Planetary nebulae constitute a popular field of investigation because they offer a unique opportunity to study the final stages of stellar evolution before a star dies. They supply challenges not only to the stellar evolution expert but also to the spectroscopist, to the student of galactic structure, and to all observers, whether one works in the optical, ultraviolet (UV), infrared (IR) or radio-frequency (r.f.) ranges. They offer insights into properties of dusty, low-density plasmas which may help unravel problems of the interstellar medium (ISM) and quasars. Finally, many are aesthetically beautiful art creations.

Since 1978 85 new objects have been classified as planetary nebulae. They are listed in Table 1 which gives the designations, names, coordinates and the references to the discovery. In the list of misclassified planetary nebulae (Table 2) 33 objects have been included. Principal criteria have been summarized which can help to distinguish planetaries from other objects. They refer to observable properties and are valid only in a statistical sense.

The application of monochromatic images and internal kinematic data to the construction of 3-dimensional models for the HII regions of planetary nebulae is discussed, and the role of models in investigating envelope evolution is commented upon.

Recent results obtained from studies of bipolar nebulae with a variety of techniques are described. Nebular polarization maps and spectropolarimetry, near-infrared spectroscopy, far-infrared photometry, radio maser and continuum work all have contributed to our knowledge of this heterogeneous class of objects. Some are certainly pre-main-sequence; others are likely to represent the rapid transition from red giant to planetary nebula. At least one dust-shrouded carbon star (CIT 6) and one visual binary with an O-star primary (MWC 349) have bipolar structure. Equatorial dusty disks must be common occurrences at different phases of stellar evolution.

NGC 2346 is a high excitation nebula which has a bipolar appearance and whose obvious central star is of too late a spectral type (A5) to account for the photo-ionization of the nebula. Spectral observations at high and low resolution in the visible, and low dispersion IUE observations in the UV are combined to explain the structure of the object. The radial velocity structure indicates a cylindrical geometry, or possibly two cavities, with the ionized material in the form of thin sheets. The variation of extinction has been investigated at many positions over the nebula; the obscuration is irregular although a ring of low extinction around the central star is apparent. The central star has previously been found to be a single-lined spectroscopic binary, and its zero velocity lies midway between the positive and negative radial velocity components of the ionized material. Visible spectrophotometry of the central star is combined with long and short wavelength IUE spectra in order to determine the parameters of the cool central star and the hot binary companion. The evolutionary status of this peculiar nebula is briefly discussed.

The nucleus of NGC 2346 (=PK 215 + 3°1; α = 7h06m49.S7, δ = −0°43′29″, 1950) was recently discovered by Kohoutek (1982a,b) to be an eclipsing binary with a deep (~ 2.m2) minimum in the visible and an orbital period of 17.2 ± 0.4 days. We observed NGC 2346 with the IUE on 4 occasions between 1982, Feb. 24 and May 13 and obtained 6 SWP and 4 LWR low dispersion spectrograms. These were taken at different phases, using the large entrance aperture centered on the nucleus.

The central star of the planetary nebula NGC 2346 is now well confirmed as a single-lined spectroscopic binary, with P = 16d (Méndez and Niemela 1981, Ap. J., 250, 240). Unexpected photometric variations were recently reported by L. Kohoutek (1982, IAU Circular 3667). From additional photoelectric measurements and visual estimates we have found that these variations are periodic, with the same period as the orbital motion of the A-type primary component. From previous observations we can ascertain that such variations did not exist before, and must have started in 1981. The light minimum occurs at phase 0.75, that is to say when the A-type component is moving towards us. Radial velocities measured on spectrograms obtained during the light minimum are more positive, by about 40 km s−1, than expected from the orbital motion; while the radial velocities corresponding to the light maximum agree with what is expected from the orbital motion. The spectral type of the A-type star does not change significantly as a function of brightness.

A discussion of radio observations for the period up to 1979 forms part of the general review given at the 1979 IAU General Assembly by Terzian (1980) and I shall be concerned here mainly with later results. With the advent of improved radio telescopes and techniques, in particular the coming into full operation of the Very Large Array (VLA), there has been an increasing interest in compact and proto-planetary nebulae and I shall discuss these observations in a separate section of this review.

The Very Large Array is a radio picture making instrument operated by the National Radio Astronomy Observatory in Socorro, New Mexico. The array, which has been in full time operation for more than 2 1/2 years, operates at four main wavelengths, 1.3, 2.0, 6.0 and 20 cm with achievable resolutions of .05, .08, .25 and 0.8 arc seconds respectively.

Advances in infrared spectroscopy have led to the detection of many forbidden fine-structure emission lines in planetary nebulae. Measurements of these lines offer sensitive probes of the physical conditions and ionization structure, and lead to improved abundance determinations. This paper reviews recent observations and discusses the ways in which infrared line measurements can contribute to our knowledge of planetary nebulae.

We have obtained 8-13 μm spectra of a sample of eight moderately extended planetary nebulae at a resolution of 0.22 μm using a 20″ circular aperture.

The study of molecules in planetary nebulae is still in its infancy; therefore, it is appropriate both to review existing knowledge and to anticipate developments that will arise as the subject matures. Briefly stated, this subject consists of observations of hydrogen, H2, and carbon monoxide, CO, molecules in a very few nebulae; the tentative identification of the hydrogen molecular ion, H2+; unsuccessful searches for various other species; some theoretical work on molecular processes relevant to planetary nebulae; and the study of molecules in possible precursors of planetary nebulae. Such a facile summary might suggest that molecules are mere curiosities of peripheral interest; on the contrary, it can be asserted that molecular studies of planetary nebulae will yield important new information about their origins, structures, and evolution.

We have observed CRL 618, an infrared object believed to be in transition between the red giant and planetary nebula stages, in seven lines of vibrationally excited H2 and the Brackett α and γ lines of ionized hydrogen. The H2 observations were made in 1979 and repeated in 1982, extending over more than four years monitoring of H2 emission from this source. The intensity of H2 emission approximately tripled between August 1977 and November 1979 and may have increased by another 15-20 per cent between November 1979 and February 1982. This increase in emission occurred during the same time period that the radio free-free flux roughly doubled (Kwok and Feldman, 1981). The object may now be in a more quiescent phase following a strong flare. The H2 lines appear to be in thermal equilibrium at ≈ 2000 K and to suffer only small extinction. The Brackett α and γ lines are more heavily obscured, with ≈ 2 mag. of 2.17 μm extinction. These results support the model that the compact H II region is behind and expanding into the dense molecular material where the H2 is found. The mass of hot H2 is, ≈ 4 × 10−4 M and of total ejecta ≈ .1 M⊙, so the mass-loss rate is probably > .

This review summarises those observations which relate to the existence of dust in planetary nebulae. The most direct evidence for dust, namely infrared emission by grains, is considered in Section 2. Section 3 describes a variety of optical and ultraviolet observations which bear more indirectly on the properties of the dust, while Section 4 discusses the power sources of the observed infrared emission. The peculiar nebulae A 30 and A 78 are considered in Section 5, and Section 6 concludes with a discussion of the various infrared features which have been observed in emission from planetary nebulae.

UV observations with IUE have led to recognition of the importance of di-electronic recombination in PN, for the calculation of ionization equilibria and for production of the features C II λ1335 and C III λ2297.

The determination of C abundances is discussed. In high-excitation nebulae one must allow for absorption of C IV λ1549 by dust. It has been shown that C/O ≤ 0.5 for five PN with a silicate IR feature and that C/0 ≳ l for four with a SiC feature.

Dust opacities τD have been deduced from observed ratios (C IV λ1549)/(C III λ2297) and (C II λ1335)/(C I λ4267). The values obtained, τD ≃ 0.1, are consistent with the observed strength of thermal infra-red emission.

Earlier UV observations of PN were made by Pottasch et al. (1978) using ANS. Results obtained using IUE support neither their downward revisions of star temperatures nor their claim that a number of stars with TZ(He II) >> TZ (H I) are optically thick for H I.

From IUE observations it is concluded that the planetary κ 648 in M 15 has C/O = 2.4 and that its central star has a temperature of 30 000 κ and a luminosity of 1700 L⊙.

Recent advances in the calculation and measurement of transition probabilities, electron excitation rate coefficients and photoionization cross sections relevant to the study of planetary nebulae are discussed. A compilation of these parameters is also presented.

A critical picture of the results obtained by means of photoionization models during the 70's is presented. Some reasons for the relative failure of these modellings are advanced and a new perspective is proposed for future investigations.