The density structure of the extended haloes of Planetary Nebulae (PN) is generally believed to reflect the previous history of heavy mass loss during the final stages of stellar evolution on the asymptotic giant-branch (AGB). In this review, we discuss different interpretations of the observed PN halo structures in the light of recent numerical simulations combining detailed AGB and post-AGB stellar evolution calculations with time-dependent hydrodynamical wind models.

An improved database of ionized haloes around PNe has been built by adding the results of an extensive observational campaign to the data available in the literature. The new observations allowed us to discovered new haloes around CN 1-5, IC 2165, IC 2553, NGC 2792, NGC 2867, NGC 3918, NGC 5979, NGC 6578, PB 4, and possibly IC 1747.

The global sample consists of 29 AGB haloes, that are believed to still contain information about the mass loss from the AGB progenitor star. Six of these haloes show a highly asymmetrical geometry that is tentatively ascribed to the interaction of the stellar outflow with the ISM.

Another 5 PNe show candidate recombination haloes. These are produced by the recombination front that sets up when the stellar luminosity drops in its post-AGB evolution. The resulting, limb-brightened shell resembles a real AGB halo, but is not related to AGB any mass loss event.

Double AGB haloes are found in at least 4 PNe.

For 11 PNe, deep images are available, but no halo is found to a level of ≲ 10-3 the peak surface brightness of the inner nebula.

These observations show us that ionized haloes are a common morphological component of PNe, being found in 70% of elliptical PNe for which adequately deep images exist. Statistical properties of the haloes are briefly discussed. Using the kinematical ages of the haloes and inner nebulae, we conclude that most of the PNe with detected haloes have hydrogen burning central stars.

We present model results of time-dependent ionization of a nebula with a halo. Fast transition of the ionization front across the halo causses its heating well above the equilibrium temperature. For a long time after this event the halo is observed as hotter than the main inner nebula, in agreement with observations of some planetary nebulae.

Bipolar Planetary Nebulae (hereafter PNe) are almost all of the Type I and have the most massive progenitors among PNe. They are mostly found at very low galactic latitudes and thus are expected to follow rather closely the differential rotation. Moreover since they are strongly suspected to constitute the youngest population among PNe, then some bipolars may have not achieved a complete galactic revolution yet and we can reasonably suppose that some signs of their recent galactic evolution still exist.

A systematic search of the wider neighborhood of Planetary Nebulae (PNe) on IRAS SkyView images revealed structures like bipolar dust blobs, curved filaments and dust-poor cavities which in several cases suggest that they are genuine and not just due to projection effects.

Ionized haloes around planetary nebulae (PNe) are a quite common phenomenon, being found in about 70% of the elliptical PNe for which adequately deep images exist (cf. Corradi, these proceedings). Physically, one has to distinguish between two different kind of haloes. AGB haloes consist of photo-ionized matter still containing information about the mass loss history during the last thermal pulse cycle on the Asymptotic Giant Branch. A second kind of halo may develop when recombination sets in as a consequence of the fast luminosity drop during the advanced post-AGB evolution. These recombination haloes, which can be confused with real AGB haloes on a pure morphological basis, are not a signature of an AGB mass-loss episode (Corradi et al. 2000).

When we discuss about MHD effects in planetary nebulae (PNe), there naturally arises a basic question: which magnetic field do we study? One possibility is the ISM magnetic field (e.g. Heiligman 1980), even more if we are concerned with moving PNe (e.g. Soker & Dgani 1997). The next possibility is the internal or stellar magnetic field (Gurzadian 1962). It is important to start this review by quoting Aller (1958): “It has been pointed out by Minkowski and others that the structural appearance of many planetary nebulae strongly suggest the presence of magnetic fields. It seems unlikely that such magnetic fields are produced ab initio in the nebular shell. Rather, they must have existed in the outer envelope of the parent star. Certain red giants stars with magnetic fields may evolve in such a way that the expansion of the shell is largely governed by the presence of such a field. Magnetic effects may actually be more important than gas pressure differentials and radiation pressure in controlling the evolution of a planetary nebula”.

We claim that cyclotron radiation at the electron gyrofrequency ωB is an efficient mechanism capable of driving highly collimated bipolar jets in magnetized nuclei of planetary nebulae.

We study the role of radiatively driven winds in the formation of PNe. The modulation of these winds due to stellar rotation, and Von Zeipel gravity darkening, are both considered. Rotation produces a wind that is faster at the poles but denser at the equator. The Von Zeipel effect, which states that the stellar flux is proportional to the effective surface gravity, results in the inverse density distribution, giving a wind that is both faster and denser at the poles. Both these effects produce bipolar nebulae for large rotational velocities of the star.

The imaging of young planetary and proto-planetary nebulae (PNe and PPNe) with unprecedented high angular resolution and dynamic range, using the Hubble Space Telescope, has led to the realisation that almost all of these objects are highly aspherical, with complex multipolar morphologies. The complexity, organization and symmetry of the morphological structures we find is forcing radical changes in, and inspiring fresh theoretical efforts to advance, our understanding of the mass-loss processes during late stellar evolution. In this paper, we review the HST data, and show some of the highlights of our imaging studies. Although the origins of many of the morphological features remains puzzling, we find that the general presence of multipolar structures support a model for PN formation in which the primary agent for shaping PNe are high-speed collimated outflows or jets which operate during the late AGB and/or early post-AGB evolutionary phase, and undergo episodic changes in their orientation (or collimated outflows operate quasi-simultaneously with different orientations).

The HST WFPC2 camera was used to obtain images of the planetary nebula BD+30°3639 at two epochs separated by 5.66 years. The expansion of the nebula in Hα and [N II] has been measured using several methods. Detailed expansion maps for both emission lines were constructed from nearly 200 almost independent features. There is good agreement between the (independent) Hα and [N II] proper motions. There are clear deviations from uniform radial expansion, with higher expansion rates in regions where the shell is faintest, such as the south-west quadrant.

HST STIS echelle spectra obtained in the C II] λ2326 multiplet provide well-resolved expansion velocities at two position angles. We find that the central velocity split is ±36.3 km s-1 at a position angle of 99°.

To determine the distance of BD+30°3639 by comparison of the angular expansion and the radial expansion, we must address the problem of the three dimensional shape of the nebula. We measured the angular expansion along the position of the 99° echelle slit, finding displacements of 4.25 mas yr-1 at the shell edge (2″.47 from the center). If the nebula were spherical, this would imply a distance of 1.80 kpc. But there is evidence that the nebula is elongated along the line of sight, which suggests that the actual distance is less. Radio continuum images from 5 and 15 GHz VLA observations provide information on the extent of the radial elongation. We fit the radio brightness variation and the echelle data by approximating the nebula as an ellipsoid. Our model has an axial ratio of 1.56, is inclined to the line of sight by 9°.7, and exhibits an expansion in the plane of the sky which is 2/3 that in the radial direction, leading to a distance of 1.2 kpc. Based on a recent model atmosphere (Crowther et al., these proceedings), this distance implies a stellar luminosity of 4250 L⊙.

Seven compact PNe with high radio surface brightness (and therefore presumably young) were observed with the HST WFPC2 under cycle 8 program 8307 (PI: S. Kwok). Observations were made with three narrow-band filters, F656N (Hα), F658N ([NII]) and F502N ([OIII]). The [NII] images of six of the nebulae are shown in Figure 1.

The study of collimated outflows (COFs) in planetary nebulae (PNe) has experienced vigorous activity since the last IAU symposium on PNe, held in Groningen in 1996, for the understanding of the origin and evolution of these outflows is directly linked to the mechanisms of PN formation. Collimated outflows in planetary nebulae (PNe) have diverse characteristics and are observed as bipolar, multipolar, point-symmetric and jet-like outflows. The Hubble Space Telescope has been instrumental in revealing the complex structure and pervasive presence of COFs in a large number of PNe during the last lustrum. Significant advances have also been achieved in the study of the molecular component associated with these outflows. On the theoretical side, models that incorporate the influence of binary cores and magnetic fields have gained particular strength in explaining the origin of COFs. This review attempts to summarize some of the main progresses in the study of COFs during the last years.

We started a program of identification of faint structures in PNe on HST archive images, based on the “à trous” wavelet transform (Starck et al. 1998). Using this technique, an image is decomposed in a new set of N images, each with the same number of pixels as the original one, but containing only the structures of a characteristic scale size (going from one image to the next, the characteristic scale vary by a factor of two). The original image c0 can be expressed as the sum of all wavelet scales plus a residual smoothed array cN by C0(x, y) = CN(x, y) + Σj=1Nwj(x, y).

High spectral resolution observations of the high speed jets of NGC 6543 are presented. When deprojected, these give velocities of Vsouth = -82.0 km s-1 and Vsouth = 63.8 km s-1.

I reconsider my study (Icke 1981) of the hydrodynamic mechanism that is responsible for the shape of the Red Rectangle. The density of this nebula is so high, and the central star(s) so cool, that momentum driven outflow is almost guaranteed. Focusing of a spherically symmetric stellar wind through an inner shock creates a biconical pattern that may be responsible for the characteristic X-shape of the outer nebula. I compute the evolution of periodic biconical outflow. Observations made available to me by Van Winckel can thus be modeled in great detail. Features that I initially thought to be problematic for my interpretation, such as the occurrence of intersecting shocks on the symmetry axis, turned out to be present on closer inspection of the Hα image.

This paper describes the structural changes occuring in the circumstellar envelopes of stars evolving from the AGB to the planetary nebula phase, with an emphasis on the role of multiple outflows.

Optical observations by the HST have shown a fantastic 3-ring structure in the circumstellar space of the SN 1987A. We attempt to reproduce the 3-ring structure from an MHD simulation which takes into account the wind-wind interaction between the slow, magnetized stellar wind generated while the progenitor was a red supergiant (RSG) and the subsequent fast stellar wind ejected from the same star after it had evolved into the blue supergiant (BSG). An important assumption in the simulation is that both the RSG and BSG winds have toroidal magnetic field and coronal holes. The simulation shows that the magnetic pinch effect is essential for the formation of the equatorial ring and that the amplification of initial density asymmetry by the effect of the dynamical interaction is responsible for the formation of two outer rings.

The formation of PNe around [WC] stars is modelled with a numerical hydrodynamics code. It is found that the PN around [WC] are expected to expand faster, and be more turbulent. It is shown that the observed lack of differences with ‘normal’ PNe, can have implications for models of the origin of asphericity in PNe.

Modeling the shells of multiple shell planetary nebulae using different model spectra for hot central stars, we found that a blackbody model leads to wrong nebular parameters. We model the density profile of the outer shells, varying the results of hydrodynamical simulations. This leads to a spatial excitation profile which reproduces well the observations.