Under nebular conditions it is expected that the [CaII] lines at 7291.470 and 7323.880 Å should be among the strongest emission lines in the red part of the optical spectrum unless the nebula is matter bounded. The lines are observed in novae, supernovae and some symbiotic systems but the only observation of these lines in planetary nebulae (PN) is for AFGL 618 where the lines are thought to be shock-excited. While higher ionization states of Ca do produce some lines observed in a few high-excitation PN, from which gas-phase Ca abundances averaging to about 5% of solar are found, the CaII lines are not observed even in PN with massive neutral envelopes such as NGC 7027. If the Ca is depleted onto dust grains it would explain the absence of the CaII lines. However it is thought that the dust is destroyed with time in the ionized region. Any observation of the CaII lines would produce better abundance estimates and would test the extent of dust destruction that is taking place in the outer parts of the PN.

A model to simulate the entire spectrum (1000 Å to 1 cm) of the high-excitation young planetary nebula NGC 7027 is presented. The ionized, dust, and molecular components of the object are modeled using geometric parameters obtained from visible, radio, infrared, and CO data. The physical processes considered include recombination lines of H and He, collisional excited lines of metals, bf and ff continuum radiations, two-photon radiation, dust continuum radiation, and molecular rotational and vibrational transitions. The dust component is assumed to be heated by a combination of direct starlight and the line and continuum radiation from the ionized nebula. The molecular component of the nebula is coupled to the dust component through the stimulated absorption of the dust continuum radiation. Specifically, we compare the predicted fluxes of the CO rotational lines and the 179.5 μm water rotational line to those observed by the Infrared Space Observatory satellite (Liu et al. 1996, A&A in press).

The high angular resolution and dynamic range of VLA radio continuum mapping of planetary nebulae has raised expansion parallax measurement to the most sensitive method for determining PN distances (Hajian et al 1995). In order to derive the distance, the proper motion in the plane of the sky must be compared to the expansion velocity measured from emission line profiles. For a spherical PN with a measured line of sight expansion velocity, the comparison is simple. However most PN are not spherical, and their axisymmetric structures may be tilted to the line of sight; the line of sight expansion velocity cannot therefore be used for distance estimation from expansion proper motion. 3-D kinematic modelling is required in order to determine the expansion velocity in the plane of the sky.

The Abell catalogue of planetary nebulae (PN) are distinguished by their large size, low surface brightness and generally faint central stars. They are thought to be old PN approaching the White Dwarf cooling track. A number have evidence for late thermal pulses (H-poor ejecta near the central star, e.g. A78) and binary central stars.

H 4-1, PNG 049.3+88.1 (α = 12h59m27.6s, δ = +27° 38′14″, 2000), was one of well known halo Planetary Nebulae (PNe). During our program to study the expanding characteristics of PNe, the long-slit spectroscopic observations were made by the cassegrain spectrograph of the 74 inch telescope at the Okayama Astrophysical Observatory in March 1995.

The causes of asynchronous variations of the intensities of OIII ions forbidden lines in the spectrum of compact planetary nebula IC 4997 are considered on the basis of the observational data of. It is shown that the rise of the intensity of line 4363 å and decrease of the intensities of N1 and N2 lines may be best explained by increase of mass-loss-rate from nucleous from 5 × 10–8 up to 2 × 10–7 M/yr within a few years (at constant nucleous effective temperature), with appropriate change of ionization structure of nebula. The arguments of existence of variable hot stellar wind are discussed. The theoretical intensities of lines are calculated by the ionization model of planetary nebulae [4], gyven the radiation fields of the nucleous and hot stellar wind with electron temperature Te= 500000 K.

“Senile” PNe are very old nebulae of extremely low surface brightness and large linear size; they offer e.g. the possibility of probing the conditions of the ISM also non-locally. We give three examples of huge nebulae in a preliminary inquire.

NGC 3242: An ESO Schmidt plate obtained in April 1996 shows the ‘wrapped’ morphology of the faint arc discovered by Deeming (1966, Astrophys. J. 146, 287) 11′ south-west of this PN. Probably this arc is part of a giant halo of very low surface brightness around NGC 3242, but up to now the detection of the halo is questionable. A spectrum of the filament obtained at Las Campanas shows [O iii]/Hα ∼ 5 compared with a value of ∼ 3 in the nebula (Perinotto et al. 1994, Astron. Astrophys. Supp., 107, 481). This high ratio is larger than that expected for the excitation class of NGC 3242, even larger than the maximum allowed for a photo-ionized nebula of more or less normal chemical abundance. Thus, due to the spectral characteristic and to the filamentary structure of the arc we suspect that shocks appear to play a role.

Confrontations of the dredge-up theory with observed patterns of chemical abundances of planetary nebulae (PNs) have been carried out by many authors (see, e.g., Kaler & Jacoby 1990, 1991; Stasińska & Tylenda 1990). Although these studies suggest that the observational abundance ratios of PNs can qualitatively be explained by the current dredge-up theory, scatters around the theoretical predictions in their diagrams are always large. This has led Ratag (1991) to conclude that there is no correlation at all between the nebular abundances and the core mass of CSPNs (see also Pottasch 1993).

The variety of different morphologies of planetary nebulae (PNs) have always fascinated researchers in the field. Recent progress in CCD imaging techniques has resulted in a large number of homogeneous optical images of more than a few hundreds PNs (Balick 1987, Schwarz, Corradi and Melnick 1992). In addition, two major radio surveys using the Very Large Array (VLA) by Aaquist and Kwok (1990, 1991) and by Zijstra, Pottasch and Bignell (1989) have revealed detailed morphologies in the radio, including many compact star-like PNs. This treasure of data sets contains useful information about morphologies, density distributions, formation and evolution of PNs, yet to be fully explored.

The interacting stellar winds (ISW) theory (Kwok, S., Purton, C. R., Fitzgerald, P. M., 1978, ApJL, 219, L125) is nowadays widely accepted in the physics of Planetary Nebulae (PNe). It received much support from the observed fast winds in the central stars of PNe (CSPN), recognized to be a quite common phenomenon (e.g., Perinotto, M., 1993, in IAU Symp. No. 155, Planetary Nebulae, eds. R. Weinberger and A. Acker, Kluwer, Dordrecht, 57). Thus, the existance of a hot bubble in the PNe structure is a cornerstone of the ISW model. The high velocities (600–3500 km s–1) of the CSPN winds, are, according to the ISW model, directly responsible for an high gas temperature in the hot bubble, which is then expected to be the source of an extended X-ray and extreme ultraviolet (EUV) radiation. The PNe should also emit infrared coronal lines (IRCL) of highly ionized species since the high temperature plasma of the hot bubble is in contact with the much colder outer shell (optical PN) and the thermal conduction will produce a region of intermediate temperatures (5 × 105–106 K). A model considering the structure of the hot bubble in PNe with taking into account the thermal conductivity effects was present by Zhekov and Perinotto (1996, A&A, 309, 648).

In order to investigate the shaping of aspherical planetary nebulae the gasdynamical conservation laws of mass, momentum and energy including radiation effects are solved numerically in twodimensional, spherical coordinates. The simulations describe the dynamical interaction of a fast, tenuous, spherically symmetric wind from the central star with the slow, dense, axisymmetric AGB-wind remnant and include photoionization, collisional ionization, collisional excitation, radiative recombination and forbidden line emission as radiation effects.

The final stages of AGB evolution still remain difficult to study theoretically. From the stellar point of view, the main theoretical problems are the evolution of the stars (involving shell flashes and dredge-up of carbon and s-process elements), pulsation, and the production of mass loss by pulsation and radiation pressure on grains. In this paper I will discuss these topics. A recent review concentrating more on the circumstellar aspects of AGB evolution is given by Habing (1996).

We review the properties of proto-planetary nebulae, highlighting the advances in our quantitative knowledge of these objects. The discussion focuses on the chemistry and morphology of their circumstellar envelopes and the chemistry and variability of their central stars.

Low and intermediate mass stars leave the Asymptotic Giant Branch (AGB) when the mass in their H-rich envelope is less than about 0.01 M⊙, and the high mass loss drops several orders of magnitude. The central star rapidly evolves to the left part of the HR diagram along a track of constant luminosity (e.g. Schönberner 1983). In principle the evolution of the central star to higher Teff and the expansion and cooling of the AGB remnant are easy to calculate. In practice several complicating factors arise which make it much more difficult to predict the morphology and properties of post-AGB stars, such as binarity, post-AGB mass loss and aspherical AGB mass loss. Binarity of post-AGB stars affects the morphology of the circumstellar environment, and it affects evolutionary timescales and surface chemical abundances of the components in the system. This review discusses some properties of binary post-AGB stars.

Recent observations of multiple shell planetary nebulae confirm the existing subdivision into two morphological types: attached halo and detached halo. The multiple shell phenomenon appears in about twenty-five percent of round and elliptical planetary nebulae of the Instituto de Astrofisica de Canarias morphological survey. The analysis of these nebulae together with that of their central stars lead to the interpretation that detached halo nebulae are formed through discontinuous mass loss at the Asymptotic Giant Branch tip. The attached halo nebulae, instead, are probably produced via dynamical nebular shaping.

This review covers recent results from ground based mid-infrared (mid–IR; 8–25μm) imaging of thermal dust emission. The mid–IR images of proto–PNe show evidence for axially symmetric superwinds. In young PNe, the dust appears to be well mixed with the gas in the ionized gas regions. In carbon rich PNe and proto–PNe, the 11.3μm feature attributed to PAHs appears more spatially extended than “continuum” dust emission and the near–IR (1-3μm) excess appears spatially coincident with mid–IR dust emission. This review is not meant to cover the broader topic of dust in PNe and proto-PNe and we refer the reader to (Barlow 1993) for the most recent broad review.

In a brief paper at the first meeting of this series, Vorontzov-Velyaminov (1968) presented the first examination of the fine scale structure found in the Helix Nebula. At that time he used the term “filaments” to describe them, although the results of improved spatial resolution during the ensuing decades (Malin 1982, Meaburn et al. 1992, Walsh & Meaburn 1993) show that the term “Cometary Knot”(CK) communicates a clearer picture of their form. The most useful groundbased images are those of Meaburn et al.(1992) made with the New Technology Telescope, which showed that each CK has a characteristic form, composed of a central dark knot, accompanied by a luminous cusp on the substellar side, and accompanied by a thin sheath of luminous material extending from the edge of the cusp to large distances. Owing to their much higher density, the Cusps are of lower ionization than the nebula as a whole, so that one sees the central dark knots, which evidently contain significant amounts of dust, in silhouette against the background nebular emission. Radio observations of the CO line show that the CK are also sources of molecular emission (Huggins et al.1992). The groundbased emission line images show that the “tails” trailing away from the Cusps lie closely on radial lines passing between the Cusp and the Central Star. Although these objects appear to be diving towards the Central Star, such an interpretation is naive, as their form must be determined by a central repulsive force arising from the star, much as true comets in our Solar System may trail their tails behind while approaching the Sun but actually follow their tails as they recede from the Sun. We also now know, from yet another excellent study by Meaburn et al. (1996) that the CK as a group are expanding away from the Central Star, although this group velocity of about 10 km s–1 is about half that value of 21 km s–1 characteristic of the nebula as a whole (Taylor 1977,Terrett 1979). The greatest breakthrough in imaging was the Hubble Space Telescope's WFPC2 (O'Dell & Handron 1996, hereafter O&H) program on a single field of view (FOV) in the north ring of the Helix in Hα, [NII], and [OIII]. One year later the same field was reimaged in [OI] and with the f547m filter, the newer observations allowing calibration of the emission line images with correction for the underlying continuum. A second, contiguous field, was also imaged, in all five filters. The results of both sets of observations are used in this paper and a combined color image is shown in the rear section of this volume. The method of calibration was the same as that used in studies of the Orion Nebula (Hu 1993). Details of the calibration and digital copies of the images can be obtained from the first author. The new observations show heretofore unseen structure which is discussed in the following sections. They indicate that there are about 3500 detectable CK in the entire Helix Nebula, with the actual number probably being much larger as strong observational selection effects operate against detection of objects far from the Central Star. The images also show that the previously known orientation along radial lines is followed, but a detailed examination shows that the tails show small local variations, as if additional, non-radial, forces are acting on them (Fig.4). O&H demonstrated that the chord diameters, measured across the cusps, decrease from about 1.8″ at 120″ distance from the Central Star to about 0.6″ at 180″. There are so many known CK that one cannot use a sequential system of designation. In this paper we will use a coordinate based system, dividing the nebula into boxes of 1″ in Declination and 0.1s of Right Ascension and dropping the values common to all of the CK. Therefore, a CK located at 22:29:42.331 −20:47:32.1 would have the designation Helix 423-732. This system will allow future studies to uniquely identify all CK with only six digits within the range of declination −20:45:00 and −20:55:00, with any objects found north or south of this range having either 4 or 5 inserted as an additional digit immediately after the hyphen. Epoch 2000 is used for the positions and the position is judged to be at the center of the dark knot.

Some supplementary methods of time series analysis are described which are used for study of periodic and aperiodic variability of the AGB stars. These are “multiharmonic fits” used to fit the periodic (asinusoidal) curve; running approximations – “parabolae”, “sines”; “asymptotic parabolae”; “linear fits”.

IR OBSERVATIONS. We have extended previous infrared imaging studies (cf. Hora & Latter 1994 ApJ 437, 281; Kastner et al. 1996 ApJ 462, 777) through wide and narrow band filters.

In these images: (a) JHK′ morphology is similar to the optical structure. The knots are brighter at the shorter wavelengths, while the core is brighter at K′. (b) [He I] 1.083 μm that traces the highly ionized region, is concentrated along the axial region and extends throughout the lobes (33″ × 9″). It shows the knots very prominently. (c) [Fe II] 1.644 μm, that traces the shock excited regions, extends []throughout the lobes; it is more prominent in the northern knot and it is very strong in the core. (d) The excited molecular H2 lines are concentrated on a wide ‘torus’ in the equatorial plane. This geometry is similar to that exhibited by most bipolar planetary nebulae where H2 has been detected (e.g., Kastner et al. 1996). No H2 emission is seen in the core. (e) Paγ shows emission in only in the core and the northern knot. (f) Brγ shows emission in the core and in knots N2, S2 and S3. (g) No significant emission in CO was obtained.

We present spectroscopic observations of the protoplanetary nebula CRL 618. The spectra in the range from 370 to 800 nm were taken at the 3.5m telescope on Calar Alto (Spain). The slit of the spectrograph covered both the brighter E-lobe and the fainter W-lobe. Using the balmer line we found an extinction of about c= 1.6 for both lobes. The emission lines show a velocity difference between the two blobs. The difference of the forbidden lines is found to be bigger than the v-difference of the permitted recombination lines in agreement with Carsenty and Solf (1982). Exceptions are the OII 732.0nm and 733.0nm and the NII 575.5nm lines. It also seems that the spatial separations of the peak emis-sivities of the forbidden lines are bigger than the separation of the recombination lines. This might be a result of the geometry and structure of the object. The permitted lines are emitted from the central source and reflected towards the observer at the inner edges of the blobs while the forbidden lines are emitted from the lobes themselves. Kelly et al. (1992) found a different I(NII 658.4nm + NII 654.8nm)/I(NII 575.5nm) ratio for the bright E-lobe than the older observations from Schmidt and Cohen (1981). This indicates an increased temperature of the bright lobe. We found now a similar temperature enhancement for the faint W-lobe. This may be caused by a shock reaching the inner edges of the blobs. This may also be the reason that the N II 575.5 nm line seems to originate at the inner blob edges, showing a smaller spatial separation. There is another indication for an increased temperature, we found some evidence for the S III 631.2nm, the He II 468.6nm and the O III 500.7 nm lines. The protoplanetary nebula CRL 618 is one of the few objects in this transition stage where dynamical processes can directly be observed.