An Etymological Dictionary of Astronomy and Astrophysics
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A type of → interstellar medium cloud in which → carbon (C) becomes almost completely molecular due to relatively high → extinction. The chemistry is qualitatively different from that of → diffuse molecular clouds, as the → electron abundance is very low (→ cosmic-ray ionization being the dominant source) and the reactive C is replaced by the very stable → carbon monoxide (CO). This regime is found only in → sightlines with A_V > 5-10 mag; not all such sightlines will contain dense cloud material and if dense cloud material is present it is likely to be surrounded by → translucent material. These clouds are typically → self-gravitating, and are most often observed by → infrared absorption and → millimeter wave emission methods. Their densities are typically at least 10⁴ cm^-3, and their → kinetic temperatures are typically on the order of 10-50 K in the quiescent regions. Most of the more than 140 currently known → interstellar molecules were found through observations of → microwave→ rotational transitions in such clouds, starting with the discovery of OH, followed by a host of other new detections such as CO, NH₃, H₂O, and H₂CO (Snow & McCall, 2006, ARA&A 44, 367).

→ dense; → molecular; → cloud.