Photodissociation region

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.^[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 a column density to prevent the penetration of far-UV photons from distant, massive stars. 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 a giant molecular cloud.^[2] PDRs are also associated with HII regions, reflection nebulae, active galactic nuclei, and Planetary nebulae.^[7] All of a galaxy's atomic gas and most of its molecular gas is found in PDRs.^[8]

The image shows the 4 primary zones of a photodissociation region: the molecular zone, the dissociation front, the ionization front, and the fully ionized flow of gas.^[1]

The closest PDRs to the Sun are IC 59 and IC 63, near the bright Be star Gamma Cassiopeiae.^[9]

History

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.^[8]