Emission-line spectra and the abundance of carbon and nitrogen ions in planetary nebulae

Relative C and N ion abundances in high-excitation planetary nebulae are evaluated from theoretical recombination spectra of the C III, N III, C Iv, N IV ions. Such nebulae are found to be overabundant in carbon, and the relative C IV abundance increases with electron temperature. The fact that lines representing normal transitions between terms with one excited electron are accompanied by transitions between terms with two excited electrons provides an opportunity to determine certain structural properties of planetaries and their interaction with their central star.