Inastrophysics,photodissociation regions (orphoton-dominated regions,PDRs) are predominantly neutral regions of theinterstellar medium in which farultravioletphotons strongly influence the gas chemistry and act as the most important source of heat.[2] They constitute a sort of shell around sources of far-UV photons at a distance where the interstellar gas is dense enough, and the flux from the photon source is no longer strong enough, to strip electrons from the neutral constituent atoms.[3] Despite being composed of denser gas, PDRs still have too low acolumn density to prevent the penetration of far-UV photons from distant, massivestars. PDRs are also composed of a cold molecular zone that has the potential for star formation.[4] They achieve this cooling by far-infrared fine line emissions of neutral oxygen and ionized carbon.[5] It is theorized that PDRs are able to maintain their shape by trapped magnetic fields originating from the far-UV source.[6] A typical and well-studied example is the gas at the boundary of agiant molecular cloud.[2] PDRs are also associated withHII regions,reflection nebulae,active galactic nuclei, andPlanetary nebulae.[7] All of a galaxy'satomic gas and most of itsmolecular gas is found in PDRs.[8]
The closest PDRs to the Sun areIC 59 andIC 63, near the brightBe starGamma Cassiopeiae.[9]
The study of photodissociation regions began from early observations of thestar-forming regionsOrion A andM17 which showed neutral areas bright ininfrared radiation lying outside ionisedHII regions.[8]
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