A SELF-CONSISTENT PHOTOIONIZATION-DUST CONTINUUM-MOLECULAR LINE TRANSFER MODEL OF NGC-7027

A model to simulate the entire spectrum (1000 Angstrom to 1 cm) of the high-excitation young planetary nebula NGC 7027 is presented. The ion ized, dust, and molecular components of the object are modeled using g eometric parameters obtained from visible, radio, infrared, and CO dat a. The physical processes considered include recombination lines of H and He, collisional excited lines of metals, bf and ff continuum radia tions, two-photon radiation, dust continuum radiation, and molecular r otational and vibrational transitions. The dust component is assumed t o be heated by a combination of direct starlight and the line and cont inuum radiation from the ionized nebula. The molecular component of th e nebula is coupled to the dust component through the stimulated absor ption of the dust continuum radiation. Specifically, we compare the pr edicted fluxes of the CO rotational lines and the 179.5 mu m water rot ational line to those observed by the Infrared Space Observatory satel lite. From the observed ratio of these lines, we derived a gas phase w ater-to-H-2 abundance ratio of 3.2 x 10(-7) assuming that the water is uniformly mixed with the CO.