Ionization structure and composition of the planetary nebula NGC 7027

The emission-line spectrum of NGC 7027 is analyzed with the aid of detailed models based on photoionization equilibrium theory. The models explain the temperature and the fractional abundances of most ions but seriously underestimate the abundances of O+ and other ions in the H+→H0 transition zone. Multidensity geometries, modifications of the stellar continuum, and internal reddening of the ionizing radiation fail to resolve this difficulty. Dynamical processes, in particular evaporating globules, are a promising solution.Chemical abundances are derived for He, C, N, O, Ne, Ar, and Fe. A C/O ratio approx.6 times larger than solar is indicated by optical recombination lines and ultraviolet lines. N/O is more than twice the solar value. Fe/O is a factor 25 lower than solar; we show that condensation of iron into grains does not conflict with the absence of internal reddening in NGC 7027