An Etymological Dictionary of Astronomy and Astrophysics
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A carbon atom → singly ionized by a photon of energy 11.3 eV. The ion C⁺ emits a → fine-structure line (²P_3/2→ ²P_1/2) at 157.7 μm when excitation conditions are satisfied (critical density ~ 3 x 10³ cm^-3). In → photodissociation regions, [C II] 157.7 μm is a major cooling line for regions exposed to significant → far ultraviolet (FUV) photon fluxes. In Galactic → H II regions, as well as in the central regions of external galaxies, the luminosity of the [C II] line is typically ~ 0.05-0.5% of the FUV luminosity and correlates well with → carbon monoxide (CO) line intensities.

→ single; → ionized; → carbon.