Radio Recombination Lines

At the time of the last review of radio recombination line research in these pages (Dupree & Goldberg 1970), the subject was still very much in its infancy and was concentrated almost exclusively on the observation and interpretation of lines from the classic bright radio H II regions, e.g. Orion A and M 17. However, with continuing improvements in the sensitivity of telescopes and receivers, it has now become possible to use radio recombination lines to explore a much broader range of phenomena. Recent areas of investigation include the study of planetary nebulae (e.g. Terzian & Balick 1972, Churchwell, Terzian & Walmsley 1976), the nuclei of some radio galaxies (e.g. Shaver, Churchwell & Rots 1976, Seaquist & Bell 1977), and the distributed ionized gas in the galactic plane, which may not be associated with identifiably discrete sources (e.g. Gottesman & Gordon 1970, Mebold et al. 1976). Besides the study of the thermodynamic structure of ionized regions, recombination lines have been used for purposes as diverse as the measurement of magnetic fields (Troland & Heiles 1977), abundance determinations in extragalactic nebulae (e.g. Huchtmeier & Churchwell 1974, McGee, Newton & Batchelor 1975), and kinematic studies of galactic structure (e.g. Mezger 1970, Lockman 1977). A new type of object, the C II region in isolated molecular clouds, has been discovered (Brown & Knapp 1974) and studied in lines of carbon (e.g. Pankonin & Walmsley 1976) and heavier elements (e.g. Chaisson 1975, Knapp et al. 1976b, Pankonin et al. 1977a). Moreover, the observations are no longer limited to single-dish measurements at centimetric wavelengths, but have now been made over virtually the entire radio spectrum, from 86 GHz (Waitman et al. 1973) to 242 MHz (Pedlar et al. 1977) using both single-dish and aperture synthesis techniques (Wellington et al. 1976)