Spectroscopic observations of the planetary nebula NGC 6818

NGC 6818 is a bright, high-excitation, fairly regular, planetary nebula wit h 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 reveal ed by the Hamilton Echelle Spectrograph at Lick Observatory (resolution sim ilar to 0.2 Angstrom) and supplemented by UV and IR data. These extensive d ata permit a determination of interstellar extinction, plasma diagnostics, and ionic concentrations. Establishing the chemical composition requires finding the ionization corre ction 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 fo r those depleted by grain formation. NGC 6818 may have originated from a st ar resembling the Sun at least in chemical composition. The central star is rather faint and possibly of the Wolf-Rayet type. Compa risons with model predictions suggest a L-star of about 3,500 L-star and T- star of at least 140,000 K. Comparison with theoretical tracks suggests an age of about 9000 yr. Spectroscopic studies show that the bright shell appe ars to consist of tangentially moving filaments of differing excitation, a complication that may explain why theoretical nebular models have difficult ies.