Planetary nebulae form one of the most important subsystems of the Galaxy. If we knew more about this subsystem, an important gap in our ideas about the galactic structure would be filled. Very briefly, we are facing the following situation:

Population I, which contributes to the total mass of the Galaxy by hardly more than 7 %, can be tracked almost over the entire Galaxy thanks to the radio observations of neutral hydrogen at 21 cm wavelength. Other data, for a wide solar neighbourhood, follow from observations of Cepheids and early-type stars.

The available photometric and spectroscopic data on the planetary nebulae in the Magellanic Clouds are discussed. Recent results regarding the distribution of the planetary nebulae in the Clouds and the kinematics of the Clouds are presented. Masses are determined for selected planetary nebulae in the Clouds, and the question about the most likely average mass is considered. The evolutionary tracks of the nuclei for two values of the average mass are compared with the actual positions in the HR diagram of the nuclei of the planetary nebulae in the Clouds.

A programme of spectroscopic observations of planetary nebulae in both Magellanic Clouds has been underway in Pretoria for some years, and some preliminary results for the LMC have been published (Feast, 1964b). More extensive observations are now available (altogether in both clouds 112 spectra of 39 objects), and these are being prepared for publication elsewhere. Below are summarized the main conclusions so far deduced.

As you know, the new and most complete catalogue of galactic planetary nebulae, compiled by Perek and Kohoutek, has recently been published. This catalogue will certainly be very useful for astronomers working on planetary nebulae.

Plates of planetary nebulae taken in October 1965 and June 1967 with the 100-inch and 60-inch telescopes on Mount Wilson are compared with photographs taken with the same instruments 40 or more years previously, to determine the amount of angular motion of material outward from the central stars. Densitometer tracings of the nebulae yield values of , the radial motion in seconds of arc per century for condensations (filaments, rings, etc.) as well as for the edges of the visible material. Preliminary data are presented for NGC 2392, 6818 and 7662. For NGC 2392 and NGC 7662 distances are derived.

A punched-card catalogue of planetary nebulae has been prepared, using data extracted from all existing catalogues. A computer program calculates distances and radii using the method of Shklovsky, in which all nebulae are assumed to have the same ionized mass, and allowance for interstellar extinction is made assuming a continuous galactic-dust distribution. The assumption made in Shklovsky's method, that the nebulae are optically thin, is considered to be satisfied if the calculated radii lie within a certain well-defined interval. The reddening constants obtained are in satisfactory statistical agreement with constants determined by other methods. The local density of planetary nebulae is in agreement with estimates of local white-dwarf densities.

An investigation of planetary nebulae has been underway at the Sternberg State Astronomical Institute in Moscow for several years. It was begun with the measurement of the emission-line intensities of planetary nebulae in an homogeneous system in absolute units. More than 300 long-exposure objective prism spectrograms were obtained with the 50-cm Maksutov telescope at the Crimean Station of the Institute and with the 70-cm meniscus telescope of the Abastumani Observatory. The dispersion of the spectrograms was 190 and 160 Å/mm at Hγ respectively.

The paper describes a photoelectric photometer especially designed for extended sources. Six bands selected by a concave grating spectrograph can be measured simultaneously. The sensitivity commutation of the electrometers is automatic and a 10' field can be observed during measurements.

Several dozen spectrograms of about 20 nebulae were obtained with the quartz spectrograph, placed at the Cassegrain focus of the 125-cm parabolic reflector of the Sternberg State Astronomical Institute Crimean Station. Nebulae with intense Balmer continuum were selected. Nearly half of them had not been studied before. Reductions were made for the nuclei spectra, for the differential atmospheric extinction and for the interstellar absorption. The A0 V stars were taken as absolute intensity standards. The work is not yet finished, but the results will be received in a short time.

La région de l'infrarouge a été fort peu étudiée jusqu'à maintenant en astrophysique. En ce qui concerne les nébuleuses planétaires les seuls travaux publiés concernent la région de l'infrarouge photographique jusqu'à 1.2 µ.

Une description qualitative des spectres d'une vingtaine de nébuleuses planétaires dans la région du proche infrarouge a été publiée en 1961 (Andrillat et Andrillat, 1961). Le spectrographe E' monté sur le télescope de 120 cm de l'Observatoire de Haute-Provence nous a permis d'entreprendre l'étude spectrophotométrique de quelquesunes de ces nébuleuses planétaires, ainsi que celle de la nébuleuse d'Orion. Ce spectrographe, pourvu d'un réseau par transmission de 300 traits au millimètre assure une dispersion pratiquement constante de 230 Å/mm dans le domaine de 5750 à 8000 Å. Les spectres obtenus sur plaques IN hypersensibilisées à l'ammoniaque, ont été calibrés photométriquement à l'aide d'un spectrographe à pénombre décrit par Barbier (1944).

Interferometric observations of profiles of He I λ 10830 emission lines in 11 planetary nebulae, and in selected regions of the Orion Nebula, are presented. In common with the Orion Nebula, the planetaries are shown to emit a P Cygni-like λ 10830 line, with the absorption component shifted toward the violet with respect to the laboratory wavelength in a frame of reference at rest in the centre of expansion of the gas. The emission components are shifted toward the red. In planetaries, the negative displacements of the absorption edges are, in general, approximately equal to the widths, β, of the emission components which, in turn, range from about 12 km/sec in IC 418 to about 28 km/sec in NGC 6210. The emission red-shift is about 0·7 β on the average, but individual shifts vary from 0·5 β in IC 2149 to 1·05 β in NGC 6826. The line widths and shifts tend to increase in nebulae with larger expansion velocities. In Orion, the absorption edges in the λ 10830 line coincide in velocity with those in the line He I λ 3888 observed against the spectra of the Trapezium stars. In planetaries, the absorption edges in the λ 10830 line appear qualitatively similar to those in the line He I λ 3888, but a coincidence in velocity could not be demonstrated.

The observed profiles indicate that the nebulae are expanding, or that they contain expanding globules or filaments. Some form of circumnebular absorbing zone may be indicated. However, it is suggested that frequency redistribution associated with resonance-like scattering in a homogeneous expanding medium might in principle (even in the absence of stratification) account for the shifted λ 10830 profiles. (See Hummer, D.G. and Rybicki, G.B. (1968), Astrophys. J. Letters (in press), for a further discussion of this point.)

No trace of He3 is evident from the profiles. Quantitative conclusions are uncertain without a model which reproduces most of the phenomena, but an upper limit of He3/He4 ≤ 0·05 or even 0·01 is suggested on the basis of conservative assumptions.

An infrared spectrometer has been constructed with a resolution of Δλ/λ≈0·02 for observations in the wavelength range from 2·8 to 15 µ using a gallium-doped germanium bolometer (Low, 1961) as the radiation-sensitive element. Observations of the planetary nebula NGC 7027 were made with this instrument in May and June 1967 at the Catalina Observing Station of the University of Arizona. It was discovered that there was a measurable continuum flux from this object in the wavelength range from 7·5–14 µ that compared in value with the strength of stellar radiation from α Lyr (A0) at about λ = 9 µ. These observations are described in detail elsewhere (Gillett et al., 1967).

Calculations show that high-excitation planetaries emit about the same total intensity of collisionally excited radiation in the unobserved ultraviolet part of the spectrum as in the visible. This ultraviolet radiation emanates from a central zone where the electron temperature is high due to absorption of He II Ly-α photons. Intensities of lines which one might expect to detect in the ultraviolet have been calculated using revised atomic data and a detailed model of the ionization and thermal structure of the central zone. Relative line intensities are presented for a wide range of excitation in the nebula. In particular, the absolute intensity of the C IV doublet at 1550 Å has been calculated for 4 selected planetaries.

Planetary nebulae are weak radio sources. The radio emission of planetary nebulae is primarily due to free-free transitions in the ionized hydrogen cloud. Very large radio telescopes and sensitive receivers are needed to detect the weak radio emission of planetary nebulae. During the last 3 years about 80 planetary nebulae have been detected at some one radio frequency, and more than 60% of these have been detected at more than one frequency. Most of the measurements have been made to measure the flux densities of the nebulae and determine their radio spectra. A few interferometric observations have also been made to determine the radio widths of planetary nebulae. In some cases the radio observations have been used to determine the electron temperatures and emission measures of planetary nebulae, and also to derive the interstellar extinction at Hβ.