An Etymological Dictionary of Astronomy and Astrophysics

A type of → interstellar medium cloud where → carbon (C), in → ionized atomic form and protected from → interstellar radiation, transforms into neutral atomic or molecular form. The chemistry in this regime is qualitatively different than in the → diffuse molecular clouds, both because of the decreasing electron fraction and because of the abundance of the highly reactive C atoms.

The translucent cloud regime is the least well understood of all the cloud types. This is partly because of a relative lack of observational data, but also because theoretical models do not all agree on the chemical behavior in this transition region. In some models, there is a zone where the abundance of C exceeds that of C⁺ and CO; in others the peak abundance of C falls below that of C+ and CO. To cope with this uncertainty, Snow & McCall (2006) propose a working definition of translucent cloud material as gas with C⁺ fraction < 0.5 and CO fraction < 0.9. This definition reflects the fact that C+ is no longer the dominant form of carbon as it converts to neutral or molecular form, but also excludes the → dense molecular clouds, where carbon is almost exclusively CO (Snow & McCall, 2006, ARA&A 44, 367).

See also: → translucent; → cloud.