An Etymological Dictionary of Astronomy and Astrophysics
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The → fine structure → emission line of → carbon at 157.74 μm, with transition ²P_3/2-²P_1/2 (rest frame frequency 1900.54 GHz). [C II] is a major → cooling line in → molecular clouds and is produced in low density → photodissociation regions (PDRs) (n_H≤ 10³ cm^-3). In higher density PDRs the [O I] 63 μm line becomes an important → coolant. The [C II] line traces → neutral gas exposed to → ultraviolet  → photons from → young stars. The [C II] line is used to probe the stellar → radiation field and find out how it affects the physical conditions of the gas. The bulk of the [C II] emission line is believed to originate from PDRs, and the remainder from → X-ray Dissociation regions (XDRs), and, in a lesser extent, by cosmic-ray-dominated regions (CRDRs), → H II regions, low-density warm gas and/or diffuse H I clouds.

→ carbon; → line.