Abundances in planetary nebulae: Hb 5

Abstract

The ISO spectra of the bilobal planetary nebula Hb 5 are presented. These spectra are combined with the spectra in the visual wavelength region to obtain a complete, extinction corrected, spectrum. The chemical composition of the nebula is then calculated in several ways. First by directly calculating and adding individual ion abundances, assuming that all the ionic lines are formed in an ionized region surrounding the ionizing star. Secondly by building an "end-to-end model" nebula in which we have included a neutral region and a photodissociation region (PDR) beyond the ionized nebula. In this way we attempt to interpret the molecular hydrogen lines observed by ISO in a more self-consistent way. In the final analysis, the model is found to be basically heuristic, but gives new insights about the PDR and the PN. The implications of these are discussed.