THE SPECTRUM OF THE PLANETARY-NEBULA NGC-6886

The relatively young, high-excitation planetary nebula (PN) NGC 6886 s hows a remarkable variety of lines from both high- and low-excitation stages, i.e., from N0 to N4+, and as a result it provides unique oppor tunities for detailed studies of physical processes, using a theoretic al model. By combining Hamilton echelle observations with UV data secu red with the International Ultraviolet Explorer (IUE) satellite, and w ith available IR data, we can obtain improved diagnostics and elementa l abundances which should supply insights into nucleogenesis processes and properties of the progenitor star. Improved theoretical nebular m odels are employed. The chemical composition of this high-excitation P N is found from ionic concentrations, and also from a theoretical mode l. Except for Ne, Si and cl, the agreement between the abundances calc ulated using the two methods is generally remarkably good. The C/H rat io is enhanced by about 15-20 per cent above the solar value, but the N/H and Ne/H, and probably Si/H ratios, are close to those of the Sun. The other 'metal'/H ratios and the He/H ratio seem to be lower than i n the Sun, and O is depleted. If pronounced T(epsilon) fluctuations ar e assumed, we can raise the abundances of O, S, Cl and Ar to approxima tely solar values, but then C, N and Ne are substantially enhanced. Si nce the distance is poorly determined, it is difficult to establish ev olutionary parameters.