1999ApJ...514..878H

NGC 6818 is a bright, high-excitation, fairly regular, planetary nebula with a rather rich spectrum. Its rather unusual kinematical structure, studied by Wilson, Weedman, and particularly by Sabbadin, is of special interest. We are here primarily concerned with its high-resolution spectrum as revealed by the Hamilton Echelle Spectrograph at Lick Observatory (resolution ∼0.2 Å) and supplemented by UV and IR data. These extensive data permit a determination of interstellar extinction, plasma diagnostics, and ionic concentrations.

Establishing the chemical composition requires finding the ionization correction factors that may be estimated empirically for only a few elements. In general we rely on a composite theoretical model. The chemical composition appears to be unremarkable. C and N appear to be more abundant than in the Sun. Most other elements seem to have roughly a solar abundance, except for those depleted by grain formation. NGC 6818 may have originated from a star resembling the Sun at least in chemical composition.

The central star is rather faint and possibly of the Wolf-Rayet type. Comparisons with model predictions suggest a L_{sstarf} of about 3,500 L_{sstarf} and T_{sstarf} of at least 140,000 K. Comparison with theoretical tracks suggests an age of about 9000 yr. Spectroscopic studies show that the bright shell appears to consist of tangentially moving filaments of differing excitation, a complication that may explain why theoretical nebular models have difficulties.