Physical conditions in photodissociation regions from observations of carbon radio recombination lines and IR fine-structure lines of CII and OI

Observations of the C56 alpha line-the first carbon radio recombination line in the millimeter waveband-in the directions of the Orion Bar, NGC 2024, and W3 are reported. The results are analyzed together with data for the IR fine-structure lines of CII and OI, enabling unambiguous determination of the temperature and density in the photodissociation regions separating the HII regions and parent molecular clouds. (C) 2000 MAIK Nauka/Interperiodica.