DENSE PHOTODISSOCIATION REGIONS (PDRs)

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Abstract
▪ Abstract All neutral atomic hydrogen gas and a large fraction of the molecular gas in the Milky Way Galaxy and external galaxies lie in PDRs, and PDRs are the origin of most of the nonstellar infrared (IR) and the millimeter CO emission from a galaxy. On the surfaces (Av < 1−3) of interstellar clouds, the absorption of far ultraviolet (FUV) photons (hν < 13.6 eV) by gas and dust grains leads to intense emission of [C II] 158 μm, [O I] 63, 146 μm, and H2 rovibrational transitions, as well as IR dust continuum and polycyclic aromatic hydrocarbon (PAH) emission features. Deeper in PDRs, CO rotational and [C I] 370, 609 μm lines originate. The transition of H to H2 and C+ to CO occurs within PDRs. Theoretical models compared with observations diagnose such physical parameters as the density and temperature structure, the elemental abundances, and the FUV radiation field in PDRs. Applications include clouds next to H II regions, reflection nebulae, planetary nebulae, red giant outflows, circumstellar gas aro...

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