying outside ionised HII regions.[3]
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