Abstract
Three decades ago, Peimbert found that in H II regions and planetary nebulae (PNe) Balmer jump temperatures are systematically lower than [O III] forbidden line temperatures. He suggested that because there are large temperature fluctuations in nebulae, as a consequence, heavy element abundances derived from collisionally excited lines (CELs) have been systematically underestimated. In this article, I review the recent abundance determinations for PNe using faint heavy element optical recombination lines (ORLs). For all the ionic species studied, ORLs yield higher abundances than CELs. The ratio of ORL to CEL abundances varies from object to object and correlates with the difference between the [O III] and Balmer jump temperatures. Temperature and/or density fluctuations alone are found to be insufficient to explain the very large discrepancies between the ORL and CEL abundances observed in a small number of nebulae. It is suggested that these extreme nebulae may contain metal-rich inclusions similar to those observed in ``born-again'' PN such as Abell30.
