Carbon radio recombination line emission from dark clouds

The author reviews the observations of carbon and sulfur radio recombination line emission from CII regions that are associated with dark interstellar clouds. Such CII regions detectable by their radio recombination line radiation share several characteristics: they are all excited by an early B-type star that is embedded within or closely associated with a dense (n > approximately 104 cm-3) neutral cloud; the exciting star is identifiable both as a near infrared point source and as an extended source of far infrared thermal dust emission; the CII region coincides with a localized region of enhanced molecular line emission. The author summarizes the physical properties (temperature and density) of the CII regions as derived from the recombination line observations and comments on prospects for further research. (Auth.)