International Ultraviolet Explorer satellite observations of seven high-excitation planetary nebulae

Abstract

Observations of seven high-excitation planetary nebulae secured with the International Ultraviolet Explorer (IUE) satellite were combined with extensive ground-based data to obtain electron densities, gas kinetic temperatures, and ionic concentrations. We then employed a network of theoretical model nebulae to estimate the factors by which observed ionic concentrations must be multiplied to obtain elemental abundances. Comparison with a large sample of nebulae for which extensive ground-based observations have been obtained shows nitrogen to be markedly enhanced in some of these objects. Possibly most, if not all, high-excitation nebulae evolve from stars that have higher masses than progenitors of nebulae of low-to-moderate excitation.

The chemical compositions of gaseous nebulae

“…The carbon abundance is particularly important because this element is manufactured in the triple-a process and is depleted in the CNO cycle, where it is mostly converted into N. There are basically two main procedures for finding N(C)/N(H): (1) Since the work of Aller and Menzel (1945), various permitted lines of C ions have been used to estimate the C abundance, most recently by Torres- Peimbert and Peimbert (1977), Kaler (1980a), French (1983), and Barker (1983, 1986. ( 2) With the International Ultraviolet Explorer (IUE), one can measure the ultraviolet, predominantly collisionally excited lines of C ill and C iv.…”

“…Fortunately, the [Ne iv] nebular-type doublet, X2423, can be measured with the IUE (Lutz and Seaton 1979). By combining such IUE measurements with those of the auroral-type transitions near X4723, which can be measured in the optical region, it is possible to estimate T e in the Ne +++ zone (Aller and Keyes 1980). High-dispersion UV measurements like those of Lutz and Seaton are required to resolve the X2422/X2426 doublet and thus enable one to solve for N e and T e simultaneously.…”