An Etymological Dictionary of Astronomy and Astrophysics
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An atom that has lost two of its external electrons, for example O⁺⁺ ([O III]).

→ doubly; → ionized.

Converted into ions.

P.p. of → ionize.

A gas composed partially or totally of → ions.

→ ionized; → gas.

Same as → H II region.

→ ionized; → hydrogen; → region.

A cloud of matter in the → interstellar medium consisting of → ionized gas, mainly → hydrogen, and → dust. Same as → H II region.

→ ionized; → nebula.

Not ionized, → neutral.

→ non-; → ionized.

The quality of an atom that has neither lost nor gained any electrons.

→ non-; → ionized.

Subject to, or produced by → photoionization.

→ photo-; → ionized.

An atom that has lost one electron and has become a positive ion.

→ single; → ionized; → atom.

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.