Abundances of Planetary Nebulae IC 418, IC 2165 and NGC 5882

Abstract

Abstract. The ISO and IUE spectra of the elliptical nebulae NGC 5882, IC 418 and IC 2165 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 nebulae is then calculated and compared to previous determinations. A discussion is given of: (1) the recombination line abundances; (2) the exciting stars of the nebulae; and (3) possible evolutionary effects. IntroductionIC 418, IC 2165 and NGC 5882 are morphologically quite similar; they are usually classified as elliptical in shape, and they are located rather far from the galactic plane, which is probably an indication that they have been formed from low mass stars. They are all reasonably small nebulae on the sky, i.e. most of their emission comes from a small diameter. IC 2165 has a circular shape with a diameter of 8 . It is surrounded by two haloes (Corradi et al. 2003) but these have very little emission compared to the nebula itself. IC 418 is slightly larger, elliptical in shape, with a size of about 14 × 12 . It too is surrounded by a low level ionized halo, which in turn is surrounded by a region of neutral hydrogen which has a diameter of about 2 (Taylor et al. 1989). Finally NGC 5882 is more or less circular with a diameter of about 13-14 ; it contains several distinctive clumps of ionized material. It is also surrounded by low level emission which extends about three times as far (Guerrero & Manchado 1999), but it is less well studied than the other two nebulae.It is possible to define an axis of symmetry for all of these nebulae.The purpose of this paper is to study the element abundances in these nebulae with the help of the mid-infrared ISO spectra. Abundances in all three nebulae have been studied for many years already, but the inclusion of the ISO spectra permits a more accurate determination. The reasons for this have The most important advantage is that the infrared lines originate from very low energy levels and thus give an abundance which is not sensitive to the temperature in the nebula, nor to possible temperature fluctuations. Furthermore, when a line originating from a high-lying energy level in the same ion is observed, it is possible to determine an effective temperature at which the lines in that particular ion are formed. When the effective temperature for many ions can be determined, it is possible to make a plot of effective temperature against ionization potential, which can be used to determine the effective temperature for ions for which only lines originating from a high energy level are observed. Use of an effective electron temperature takes into account the fact that ions are formed in different regions of the nebula. At the same time possible temperature fluctuations are taken into account.Use of the ISO spectra have further advantages. One of them is that the number of observed ions used in the abundance analysis is approximately doubled, which removes the need for using large "Ionization Correction Fact...

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