The fate of the Hell Lya photons, produced during recombinations of the He++ ions and electrons, is important in considerations of the ionization and thermal structure of high excitation nebulae (Flower, 1968, IAU Symp. No. 34, Planetary Nebulae, p. 77). Hummer and Seaton (1964, M.N.R.A.S. 127, 217) effectively supposed that these photons are absorbed by H° in the central (He++) zone, whereas detailed line transfer calculations (Weymann and Williams, 1969, Ap. J. 157, 1201; Harrington, 1972, Ap. J. 176, 127) suggest that a large fraction of the photons escapes from the central zone. We consider the effects of the absorption of these photons on the ionization and thermal structure of the outer (He+, H+) zone of a nebula which is optically thick in the HI Lyman continuum. (Paper will appear in Astronomy and Astrophysics.)

Coudé échelle studies of the symmetrical planetary southern nebula NGC 3918 at a resolution of 20 km/sec have shown a high degree of stratification of velocity of expansion with ionisation. The greatest velocity of expansion, 28 km/sec is observed in the [Mgl] line at 4751 Å

The mean physical parameters were estimated for the planetary nebulae of the two groups, regarded as a whole: planetaries of a general Galactic field (GGF), and those, belonging to a large group seen in the Galactic centre direction (CG).

This review begins with a brief summary of atmospheric models that are of possible relevance to the central stars of planetary nebulae, and then discusses the extent to which these models accord with the observations of both nebulae and central stars. Particular attention is given to the significance of the very high Zanstra temperature implied by the nebular He II λ4686 Å line, and to the discrepancy between the Zanstra He II temperature and the considerably lower temperatures suggested by the appearance of the visual stellar spectrum for some of these objects.

Central stars of planetary nebulae have long presented a difficult problem for observers because most of them are fainter than tenth magnitude and because emission lines from the nebulosity interfere with spectroscopy of stellar features. With the advent of numerous large telescopes and space telescopes it is possible to obtain detailed observations of many central stars. More observing time devoted to central stars has resulted in accurate observations of emission and absorption features in their optical spectra and in the discovery of binary systems among them. Satellites have made possible observations in the ultraviolet. As usual, recent studies have raised as many questions as they have answered. This review will concentrate on a few topics of current interest. The reader should consult Aller (1976) for a comprehensive review on central stars and for an excellent summary of magnitudes and spectral classifications of individual central stars.

The fundamental feature of the evolution of the central star of planetary nebulae (CPN) is the cooling of the core towards a white dwarf. The basic observations were made by O'Dell (1963), Seaton (1966), and Harman and Seaton (1964); namely, they discovered a correlation between the linear size of the nebula and the position of the CNP in the HR diagram. Typical values for the velocity of expansion are 10–30 km/sec and the largest size observed for a nebula is ∼0.6 pc and hence the lifetime is 2–6 × 104 years. From a theoretical point of view the important fact about the Harman-Seaton sequence (HS) is the implication on the evolution of the central star. The star first heats up, L and Te increase, then L decreases. But Te keeps increasing until finally Te decreases also (shown schematically in Figure 2). Thus the HS is an evolutionary sequence and not a locus of stars.

Young central stars of planetary nebulae burn hydrogen and helium in the shell sources. Within less than 104 years nuclear fuel is exhausted and old central stars cool off at almost constant radius to the white dwarf stage.

Young, i.e., luminous central stars should follow complicated loops on the H-R diagram as a result of helium shell flashes. FG Sagittae is the example. This should be a typical behaviour and a systematic search for light variations on a time scale of years or decades among other luminous entral stars should be undertaken. Because of rapid evolution of their nuclei many planetary nebulae may be far from the ionization equilibrium. This should be taken into account when the effective temperature and bolometric luminosity of the central stars are estimated.

A faint companion to HD 87892 (V=10.07, B-V=+0.09, U-B=+0.10, Sp.A2) has been discovered with the 3.6 m telescope (prime focus) of the European Southern Observatory at La Silla having the following parameters: separation ψ = 1″.65, position angle θ = 226°.3. The companion was detected on plates taken in the U system (IIIa-J baked + UG1) and appears to be about ΔmU ≃ 4.5 mag fainter than the main component; it is almost invisible in the nearly visual system having Δmvis ≃ 4.8 mag. Contrary to the eccentric position of HD 87892 the companion lies in the axis of symmetry of the nebula. For the given reason the companion can be considered as the actual planetary nucleus. Assuming that its temperature is approximately T* ≈ 105 °K and the distance of the nebula d ≃ 0.9 kpc, the following parameters of the nucleus were estimated: L*/L ≃ 110, R*/R ≃ 0.035. The physical association of HD 87892 with its visual companion is probable. (Paper will appear in Astronomy and Astrophysics.)

An interesting new phenomenon was encountered while evolving a star with a core mass, Mc of 0.8 M0 and with a small envelope mass (0.015 M0) away from the red giant branch towards the nuclei of planetary nebulae, while taking the helium shell flashes into account. It was found that the top of the intershell carbon pocket (the carbon-enriched region in between the hydrogen- and helium-burning shells left behind by the flash) was expanded outwards and cooled immensely; namely, cooled to near 20,000°K! This means that the intershell carbon pocket was lifted out to near the photosphere, right into the shallow outer convective envelope surrounding the hydrogen- and helium-ionization zones! The carbon opacity at these cool temperatures is great. It seems likely that all the layers from the outer regions of the intershell carbon pocket right up to the surface will become convective. This would be a totally new type of deep convective envelope with a vast number of fascinating implications. Careful checks of this new phenomenon are now underway. (Supported in part by the National Aeronautics and Space Administration [NSG 7195].)

Evolution of an electron-degenerate carbon-oxygen star of 1.08 M0 was computed assuming that hydrogen-rich gas was accreted at 1.58 × 10−7 M0y−1. Such a star mimicks the evolution of FG Sge, if a part of the ejected mass accretes back onto the central star. When a hydrogen envelope of 4 × 10−6 M0 was formed, a hydrogen-shell flash began. Succeeding 300 hydrogen-shell flashes were suppressed artificially to compute the growth of helium zone. After 8100 years of accretion, helium-shell flash began. Helium convection zone was found to reach the bottom of the hydrogen envelope, because the entropy barrier was low for the thin envelope. Protons were mixed into the helium zone, which produced neutrons and s-process elements. The mixing triggered also the hydrogen-she11 flash. Hydrogen convection zone reached a mass shell of 10−8 M0 measured from the surface. After the helium-shell flash ceased, the hydrogen envelope expanded greatly and a deepening surface convection dredged up the s-process elements. (Paper to be submitted to Publ. Astron. Soc. Japan.)

In view of the spectral similarity and the probable evolutionary relation between certain planetary nebulae (PN) and other objects that are generally binaries (symbiotic stars, Be stars, or slow novae), it is probable that many nuclei of PN are double.

The study of several planetary nebulae, suspected earlier of variability, was continued during the last years at the Sternberg Astronomical Institute.

Ultraviolet photometric observations of IC 418 taken with the Netherlands Astronomical Satellite (ANS) indicate variability in the central star brightness. The variations occurring on the time scales of 5 hours are only 4% at 3300 Å but increase to about 15% at 1550 Å. The observations were not made frequently enough to determine a periodicity, if one exists. There are also 5–10% changes in brightness observed in A78 and NGC 6543, these are probably due to spectral changes in the central star spectra. About 10% type variations were found in VV 1–7 and He 2–131 but their interpretation is not clear.

With the new measurements of the central star between 1500 Å and 3300 Å, practically the entire continuous emission has been measured or can be deduced if the nebula is optically thick for hydrogen and helium ionizing radiation. The effective temperature can then be found directly, if the ratio of the radius to the distance of the star is known. This latter quantity can be determined with different assumptions: that the brightness temperature in the visual is equal to the effective temperature is a reasonable assumption, with the advantage that even if it is not quite correct, the resultant effective temperature is only affected in a small way.

The determination of total luminosity and effective temperature recently reported on sometimes (but not always) differ in a substantial way from the determination of these quantities from Hell Zanstra method. For example, differences in luminosity exceeding a factor of 10 have been noted. The resultant effect on the Hertzsprung-Russell diagram is presented and discussed. (Paper will appear in Astronomy and Astrophysics.)

Nuclei of planetary nebulae are suspected to go through thermal pulses. A time scale of such pulses is of the order of 10–103 years and so it is comparable with the time of recombination in a typical planetary nebula. Theoretical models have been constructed to study evolution of ionization structure of nebulae in which the spectrum of ionizing radiation varies with time. Resulting intensities of emission lines are compared with those produced by stationary model nebulae.

Results are presented of a spectroscopic survey of sixteen peculiar central stars (i.e., central stars which have absorption spectra and continuua which are too cool to account for the superimposed emission line spectrum). Classifications on the MK system are presented for the absorption spectra. Intensities relative to H are presented for some of the strongest emission lines. The relationships between peculiar central stars and other types of emission line objects such as Be stars, symbiotic stars and P Cygni stars will be discussed. Three interpretations for peculiar central stars will be considered: 1) Some of them are binary nuclei of planetary nebulae, 2) Some of them are manifestations of the protoplanetary nebula phase, 3) Some of them are not related to planetary nebulae. (Paper will appear in Astronomy and Astrophysics.)

PN can be divided into four types depending on their chemical composition. In order of decreasing heavy element abundances the types are: I) He and N rich, II) intermediate population, III) high velocity, and IV) halo population. The type II PN are overabundant in N and C relative to the Orion Nebula. Well defined gradients across the galactic disk of He, N and O are derived from type II PN; the oxygen gradient is similar to the metallicity gradient derived from GK giants and F main sequence stars. By comparing the O, Ne and S abundances of PN of types III and IV with the Fe abundances of stars of similar population it is found that the O, Ne and S enrichment in the Galaxy probably took place before the Fe enrichment.

The determination of the chemical compositions of gaseous nebulae in general and of planetary nebulae in particular is a difficult undertaking. The zeroth-order approximation is straightforward, the first approximation is challenging, and the second approximation is almost intractable.

Various abundance problems have been well reviewed by the two previous papers. The study of the compositions of planetary nebulae has been quite fruitful. Nebulae can be used to study elements which are difficult to study in stars. In particular, it has been shown that planetaries can contain matter which has undergone nuclear processing, which in turn can enhance the abundances of some elements, notably He and N. In addition, planetaries can be used to study composition gradients in the galaxy. Of particular relevance to these topics are papers by Peimbert and Torres-Peimbert (1971) (hereafter referred to as PTP) and Boeshaar (1975) on enhancement of N/O; by Kaler (1973) on He/H ratios; by Barker (1976) on He and N; by D'Odorico, Peimbert and Sabbadin (1976), Aller (1976) and Torres-Peimbert and Peimbert (1977) (hereafter referred to as TPP) on abundance gradients.