Context. Models have been made of stars of a given mass that produce planetary nebulae that usually begin on the AGB (although they may begin earlier) and run to the white dwarf stage. While these models cover the so-called dredge-up phases when nuclear reactions occur and the newly formed products are brought to the surface, it is important to compare the abundances predicted by the models with the abundances actually observed in PNe. Aims. The aim of the paper is to determine the abundances in a group of PNe with uniform morphological and kinematic properties. The PNe we discuss are circular with rather low-temperature central stars and are rather far from the galactic plane. We discuss the effect these abundances have on determining the evolution of the central stars of these PNe. Methods. The mid-infrared spectra of the planetary nebulae NGC 1535, NGC 6629, He2-108, and Tc1 (IC 1266) taken with the Spitzer Space Telescope are presented. These spectra were combined with the ultraviolet IUE spectra and with the spectra in the visual wavelength region to obtain complete, extinction-corrected spectra. The chemical composition of these nebulae is then found by directly calculating and adding individual ion abundances. For two of these PNe, we attempted to reproduce the observed spectrum by making a model nebula. This proved impossible for one of the nebulae and the reason for this is discussed. The resulting abundances are more accurate than earlier studies for several reasons, the most important is that inclusion of the far infrared spectra increases the number of observed ions and makes it possible to include the nebular temperature gradient in the abundance calculations. Results. The abundances of the above four PNe have been determined and compared to the abundances found in five other PNe with similar properties studied earlier. These abundances are further compared with values predicted by the models of Karakas (2003). From this comparison we conclude that the central stars of these PNe originally had a low mass, probably between 1 M and 2.5 M . A further comparison is made with the stellar evolution models on the HR diagram, from which we conclude that the core mass of these PNe is between 0.56 M and 0.63 M . Conclusions. A consistent picture of the evolution of this group of PNe is found that agrees with the predictions of the models concerning the present nebular abundances, the individual masses, and luminosities of these PNe. The distance of these PNe can be determined as well.

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.

Aims. We determine the chemical abundances and other parameters of the nebula NGC 6826 and its central star.Methods. We present new ISO spectra and combine them with archival IUE and optical spectra from the literature to get a complete, extinction-corrected, spectrum. The chemical composition of the nebula is then calculated in two ways, first by directly calculating and adding individual ion abundances, and second by building a model nebula that will reproduce the observed spectrum.Results. The results of these two methods are compared. In addition, we discuss the star exciting the nebula.

ISO spectra of the bipolar planetary nebula Mz 3 are used to determine the element abundances in the bright lobes of the nebula. The ISO spectra alone are sufficient to determine nitrogen, neon, argon, sulfur and iron abundances. These spectra are combined with spectra in the visual wavelength region (taken from the literature) to obtain an extinction corrected spectrum which is used to determine the abundance of oxygen and some other elements using a classical determination. We have tried abundance determination using photoionization modeling using cloudy, which is essential for determining the helium, silicon and chlorine abundances. It was found difficult to model the entire spectrum. New information about the central star could be determined. The abundances determined are found to differ somewhat from earlier results using only visual spectra. The reasons for this difference are discussed. An elevated helium abundance is found, agreeing with the determination of Smith 2003. Taken together with the high nitrogen abundance found, it is concluded that the exciting star of Mz 3 had a high progenitor mass. IntroductionMz 3 (Menzel 3) is an intrinsically bright young bipolar planetary nebula (PN 331.7-01.0). It extends for more than 50 along its major axis, although its two bright polar lobes extend about 12 in an almost north-south direction on either side of its bright unresolved nucleus. It has been well studied in recent years. The visual and infrared spectrum has been studied by Zhang & Liu (2002), by Smith (2003) and by Smith & Gehrz (2005). A radio interferometric study of the nebula at four wavelengths between 3.5 cm and 21 cm has been made by Bains et al. (2004); the 6 cm flux density they find, about 630 mJy indicates that the nebula is one of the strongest emission sources in the sky. It is therefore reasonably close although there is no reliable estimate of its distance.Narrow band HST images of the nebula have been studied by Guerrero et al. (2004) and Santander-Garcia et al. (2004). It is clearly a very complicated nebula. Besides the bright inner lobes, there are also cylindrical and conical lobes and an equatorial ellipse. These structures extend much further than the bright conical lobes and are at a much lower intensity level. None of these additional structures is seen in the radio maps of Bains et al. (2004). In this paper we concern ourselves only with the bright inner lobes and the central source.Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.The nebula is located close to the galactic plane, in a region of the sky with a rather high extinction. It is difficult to obtain an exact value of the extinction but in this direction it probably lies between 1 and 2 mag/kpc (see e.g. Lucke 1978). The extinction of the nebula is rather high: much of it is interstellar but some of it is local to the nebula. We shall discu...

Abstract. The planetary nebula IRAS 07027-7934 has a compact ionized core surrounded by a large cloud of matter believed to be of neutral gas and dust particles. A photoionization model of this ionized core is presented in this paper. The parameters T eff , R * , and distance of the central star along with the radius of the nebula are derived by photoionization modeling. The nebula is found to have a much lower electron temperature than was estimated earlier but the electron density is very high, in broad agreement with previous result. The abundances derived for the first time point to a C/O ratio of >5 for the nebula, and the internal extinction caused by the ionized core, E(B − V), is 2.