This result suggests that the thick disk is ancient, the result of a minor merger with a companion. In some nebulae O II recombination lines give much larger oxygen abundances than the corresponding collisionally-excited lines. This is very puzzling and could indicate that either forbidden lines or recombination lines give seriously erroneous results. A new study of o II recombination lines in a larger sample of planetary nebulae reveals some intriguing correlations. The discrepancy between O II abundances and abundances is anticorrelated with nebular size and surface brightness, suggesting nebular evolution plays a key role.
Lines arising primarily from dielectronic recombination show the largest discrepancies.
The combined evidence suggests that the recombination lines may be enhanced by high-temperature dielectronic recombination in a hot bubble produced by the fast stellar wind from the central star. Several H II regions in the Local Group have been found to have surprisingly strong nebular He II emission, which requires 54 eV photons for ionization. Fast shocks can also produce He II emission. N44C is peculiar in that the only identified ionizing star is an O7 star, apparently too cool to produce the He II emission. HST and ground-based spectroscopy show that the nebular abundances are normal relative to LMC averages, revealing no evidence for an evolved vanished companion.