The ionization structure of planetary nebulae. III. NGC 7009.

Abstract

Barker has found (1980, 1982) that combined optical and ultraviolet observations in several different regions of one planetary nebula, NGC 6720, can be used both to estimate particularly reliable total elemental abundances and to learn more about the physical conditions in the nebula. It is important, however, that all the observations be made in the same regions of a nebula. The decision was made to conduct optical and ultraviolet observations with similar entrance apertures in a number of positions in several extended planetary nebulae. For the present investigation, NGC 7009 was chosen. In connection with these studies, optical estimates of the abundances of a number of elements in NGC 7009 have been confirmed by UV measurements of their higher ionization stages. This is particularly gratifying for N, because previous studies suggested that this element could not be adequately measured from optical observations alone.