Explain the abundance discrepancy between optical recombination and collisionally excited lines

Establish the physical mechanism responsible for the systematic excess of elemental abundances derived from optical recombination lines over those derived from collisionally excited lines in ionized nebulae.

Background

Nebular abundances can be estimated using optical recombination lines (ORLs) or collisionally excited lines (CELs). These two methods often yield discrepant results, conventionally quantified by the abundance discrepancy factor (ADF). NGC 7009 is not an extreme example, but its low-ionization structures still show ADF values between approximately 1.8 and 2.5 for oxygen.

The paper notes that several explanations have been proposed, including temperature fluctuations and chemically distinct gas components, but no explanation has been established as generally sufficient. Resolving this issue is important for obtaining reliable chemical abundances and understanding the physical structure of ionized nebulae.