I review recent results of mm-wave interferometric observations of molecular envelopes of young and proto planetary nebulae. Spatial resolutions of mm-wave interferometers now reach from one to a few arcseconds; together with line observations reveale very small structures in molecular envelopes of planetary nebulae. These small scale structures are probably caused by an instability of the radiation-driven, gas-dust outflow. Implications of these observations and theories are discussed.

This paper will review a selection of the work that has been published on the calculation of atomic parameters, relevant to PN, in the period since the 1992 Symposium. It will also attempt to address the accuracy of theoretical results, where possible. The review will concentrate primarily on results obtained using the powerful electron-scattering codes that describe resonance phenomena with ever increasing precision. Calculations of oscillator strengths and photoionisation cross-sections will not be discussed.

Shall we leave this millennium with a template spectrum for PNe based on photographic plates taken nearly half a century ago? Is photoionization being demolished by shock waves in PNe? Is icf getting on well with t2?

Type I PNe were defined in 1977; during the last 19 years evidence has accumulated indicating that indeed these objects form a distinct physical group, corresponding to the high mass end of the stellar progenitors of PNe. In this review the definition of PNe of Type I is reanalyzed. Their properties in our Galaxy and other galaxies are discussed.

The study of the morphology of planetary nebula (PN) is fundamental for addressing several questions in the context of stellar evolution. An AGB star can loose most of its mass due to strong stellar winds. Kwok et al. (1978) proposed that the interaction of a low-density fast wind with a high-density slow wind, will form the PN. This model can account for the round observed PNe with a great degree of symmetry. However as we will see later, round PNe are not the most common ones. Therefore a mechanism for causing asymmetry has to be invoked. Several processes have been proposed by different authors.

In this review we focus on new observations and theory that directly pertain to understanding the hydrodynamic evolution of planetary nebulae (PNe). The recent history of the field lead us to break the review into two parts, macroscopic and microscopic structures, since both the size scales and the dominant physics appear to cleave nicely into more-or-less distinct groups.

The presence of high-velocity, collimated outflows in planetary nebulae (PNe) has been observationaly established in recent years. Furthermore, these collimated outflows, or jets, are usually found to be symmetric with respect to the nucleus. In some cases, their morphology and spatial distribution indicates episodic outbursts and rotation of the symmetry axis. In these cases, the bipolar, rotating, episodic jets are identified as BRETs. The existence of bipolar jets in PNe has been considered perplexing since the mechanisms for their formation are unclear. However, substantial progress in their study has been made in the last few years, from both theoretical and observational grounds. Consequently, new results in the field are rapidly coming out, as it can be noticed from the number of related works in this symposium. We still need to reach solid answers in many aspects of this field, but its influence in the study of PNe has already modified the traditional views of PNe shaping and evolution. The main characteristics of some of these phenomena are discussed here, together with prototypical cases, including a compilation of PNe with BRETs charateristics.

We present first results of a project to construct and study spatio-kinematical models of selected Planetary Nebulae (PNe). Deep monochromatic CCD images of A8, A4, M2-55, and J-320 were made with the Calar Alto 2.2m telescope and the two interference filters 5005/90 and 6574/104 for the [O III] and Hα+[N II] emission lines. The images were corrected for bias and dark, and flatfielded using MIDAS software, and also corrected for atmospheric extinction to derive the total flux in each emission line. The observed parameters are summarized in Table 1. – A8 (PNG167.0–00.9) has a very symmetric, round appearance with large diameter. The Hα+[N II] image clearly shows a round, limb-brightened ring which is less apparent in the [O III] image. Due to its appearance we can classify it as a late round type according to Balick (1987). −M2-55 (PN G116.2+08.5) has a box-like, quadratic and highly structured inner part (slightly more roundish in [O III]), with four maxima at the corners. The faint outer parts seem to consist of two ovals that are perpendicular to each other. Together, they again resemble a square tilted 45° against the inner one. The Hα+[N II] image is larger than [O III]: The square (distance of maxima) by some 20 percent, the faint ovals by about 10 percent. We can classify this PN as a middle elliptical type. – A4 (PNG144.3–15.5) is a spherical shell, with some definite structure in the brightest parts. This structure forms roughly a ring halfway between the center and the rim. A4 can be classified as an early round type. Both images are quite similar, the Hα+ [N II] maximum lying slightly, about 20 percent, farther from the center. In the very faint outer regions, the [O III]/(Hα+[N II]) ratio seems to increase by a factor of about 5 above the average which is otherwise constant within 10 percent over the whole image. -J-320 (PNG190.3–17.7) is elongated, with one maximum on the main axis, slightly off-center. The Hα+[N II] image has an interesting structure which is not present in [O III]: Two stripes of decreased intensity that are roughly parallel to and on both sides of the main axis. At present, we have no explanation for this feature. J-320 has ansae (small knots found in pairs on opposite sides and at equal distance from the nucleus) usually seen best in the light of low-ionization emission lines. Balick (1987) classified this PN as early type butterfly.

Near-infrared (1-3μm) emission lines of molecular and ionized hydrogen are excellent tools for investigating the morphology, energetics and kinematics of planetary nebulae, especially those PNe which contain large amounts of dust and are thus obscured at shorter wavelengths. The southern planetary nebula NGC 3132 was imaged with UNSWIRF (University of New South Wales Infrared Fabry-Perot) and IRIS on the 3.9m AAT Images in the H2 v=1-0 S(1) and H2 v=2-1 S(1) lines at 2.12μm and 2.25μm, and in Hii Brγ at 2.16μm are presented.

We have made multi-wavelength observations of the young planetary nebula NGC 7027. The C-band continuum radio map shown in Figure 1 was obtained from the combination of MERLIN and VLA1 observations (M. Bryce et al., 1996). This map has the highest spatial resolution of any radio images of this object to date and has been compared with the HST archive picture of NGC7027, taken using WFPC2 with a 400 å wide F547N filter, to reveal the differences in the emission at the disparate wavelengths. The two maps, while being superficially different, show emission regions that are similar in size. The contours of radio brightness often coincide with prominent optical features.

Hubble Space Telescope narrowband images of four PNe resolve the microstructures in four PNe known to contain FLIERs (e.g. Balick et al 1994). FLIERs are seen to fall into two morphological groups: very low ionization clumps of small knots with cometary-like tails, and sinewy transverse filaments with fainter extensions pointing outward from the nucleus. Generally speaking, FLIERs exhibit [NII]λ6584/Hα > 1. They are marginally detectable in Hα above the bright background of nebular emission, and all but invisible in [O III]λ5007A. In every case the star-facing side of the structure is on the order of a pixel in width (about 1015 cm). A decreasing ionization gradient is seen [O I]λ6300A, [S II]λ6717 + 31A, and [N II]λ6584A images.

We study the physical structure of H II regions, including Planetary Nebulae (PNe), through detailed analysis of the ionization structure and spectra of iron in relevant ionization stages. This results from our work in the Ohio State group on: (i) calculation of new and improved atomic data for Fe I-VI (photoionization cross sections, recombination rates, collision strengths, and transition probabilities); (ii) detailed study of ionization, recombination, and excitation mechanisms and radiative transfer effects under nebular conditions; (iii) study of the physical structure and kinematics of nebulae and their ionization fronts. This includes further examination of high density regions deduced from the spectroscopic analysis of optical [Fe II], [Ni II], and [O I] emission (Bautista etal 1996; and references therein). The study also addresses the issue of chemical abundances in fully ionized and partially ionized zones of nebulae.

Binarity in central stars is often thought to be the most likely explanation for the bipolar and axisymmetric shapes seen in planetary nebulae. Theoretical computations show that low mass stellar companions — possibly even planets, merged nuclei and long period systems will also produce bipolar-shaped nebulae. As such all known short period binary central stars are expected to have an axisymmetric nebula surrounding them.

During Cycle 5 of the HST General Observer program we have been carrying out a “snapshot” survey of central stars of planetary nebulae. The snapshots are short exposures in the V (F555W) and I (F814W) filters, taken with the Wide Field Planetary Camera (WFPC2) during brief scheduling opportunities.

Density distributions of four planetary nebulae have been calculated in cylindrical coordinates (r,z,ϕ) using the iterative deprojection algorithm by Bremer (1995). Narrowband filter CCD images of NGC 2346, NGC 6563, NGC 6826 and NGC 7009 in the Hα line (6562.8 å) were corrected for cosmic ray hits, background/foreground stars were removed, and the position angles PA of their symmetry axes were determined with a maximum correlation criterion. For NGC 6563, the axis was shifted manually to the south-east to put it on the central ridge (the PA was conserved). For deprojections in cylindrical coordinates, an inclination angle i (angle between the object axis and the line of sight) must be specified. Literature values based on optical spectra were available for NGC 2346 (Walsh 1983) and NGC 7009 (Preston and Willson 1990), for NGC 6563 and NGC 6826 lower limits were derived from deprojections.

X-ray emission from planetary nebulae (PNe) may originate from two sources: central stars which are 100,000–200,000 K will emit soft X-rays, and shocked fast stellar winds reaching 106–107 K will emit harder X-rays. The former are point sources, while the shocked winds are expected to be extended sources emitting continuously out to the inner wall of the visible nebular shell (Weaver et al. 1977; Wrigge & Wendker 1996).

Superb–seeing (0″ .40 FWHM) narrowband images and high resolution (R=60000) spectra of the asymmetrical planetary nebula IC 4593 have been used in order to study its detailed morphology and kinematics.

In this work we present a summary of the results of a long term observational program aimed to derive physical parameters and chemical abundances of southern PNe developed in the last years at the University of São Paulo, Brazil. Results shown here were derived from 74 nebulae, all of them observed (at least twice) with the 1.60m telescope at the Pico dos Dias Observatory, Brazil, using a Cassegrain spectrograph, a 300 lines/mm (4.4 å/pixel) grating, and UV-coated CCD detectors. Observational runs took place between 1988 and 1995. Data reduction followed the standard procedure of bias and flat-field corrections, extraction of the spectrum, wavelength calibration, and flux calibration through spectrophotometric standard stars observed each night. The table below summarizes our results. Nebulae were classified among the Peimbert types and mean abundances are shown for each type, using the notation ∊(X)=log(X/H)+12.

We have acquired monochromatic imaging of the bipolar outflow source NGC 2440 in 12 ionic transitions using the 1.5 m Telescope at the Observatorio de San Pedro Mártir, Ensenada, Baja California, Mexico. As a result, we have been able to undertake detailed analysis of the shell ionisation structure. Then, we found that excitation is high close the center, extending towards the exterior at located radial positions. Edges and ansae are zones of low excitation indicating probably shock regions, in particular the outer shell.

During more than twenty years, we are making a continuous effort to provide Stark-broadening parameters needed for research of astrophysical, laboratory and laser produced plasma. A review of our results is presented in Dimitrijević, 1996). Such data are of interest for the consideration of a number of problems in astrophysics, physics and technology as e.g. for stellar plasma diagnostic, opacity calculations, the investigation/modelling of stellar spectra or a particular line, laboratory plasma diagnostic, laser produced plasmas, thermonuclear research, plasma technology, as well as for different examinations of regularities and systematic trends for e.g. homologous atoms (Dimitrijević and Popović, 1989) or in general (Purić et al. 1991).