A review is presented of charge exchange reactions of multiply charged ions with atomic hydrogen and helium at thermal-eV energies, typical of the physical conditions encountered in planetary nebulae. The basic features of the processes are analyzed in the framework of the molecular model of atomic collisions. A discussion is given of the different theoretical approaches to the calculation of the collision cross sections. A comparison with recent experimental data is included.

The role of resonances in the photoionization cross-sections of ions is discussed in relation to the radiative recombination coefficient. The calculation of dielectronic contributions to the recombination coefficient is reviewed for nebular conditions, and the importance of autoionizing states close to the first ionization threshold is illustrated. For those ions for which dielectronic recombination is not important at nebular temperatures, an assessment of the accuracy of the best available recombination coefficients is given.

In view of the enormous importance of the UV observations of planetary nebulae made possible by the IUE, this review will concentrate primarily on the formation of resonance lines in nebulae; an important special case is that of He II Lyα and its role in the Bowen mechanism. Special attention is given to the effects of dust on the line and continuum formation.

Computed models are now recognized as useful tools for interpretation of the spectra of planetary nebulae. However, even the most detailed models need geometrical parameters such as filling factors which are poorly determined by observations. Some effects may be seen more clearly by modeling the stratification than by just using total fluxes. A simple model for NGC 6720 is presented which reproduces the behavior of (Ne III) λ3869 observed by Hawley and Miller (1977), clearly showing the effects of charge transfer. The behavior of C II λ4267 remains puzzling. Finally, we comment on the interaction of high velocity stellar winds with nebular shells. Non-equilibrium particle distributions at the contact between the shocked stellar wind and the nebula may result in the rapid cooling of the shocked gas.

Numerical models have been constructed for twelve ionization bounded, medium to high excitation planetary nebulae. In most objects the excitation sensitive line ratio (O III) λ 500.9 nm / (O II) λ 372.7 nm is predicted to be too low as compared to observations. A similar systematic discrepancy is observed for [S III] λ 953.2 nm / (S II) λ 672.0 nm. We investigated the following effects on the ionization structure of the nebulae: 0++ + HO → 0+ + H+ charge exchange reaction, energy distribution of ionizing radiation and density distribution of gas in the nebular shell. The results show that density distribution is the most important factor determining the 0 III/O II and S III/S II line intensity ratios. While a factor of ten decrease in the charge exchange coefficient is required to explain the systematic discrepancy, a reduction of nebular radius by a few percent - truncated nebula (quasi density bounded model, but nebula still optically thick to Lyman photons) - suffices to produce the correct 0 III/O II ratio. Also, a density gradient of n ∼ r−1 to r−2 yields much better agreement with observations. Realistic variations in stellar spectrum hardly affects the 0 III/O II line intensity ratio.

Massive central stars (M > 1 Mo) of planetary nebulae burn nuclear fuel on a time scale of hundreds or tens of years which is shorter than the recombination time in a typical planetary nebula. Consequently the ionization and thermal structure of a nebula with such a nucleus is expected to be far from equilibrium conditions. The greatest chance of observing such a nebula is when the central star cools down to the white dwarf region. Time-dependent photoionization models suggest the following non-equilibrium effects to be expected at this stage. Firstly, the nebula shows a double shell structure, i.e. a bright, inner ring is surrounded by a faint, extended halo best seen in the HI lines and infrared lines from low-ionization species, such as (Ne II) 12.8 μ. Secondly, the low-excitation emission ((O II), (Ne II), (S III)) is enhanced relative to the high-excitation ((O III), (Ne III), (S III)). Thirdly, different modifications of the Zanstra method result in significantly different temperatures for the central star with a general rule that THI > THeII > THeII/HI The He II Zanstra method gives the most reliable result. Fourthly, the electron temperature derived from the (O III) lines is appreciably higher than that obtained from the (N II) lines. It is suggested that NGC 7027 and NGC 2440 possess massive central stars and that the above time-dependent effects are currently observed in these nebulae.

The general properties of PN of Type I are reviewed. A list of 29 PN of Type I is presented, most of them are bipolar. Their bipolar nature might be a direct consequence of the large masses and angular momenta of their progenitor stars. PN of Type I are He and N rich, their observed chemical abundances are compared with theoretical predictions. A group of Type I PN candidates is presented.

The intensities of the (S II) 6717, 31 Å and (S III) 9532 Å lines have been measured for the first time in the three known extreme halo planetaries. Preliminary estimates of the S/H ratios are: (4.7 ± 6.5) x 10−7 for 108 – 76o1, (1.4 ± 0.6) x 10−7 for K 648, and (2.0 ± 1.3) x 10−7 for 49 + 88o1. The S/H ratio in K 648 (the planetary in M 15) is 0.014 of that measured in the Ring Nebula (Barker, 1980, Ap. J., 240, 99), consistent with the Fe/H abundance of about 1/100 solar found (Cohen, 1978, Ap. J. 223, 487) in the stars in M 15. The average of the S/O ratios in the halo planetaries is an eighth the value in the Ring Nebula, similar to the low Ar/O ratios found previously (Barker, 1980, Ap. J. 237, 482). The implication is either that the abundances of lighter elements such as O have been enhanced by nuclear reactions in the planetary progenitors or that S and Ar were synthesized galactically at a slower rate than lighter elements. In either case, it appears that S and Ar may be more representative of the true heavy metal abundances in planetaries than O.

To date, we have been able to gather information on the abundances of 16 elements, ranging from helium to iron, relative of course to hydrogen, the seventeenth. Of the lightest 26 elements only the lithium, beryllium, boron trio, aluminum, and the quintet of metals from scandium to magnanese have not been treated. The results of decades of labor on galactic planetaries are presented as succinctly as possible in Table 1, where the elements are shown in order of atomic number. I will take as a general approach that He/H and O/H have readily recognizable gradients and variations, and that the other elements either generally vary in concert with oxygen, or are best studied with respect to that atom. Column (2) classifies the element according to its most prominent behavior. The well-studied ratios that are generally constant, for which a true mean can be derived, are designated “C.” Those that are probably constant, but which are not well studied, are noted as “c.” The four elements with abundances significantly under solar, which are probably depleted from the nebular gas by grain formation, are called “D.” The letters “G” and “E” denote those for which vertical galactic gradients and/or enrichment by the parent star have been clearly established.

Gas-phase abundances of C, Mg, Si, Ca, and Fe have been measured for a number of planetary nebulae on the basis of optical, ultraviolet, and infrared emission-line intensities. The abundances of Si, Ca, and Fe show characteristic depletions of one to two orders-of-magnitude as a result of grain formation. Magnesium shows a similar depletion in the outer parts of several planetary nebulae, but it is undepleted in their inner parts. Carbon is not detectably depleted by grain formation. Efficient condensation of refractory elements can easily occur during the early stages of formation of a planetary nebula; but the observed, residual gas-phase abundances are not understood. Observations of molecules in the envelopes of late-type stars may provide useful clues.

Several physical processes taking place during the red (super)giant phase of intermediate-mass stars have direct observational consequences for the subsequent nebular stage. These processes include: the regular wind and the envelope ejection, the thermal pulses during the AGB phase, the dredge-up processes, and the dust formation in expanding circumstellar envelopes. In this paper it is briefly discussed how such processes affect the mass range of PN nuclei and their evolution, and the PN lifetime, composition and dust content. The last section is devoted to a cursory discussion of PNe which can be generated by binary stars.

Two main observable phenomena are directly connected with the dynamical behaviour of Red Giant (R.G.) envelopes: The variability of Mira stars and Planetary Nebulae (P.N.) ejection.

Infra-red (JHK) photometry of long period variables (LPV) in the Magellanic Clouds has shown that the LPV's can be divided into core helium burning supergiants and asymptotic giant branch (AGB) stars. Application of the pulsation theory allows masses to be derived for the LPV's while stellar evolution theory allows core masses to be derived for the AGB stars. By considering evolution of the LPV's in the (Mbol,P) diagram, estimates of planetary nebula mass and planetary nebula nucleus mass are derived as a function of initial mass. Spectra of the LPV's suggest that many low mass planetary nebulae in the Magellanic Clouds should be carbon rich while the more massive nebulae may be nitrogen enhanced.

Rapid mass-loss is observed in many late-type stars, yet the mass-loss mechanisms operating are not well understood. A survey of molecular emission from circumstellar shells has been carried out using millimeterwave molecular lines and suggests that radiation pressure alone may be inadequate to explain the observed mass-loss, especially in the case of carbon-rich objects which may display rates in excess of 10−5 M⊙/yr. Recent near-IR molecular line observations provide evidence for ejected material at several different velocities along the line-of-sight and may indicate the additional mass-loss mechanism at work. Resonantly scattered IR radiation spatially displaced from the central IR continuum source has now been observed for the first time and sheds new light on the IR absorption-line results, providing information about material within 1016 cm of the central star. These results are discussed along with recent high-resolution millimeterwave observations.

Over the last decade, stellar mass loss has become recognized as an important factor in the evolution of stars. The magnitude of the mass loss rate is found to be greatest amongst high luminosity stars at both the red and blue sides of the HR diagram. Planetary nebulae (PN), which result from an evolutionary phase during which the parent star traverses from the red side to the blue in a relatively short time scale, are likely to be affected by these processes. In this talk, I shall review the mass loss processes relevant to the PN phase and discuss their affects on the formation and evolution of PN.

A planetary nebula consists mainly of gas ejected slowly by a red giant. Its dynamics is dominated by the hot central star which is left behind later. In particular a fast wind from this star forms a bubble of hot gas which fills the inner part of the nebula and pushes the envelope into a shell. This shell remains only partly ionized for a considerable time. Its non-ionized part is subject to a Rayleigh-Taylor instability, and is expected to break up into fragments which remain behind in the HII part of the nebula.

Numerical models of planetary nebulae (PN) dynamical evolution are calculated under the assumption of spherical symmetry and discussed in the light of infrared, radio and optical observations. The set of hydrodynamical equations is solved simultaneously with equations for nongrey radiative transfer.

The suggestion that the shells of planetary nebulae may be formed at the interface between two stellar winds of different velocity (“Not with a bang, but a whimper”) (Kwok, Purton and FitzGerald, 1978, Astrophys. J. 219, L 125) is investigated using the ‘beam scheme’ (Sanders and Prendergast, 1974, Astrophys. J. 188, 489) adapted to a system with spherical symmetry. Initial conditions include the remnant wind from the red giant phase and a highspeed wind from the hot nucleus ( (two tests), v = 103km s−1). The collision began at a radial distance of 200 AU. A third test at 50 AU indicated that the end result was insensitive to the details of the transition from one mass-loss mechanism to the other.

The symbiotic nature of this object has been investigated by optical spectroscopy and photometry in the near infrared region. There are three components, which are (i) the ionized expanding envelope consisting of two layers of low and high excitation species, (ii) the late type component corresponding to a blackbody of 2500 K and indicated by TiO absorption band, (iii) the hot remnant star whose temperature is estimated indirectly as 150 000 K or more.

The spectra of symbiotic stars generally display many emission lines seen in moderate to high-excitation planetary nebulae, but are superposed upon a strong continuum characteristic of a cool star, typically of type M. Furthermore, the spatial distribution of symbiotics has been noted to resemble that of planetaries. These similarities suggest that the symbiotic stars and planetary nebulae might have some relationship (causal or otherwise) or that they might arise from similar progenitors.