La nébuleuse planétaire ayant le plus grand diamètre apparent est la nébuleuse Helix; elle se prête très bien, en raison de ses larges dimensions (15′ × 10′), aux observations avec les techniques interférentielles courantes: la photographie monochromatique en lumière Hα ou [NII] λ6584 et l'interférométrie Perot-Fabry. On peut ainsi obtenir non seulement la morphologie de cette nébuleuse, mais aussi le champ des vitesses radiales au moyen des anneaux d'interférence des mêmes raies (Courtès, 1951, 1952, 1960).

Thirty years ago Baade discovered in the Helix Nebula extremely thin radial filaments, but since then their existence has been neglected. Data presented below were obtained by studying a copy of a plate obtained in 1954 in the light of Hα with the 200-inch telescope. Our goal is to draw attention to these filaments, which are of great importance for the understanding of planetary nebulae in general.

The planetary nebula NGC 6543 has been described (Curtis, 1918) as “quite irregular, of helical form”. Its high surface brightness and its dimensions (16′′ × 22′′ of arc) make it an ideal object for observation with the multislit technique used by Wilson (1950) to study other planetaries. In fact, had NGC 6543 been accessible to the 100-inch Mount Wilson Coudé, undoubtedly it would have been included in Wilson's survey. The interest in NGC 6543 arises from the fact that no information is available regarding the motions in a planetary nebula with some sort of helical appearance. The obvious question that arises in this respect is whether the nebular material is actually arranged in space on an open helix, rather than in a flat spiral. The related problems of its lifetime and stability also may be elucidated by studying in detail the internal motions.

This research was undertaken with the idea of measuring as accurately as possible the internal-velocity distribution in planetary nebulae, in order to compare the observational measurements with hydrodynamical models of expanding nebulae. Much of the work was done in collaboration with J. S. Miller and D.W. Weedman. All the observational data were obtained photographically with the Coudé spectrograph of the 100-inch telescope at Mt. Wilson, using an image rotator, a 900 line/mm grating, and an F/5-2 camera, giving a dispersion of about 4 Å/mm in the blue and about 6 Å/mm in the red. The measured velocity resolution is approximately 5–6 km/sec. The data for five nebulae have been published (Osterbrock et al., 1966) while data for three more, NGC 2392, NGC 3242, and IC 418 are discussed here for the first time.

Je voudrai bien parler la langue classique de l'interférometrie, mais le temps nous est cher et je passerai à l'Anglais.

The gas-dynamical equations for conservation of mass, momentum and energy, the ionization equation, and the equation of transfer for the ionizing radiation have been solved in a manner which approximates conditions in planetary nebulae. The ionized and neutral gas are assumed to heat and cool according to a linearized rate proportional to

Many features observed in solar prominences, planetary nebulae and the terrestrial magnetosphere (e.g. the filamentary structure, suspended clouds and arcs or toroids hovering above the equator) lead to the conviction that magnetic fields play a significant role in these objects. The magnetic mechanism (Woyk, 1967) described briefly below may be able to explain the shapes of all known planetaries. The field intensity sufficient for supporting the mechanism described need not exceed about 10−6–10−4G.

That the characteristic, quasi-symmetrical structure of planetary nebulae may result from the presence of imbedded magnetic fields has undoubtedly occurred to many astronomers. Gurzadian (1962), for example, employed the widely used equation where p is the pressure of the ionized gas and H the magnetic-field intensity. This equation specifies that the sum of the gas and magnetic pressures should be constant for a given value of the radius.

The linear response and high information gain of the electronographic process make it an important new technique for the observation of monochromatic intensity distributions in planetary nebulae. Preliminary results for NGC 6720, obtained with the U.S. Navy electronic camera and the 61-inch astrometric reflector, are presented.

The orientation of the major axes of planetary nebulae was investigated and the results are given in Figure 1. The solid line gives the distribution of 132 planetary nebulae as a function of the angle Δϕ between the major axis of the nebula and the galactic equator. It can be seen that the distribution is not uniform, and is not symmetrical about Δϕ = 0. The dashed line gives the distribution of the same nebulae as a function of the angle between major axis of the nebula and the direction of the interstellar magnetic field, which is determined from the stellar polarization measurements of stars close to the individual nebulae. This distribution is less uniform than the first one but is symmetrical about Δϕ = 0. These results lead to the conclusion that the orientation of planetary nebulae is partly determined by the interstellar magnetic field.

After a brief discussion of the determination of the effective temperature, the surface gravity and the chemical composition of the central stars, we describe the typical problems arising in the computation of static and dynamical models of the very hot atmospheres of these objects. Special attention is paid (1) to the strong non-greyness of the atmosphere, (2) the electron-scattering contribution to the source function, (3) the contribution of the higher ions of C, N, O and Ne to the absorption coefficient, (4) the importance of radiation-pressure effects. The application of the computed energy distribution to the determination of the ionization stratification of the nebula and to the calculation of the bolometric correction is discussed.

A formation of emission edges due to the Schuster effect does not seem to occur in any of the static non-grey model atmospheres which have been computed so far. The reason is that the steep temperature gradient in the uppermost layers of these models strongly favors the formation of absorption edges. Using very simple dynamical models by Schmid-Burgk (1967) we discuss the possibility of a considerable flattening of this temperature gradient by a hydrodynamic outflow driven by radiative acceleration. We argue that the Schuster effect may be much more important, if these atmospheres are not in hydrostatic equilibrium.

The position of central stars of different spectral type in the Teff-g plane is discussed.

The data, which we have at present for the planetary nebulae central stars, are in general insufficient and uncertain. Even the photographic magnitudes of the brightest nuclei are usually given in the known catalogues with accuracies of 2m or 3m. Therefore, the determination of central stars' magnitudes by the photoelectric method seems to be of great importance.

Currently at Harvard, we are engaged in a program for measuring multi-color magnitudes of the central stars of planetary nebulae. We plan to determine magnitudes of all favourable planetaries having central stars brighter than 16th photographic magnitude. The results, when completed, will give us improved information on (1) the amount of interstellar extinction for the individual objects, (2) the luminosities and colors of the central stars, (3) interstellar reddening corrections for spectrophotometric studies, and (4) the existence of variability among the central stars.

It is well known that the central star of the planetary nebula NGC 1514 (α50 = 4h06m.1, δ50 = +30°39'; lII= 165·5, bII = −15·3) differs from the other planetary nuclei by its high brightness (relative to the nebula) and late-type spectrum. The difference B∗ – Bn = 1m.9, and especially the A0-spectral type (Chopinet, 1963) are quite atypical for central stars. For these reasons we began a complex study of this object (Kohoutek, 1967; Kohoutek and Hekela, 1967) on the basis of the following sources of observational material:

1. (1) Palomar Sky-Survey prints (Schmidt-camera, Palomar Observatory);

2. (2) direct photographs in UBV system with the 2-m Schmidt-camera in Tautenburg (1962);

3. (3) photoelectric UBV photometry with the 65-cm reflector at Ondřejov (1964–65),

4. (4) (4) spectrograms of the central star using the 122-cm reflector in Asiago (1965), and relatively infrequent observational data from the older literature.

We would like to report on some spectroscopic observations made of the central star of the planetary nebula NGC 1514, which we think can support the binary hypothesis now advanced by Kohoutek.

I would like to make a few comments on the problem of the determination of the temperatures of nuclei of planetary nebulae.

It can be shown that in a high-excitation planetary nebula two independent Strömgren zones of heavy elements can be found. The first is a He III zone; the second corresponds to a zone of the luminescence of Ne V forbidden lines and originates due to combined absorption by the heavy ions Ne IV, O V and N V. If so, a relatively independent method of the determination of the color temperature of nuclei can be suggested.

At present there is some basis for questioning the usual presentation of the nuclei of planetary nebulae as being ordinary stars. If the quantity of mass released by a star during its formation or at some stage of its evolution serves as a criterion for its not being stationary, then the nuclei of planetary nebulae are hardly the most non-stationary objects. A strong deviation from Planck's radiation law may also serve as a criterion for non-stationarity when the deviation occurs in the far short-wave region of the spectrum in nuclei and similar objects (e.g. stars of Wolf-Rayet which are not nuclei of planetary nebulae). Finally, the existence of synchrotron radio radiation in some planetary nebulae must be explained by the exceptional activity of their nuclei. It would seem that the nuclei of these nebulae are suppliers if not of typical relativistic electrons then at least of electrons with high energies.

The central stars of planetary nebulae represent seemingly well-defined late stages of stellar evolution. Theoretical investigations with predictions of evolutionary tracks impose difficult requirements for observational data. Measurements of spectral energy distributions, of colors, and of magnitudes, and spectroscopic observations are all urgently needed.