The relative abundances of H, He, Ne, O, and N have been determined in thirty planetary nebulae of widely-differing kinematical properties. Helium abundances are found to be affected by incomplete ionization of He, an effect that is strongly correlated with the abundance of S+. The He and N abundances appear to be positively-correlated, suggesting that nebular material has been contaminated through mixing with CNO-processed material. There is some evidence for a radial galactic abundance gradient in He, N, and possibly O and Ne; abundances increase toward the galactic center. This gradient is evident only from the nebulae that appear to be moving in circular orbits. All nebulae (except the previously studied planetary in the galactic halo and the one in the globular cluster M 15) have nearly the same abundances of O and Ne as galactic HII regions and the sun, and there is little correlation between these abundances and the kinematical properties of the planetaries. Apparently either Ne and O are not representative of the true heavy metal abundance, or the vast majority of planetaries belong to a metal-rich population. The latter possibility seems more likely in view of recent observations of the S++ abundances of nineteen of the nebulae, which show them to have generally solar S abundances. (Paper will appear in The Astrophysical Journal.)

With the Lick Observatory 3m telescope and image dissector scanner we have obtained new spectrophotometric observations of three halo planetary nebulae; 49 + 88°1, 108 − 76°1, and K 648. Based on the observed line intensities we are able to compute temperatures, densities, and abundances of He, 0, N, and Ne. Our data show that the helium abundance in the halo planetaries is normal with respect to disk planetaries. Oxygen is consistently less abundant in the halo planetaries by factors of 4, 12, and 20. The ratio N/0 in 49 + 88°1 and K 648 is typical of disk planetaries while N/0 in 108 − 76°1 is an order of magnitude larger. This suggests to us the possibility that varying amounts of mixing of CNO processed material has taken place in the progenitor stars. We do not detect [S II] or [S III] emission in K 648 or 108 − 76°1. In 49 + 88°1 [S II] and [S III] are present but weak and we derive an S abundance which is roughly a factor of 10 smaller than for disk planetaries. K 648 has the lowest abundances of 0, N, and Ne of all these halo planetary nebulae.

Results are most complete for NGC 185–1, where the electron temperature is 17,900±1600°K. Compared to typical galactic planetary nebulae the abundance ratio in NGC 185–1 of helium/hydrogen (0.24±0.08) is approximately twice that normally found, the abundance ratio of oxygen/hydrogen (7.7±1.3)10−5 is approximately one-tenth that normally found, and the ratio of nitrogen/hydrogen (1.4 × 10−4, poorly known) is approximately equal to that normally found.

A satisfactory model of the formation of planetaries should explain the observations indicating that nitrogen and carbon in planetaries are overabundant by about one order of magnitude. These large abundances imply that substantial fractions of the galactic nitrogen and carbon originate from planetaries; one should then also investigate if the low value of the ratio C13/C12 measured on the Earth is compatible with an assumed dominant rate of the CNO cycle in the formation of planetaries.

The relatively strong emission lines belonging to the singly ionized aluminum, titanium and magnesium are discovered on the shortwave spectrogram of planetary nebula IC 2149 obtained with the help of space observatory “Orion-2”. These lines are: 2669 AIII, 3080 Till and 2800 MgVI. At the same time this is the first evidence on the presence of chemical elements of aluminum, titanium and even magnesium (if we do not have in view the early identification of one very faint line, 4571 Mgl, in the spectrum of NGC 7027) in the composition of the planetary nebulae. As a result, the number of chemical elements discovered in planetary nebulae until now reaches up to 19 (from 16).

Since the last IAU Symposium on Planetary Nebulae at Tatranska-Lomnica, a number of significant theoretical and observational advances have been made in the study of nebular dynamics. The following review emphasizes the evolution of nebulae after they have been ejected from the central star and have become optically thin to most radiation.

When we look at planetary nebulae, we are by definition looking at similar objects and yet, the pictures which we exhibit show many dissimilar features. In fact, individual planetary nebulae have names that are associated with their peculiar features. There are, for example, the Helix, the Dumbbell, the Ring and the Eskimo.

The question of dust in planetary nebulae dates back a decade to the first then unexpected discoveries of strong infrared (IR) excesses in planetary nebulae (Gillett et al. 1967; Woolf 1969; Gillett and Stein 1969, 1970; Neugebauer and Garmire 1970; Willner et al. 1972). It has been clear from the outset that a substantial fraction, if not most, of the nebular luminosity is somehow converted to IR radiation by the dust. Two important questions are whether, and to what extent, dust influences the ionization structure and chemical properties of the nebula.

In this review I shall concentrate on theoretical aspects of dust in planetary nebulae (PN), since Dr. Balick has reviewed observations in the preceding paper. I shall consider the questions, how much dust is there in PN? What is its composition? What effects does it have on the ionization structure? On the dynamics of the nebula?

Image-tube photographs taken through narrow-band interference filters have been used to obtain monochromatic surface brightnesses of the Ring Nebula (NGC 6720) in the strongest emission lines of H I, He I, He II, [N II], [O I], [O II], [O III], and [S II]. These data have been used to analyze the spatial distribution of the various ionized species within the nebula. The bulk of the observed emission is shown to arise from a network of neutral filaments whose inner surfaces are ionized by the incident stellar radiation. The filamentary network is embedded in a low-density, high-temperature medium which is the source of most of the observed high-excitation lines. The ionization structure of several prominent filaments is examined in detail and comparisons are made to recent ionization structure calculations.

New observations have been made of the [OIII] 5007Å ine profile from the central and outer regions of 6720 in order to study in detail the radial velocity structure of the nebula. This data was obtained using a Fabry-Perot Interferometer working at a resolution of 67,000. The line profile is clearly s p l i t in the centre indicating the presence of a gaseous envelope surrounding, and in radial expansion from, the central star at avelocity of 25 ±5 km/sec. Irregularities in the emission intensities indicate local patchiness in this shell.

There i s increasing evidence that multiple shell structures occur often in planetary nebulae and emission nebulae surrounding Oef and Wolf-Rayet Stars, yet the multiple shell structures in the two classes of objects are very different. In planetary nebulae they are defined by an internal structure filled by [OIII] 5007 emission, and an outer shell that is most noticeable in the light of singly-ionized species such as [Oil], [SII] or [Nil]. Planetary nebulae with multiple shell structure are well exemplified by NGC 6853 (The Dumbbell Nebula) and NGC7293 (The Helix Nebula). The most pronounced effect is [OI] condensations plus [NII], [OII], [SII] “shadows” and complementary [OIII] structure.

We have investigated the dynamical effects of radiation pressure occurring in evolved planetary nebulae, for a wide range of central star and nebula parameters. Detailed model atmosphere calculations of the incident stellar flux were used to determine the temperature, ionization structure, and radiative acceleration throughout the nebula. Our computations included the effects of dust, a provision for charge exchange reactions, and all bound-bound, bound-free, and free-free processes that contribute significantly to the total radiation pressure. We have estimated the effects of the radiative force on nebular condensations and, as an example, have applied the results to the problem of the numerous radial filaments in NGC 7293. This work was supported by the U.S. ERDA under contract No. W-7405-Eng-48.

From an analysis of infrared, optical, ultraviolet and radio data of a number of planetary nebulae, it is found that the dust mixed with the ionized gas mainly consists of relatively large, graphite-like grains.

My interest in the origin of planetary nebulae goes back to 1967 when I proposed that the envelopes of giants with degenerate carbon-helium cores were dynamically unstable and would be completely ejected to form a planetary nebula. Essentially the same proposal was made almost simultaneously by Lucy (1967) and Paczyński (1967). For some years after that I worked on the dynamical ejection with W. van der Reijden, but for the last few years my interests have been in other areas, principally solar physics and gravitation theory. What then can I contribute to a discussion on Planetary Nebulae? Not a detailed review of work in this area; for that, the reader is referred to review articles by Salpeter (1971), Osterbrock (1973) and Miller (1974).

Although rare, planetary nebulae have been extensively studied by astronomers. This affection is certainly due in part to the beauty of the gaseous nebulae but it is also due to the belief that many, probably most, stars of intermediate mass (1–4 M) become planetary nebulae at least once during their lifetimes. If the planetary nebula is an (almost) inevitable stage in stellar evolution, it is important to determine its evolutionary precursors and followers. The latter are likely to be the white dwarfs and the former are generally believed to be red giants. With the advent of infrared and radio techniques it now appears possible, for the first time, to specify the immediate progenitors of planetary nebulae.

This symposium is explicitly devoted to Planetary Nebulae which are associated to “dying” stars of moderate mass. All other bright nebulae in the Galaxy can be considered to be related to planetary nebulae in the sense that they are similar to planetary nebulae in many respects. However, in most cases, these “relatives”-nebulae correspond to astro-physical situations (as characterized by the evolutionary stage, chemical composition, physical parameters, etc.) which are completely different. A large variety of nebulae can be found in the sky. One goes from point-like objects like K3-50, which is a very compact HII region, or V1016 Cyg, possibly a proto-planetary nebula, through moderate size nebulae like the Crab Nebula (the famous supernova remnant, 5′ size) or the Orion Nebula (the famous HII region around the Trapezium stars, 3′ core), to large nebulae like the Trifid (HII region of 20′ size) or NGC 6888 (which is associated to a WN6 star, about 30′) and extremely large like the North America Nebula (NGC 7000, an emission-reflection nebula of about 2° size) and the Gum Nebula (probably a supernova remnant of approximately 36° diameter). We see that not only might they be different in size but also, and drastically, different in their physical nature. It is interesting to note, however, that as a rule nebulosities are found to accompany stars only in the early phases and in the very late phases of evolution. At this stage of the symposium we should have learned enough about nebulae related to old stars. Therefore, I have decided to talk about nebulae related to stars at the other end of evolution, that is classical HII regions. For convenience, I will use here the term “HII region” meaning implicitly an ionized nebula associated to, and ionized by, a young and massive star. By definition, these nebulae are interesting because they represent the first manifestation of new-born stars of high mass. In fact, in many cases one can detect an HII region (in the radio and/or in the infrared) well before being able to directly observe the exciting star.

We report some results of an extensive study of dynamically and/or pulsationally unstable stellar envelopes at the red giant stage.

According to a model presented elsewhere the progenitors of plane-taries are fully degenerate stars with hydrogen-rich outer shells and internal temperatures lower than those of observed white dwarfs. Apparently, these stars have gone through the white dwarf stage and evolved into invisible black dwarfs. This suggests that some among observed white dwarfs may be stars with hydrogen-rich envelopes that have still to go through the planetary stage. Tentatively, we identify these stars with light white dwarfs of DA spectra and consider possible tests of this tentative identification:

From a large body of data gathered on the 3.9 metre AAT and the 2 metre and 76 cm telescopes at Mount Stromlo including absolute photometry, quantitative spectrometry and studies of internal velocity fields, the Herbig-Haro objects are shown to be radiating shock fronts in the ISM. Physical conditions are found by comparison with theoretical models of plane-parallel shock fronts. Shock velocities in the range 30–150 km/sec are observed, and the densities in the interstellar clouds in which the objects are embedded found to be 80–400 cm". Extinction at Hg ranges up to 4.4 mag. Parameters are found to be consistent with the radiation pressure driven breakup of a dust cocoon around a newly formed B-type star.