Carbenes in the Interstellar Gas

Carbenes are a significant trace constituent of the gas in the interstellar medium (and in the expanding shell around at least one star), representing about one-sixth of the molecular species that have been identified in space. The identification of one of the most abundant and widespread interstellar carbenes, the cyclopropenylidene ring, C3H2, is described, together with recent laboratory work on the excited vibrational states of this molecule and on the geometrical structure of one of its isomers, the carbon chain carbene H2C3. A number of additional free carbines which might be detected in space are considered. There are at least two reasons why carbenes are comparatively conspicuous in astronomical sources relative to other reactive molecules: one is their high polarity; a second is their production via the same ion-molecule reactions that make known stable species in space (or very similar reactions). Finally, it is pointed out that cumulene carbon chains somewhat longer than those so far detected in space are promising candidates for the carriers of the interstellar optical diffuse bands.