Noncollisional excitation of low-lying states in gaseous nebulae

It is found that low-lying metastable levels in some species traditionally treated as being excited by electron collisions may also be significantly populated by recombination processes. This is predominantly due to a large effective dielectronic recombination coefficient to these levels as well as direct radiative recombinations to these levels. Low-excitation states of the various ionic species of C, N, and O are examined. By far the most dramatic change among the forbidden lines occurs for the [O II] lines. Implications for O and C abundance determinations in H II regions and planetary nebulae are discussed.