A concise summary of the recent international workshop held in honour of the career of Agnès Acker is presented together with a brief exposition of her productive career in the study of Planetary Nebulae, the expelled, ionised outer shrouds of dying stars.

Planetary nebulae (PN) are powerful tracers of both stellar and Galactic evolution. The capacity of PN to perform these studies is critically dependent on the size of the population, a major problem with a remarkable advance thanks to Quentin Parker and his team, who from 1997 to 2008 discovered an unprecedented sample of ∼1250 PN with the deep, high resolution AAO/UKST SuperCosmos Halpha Survey (SHS) of the Southern Galactic Plane (Parker et al. 2005), doubling the sample collected over the previous century, and leading to ∼2700 for the number of known PN today. A highly productive collaboration between Quentin and I has been established since 2001. Our complementary levels of expertise and facilities constitute the Macquarie/AAO/Strasbourg H-α Planetary Nebulae Project.

The new MASH PN were added to the Centre de Données de Strasbourg as a new PN database continuously updated, and detailed in Parker et al. (2006) and Miszalski et al. (2008). In the framework of a cotutelle agreement between the Strasbourg and Macquarie universities, two PhD projects based on MASH PN have been conducted under the supervision of Quentin and myself, both projects focusing on the mysterious crowded region of the Galactic Bulge. Alan Peyaud proposed new constraints on late stages of stellar evolution and on the dynamics of the Galactic Bulge (defence 21 December 2005, Strasbourg). Brent Miszalski discovered ∼360 new PN (MASH-II) completing the largest and most representative sample of PN towards the Galactic bulge (defence 15 August 2009).

The total number of true, likely and possible planetary nebulae (PN) now known in the Milky Way is nearly 3000, double the number known a decade ago. The new discoveries are a legacy of the recent availability of wide field, narrowband imaging surveys, primarily in the light of H-alpha. In this paper, we summarise the various PN discovery techniques, and give an overview of the many types of objects which mimic PN and which appear as contaminants in both Galactic and extragalactic samples. Much improved discrimination of classical PN from their mimics is now possible based on the wide variety of high-quality multiwavelength data sets that are now available. We offer improved taxonomic and observational definitions for the PN phenomenon based on evaluation of these better diagnostic capabilities. However, we note that evidence is increasing that the PN phenomenon is heterogeneous, and PN are likely to be formed from multiple evolutionary scenarios. In particular, the relationships between some collimated symbiotic outflows and bipolar PN remain uncertain.

Extragalactic surveys in the emission line of [O iii] λ5007 have provided us with the absolute line strengths of large, homogeneous sets of planetary nebulae. These data have been used to address a host of problems, from the measurement of the extragalactic distance scale, to the study of stellar populations. I review our current understanding of the [O iii] planetary nebula luminosity function (PNLF), and discuss some of the physical processes that effect its structure. I also describe the features of the Hα PNLF, a function that, upon first glance, looks similar to the [O iii] PNLF, but which includes a very different set of objects. Finally, I discuss recent measurements of α, the number of PNe found in a stellar population, normalized to that population's bolometric luminosity. I show that, contrary to expectations, the values of α found in actively star-forming spirals are essentially the same as those measured in late-type elliptical and lenticular systems. I discuss how this result sheds light on the physics of the planetary nebula phenomenon.

Recent Hα surveys such as SHS and IPHAS have improved the completeness of the Galactic planetary nebula (PN) census. We now know of ∼3000 PNe in the Galaxy, but this is far short of most estimates, typically ~25 000 or more for the total population. The size of the Galactic PN population is required to derive an accurate estimate of the chemical enrichment rates of nitrogen, carbon, and helium. In addition, a high PN count (>20 000) is strong evidence that most main-sequence stars of mass 1–8 M⊙ will go through a PN phase, while a low count (<10 000) argues that special conditions (e.g. close binary interactions) are required to form a PN. We describe a technique for finding hundreds more PNe using the existing data collections of the digital sky surveys, thereby improving the census of Galactic PNe.

We present the results of the search for candidate Planetary Nebulae interacting with the interstellar medium (PN–ISM) in the framework of the INT Photometric Hα Survey (IPHAS) and located in the right ascension range 18–20 h. The detection capability of this new Northern survey, in terms of depth and imaging resolution, has allowed us to overcome the detection problem generally associated to the low surface brightness inherent to PNe-ISM. We discuss the detection of 21 IPHAS PN–ISM candidates. Thus, different stages of interaction were observed, implying various morphologies i.e. from the unaffected to totally disrupted shapes. The majority of the sources belong to the so-called WZO2 stage which main characteristic is a brightening of the nebula's shell in the direction of motion. The new findings are encouraging as they would be a first step into the reduction of the scarcity of observational data and they would provide new insights into the physical processes occurring in the rather evolved PNe.

Since various structural components of planetary nebulae (PN) manifest themselves differently, a combination of optical, infrared, submillimetre and radio techniques is needed to derive a complete picture of planetary nebulae. The effects of projection can also make derivation of the true 3-D structure difficult. Using a number of examples, we show that bipolar and multipolar nebulae are much more common than usually inferred from morphological classifications of apparent structures of planetary nebulae. We put forward a new hypothesis that the bipolar and multipolar lobes of PN are not regions of high-density ejected matter, but the result of ionization and illumination. The visible bright regions are in fact volumes of low density (cleared by high-velocity outflows), through which UV photons are being channelled. We suggest that multipolar nebulae with similar lobe sizes are not caused by simultaneous ejection of matter in several directions, but by leakage of UV photons in those directions.

Planetary nebulae (PNe) can be roughly categorized into several broad morphological classes. The high quality images of PNe acquired in recent years, however, have revealed a wealth of fine structures that preclude simplistic models for their formation. Here we present narrow-band, sub-arcsecond images of a sample of relatively large PNe that illustrate the complexity and variety of small-scale structures. This is especially true for bipolar PNe, for which the images reveal multi-polar ejections and, in some cases, suggest turbulent gas motions. Our images also reveal the presence or signs of jet-like outflows in several objects in which this kind of component has not been previously reported.

The three main methods currently in use for estimating the excitation class of planetary nebulae (PNe) central stars are compared and evaluated using 586 newly discovered and previously known PNe in the Large Magellanic Cloud (LMC). In order to achieve this we ran a series of evaluation tests using line ratios derived from de-reddened, flux-calibrated spectra. Pronounced differences between the methods are exposed after comparing the distribution of objects to their derived excitation. Line ratio comparisons show that each method's input parameters have a strong effect on the estimated excitation of a central star. Diagrams were created by comparing excitation classes with Hβ line fluxes. The best methods are then compared to published temperatures using the Zanstra method and assessed for their ability to reflect central star effective temperatures and evolution. As a result we call for a clarification of the term ‘excitation class’ according to the different input parameters used. The first method, which we refer to as Exneb relies purely on the ratios of certain key emission lines. The second method, which we refer to as Ex* includes modeling to create a continuous variable and, for optically thick PNe in the Magellanic Clouds, is designed to relate more closely to intrinsic stellar parameters. The third method, we refer to as Ex[Oiii]/Hβ since the [Oiii]/Hβ ratio is used in isolation to other temperature diagnostics. Each of these methods is shown to have serious drawbacks when used as an indicator for central star temperature. Finally, we suggest a new method (Exρ) for estimating excitation class incorporating both the [Oiii]/Hβ and the HeII λ4686/Hβ ratios. Although any attempt to provide accurate central star temperatures using the excitation class derived from nebula lines will always be limited, we show that this new method provides a substantial improvement over previous methods with better agreement to temperatures derived through the Zanstra method.

We have recently confirmed the planetary nebula (PN) nature of PM 1–242, PM 1–318 and PM 1–322. Here we present high-resolution long-slit spectra of these three PNe in order to analyze their internal kinematics and to investigate their physical structure. PM 1–242 is a tilted ring and not an elliptical PN as suggested by direct images. The object is probably related to ring-like PNe and shows an unusual pointsymmetric brightness distribution in the ring. PM 1–318 is a pole-on elliptical PN, instead of a circular one as suggested by direct images. PM 1–322 is spatially unresolved and its spectrum shows large differences between the forbidden lines and Hα profiles, with the latter showing a double-peaked profile and relatively extended wings (FWZI ~325 km s−1). These properties are found in other PNe that are suspected to host a symbiotic central star.

The emission nebula around the subdwarf B (sdB) star PHL 932 is currently classified as a planetary nebula (PN) in the literature. Based on a large body of multi-wavelength data, both new and previously published, we show here that this low-excitation nebula is in fact a small Strömgren sphere (Hii region) in the interstellar medium around this star. We summarise the properties of the nebula and its ionizing star, and discuss its evolutionary status. We find no compelling evidence for close binarity, arguing that PHL 932 is an ordinary sdB star. We also find that the emission nebulae around the hot DO stars PG 0108 + 101 and PG 0109 + 111 are also Strömgren spheres in the ISM, and along with PHL 932, are probably associated with the same extensive region of high-latitude molecular gas in Pisces–Pegasus.

Since the IUE satellite produced a vast collection of high-resolution UV spectra of the central stars of planetary nebulae (CSPNe), there has not been any further systematic study of the stellar winds of these stars. The high spectral resolution, sensitivity and large number of archival observations in the FUSE archive allow the study of the stellar winds of CSPNe in the far-UV domain where lines of species spanning a wide excitation range can be observed. We present here a preliminary analysis of the P Cygni profiles of a sample of ∼60 CSPNe observed by FUSE. P Cygni profiles providing evidence for fast stellar winds with velocities between 200 and 4300 km s−1 have been found in 40 CSPNe. In many cases, this is the first time that fast stellar winds have been reported for these planetary nebulae (PNe). A detailed study of these far-UV spectra is on-going.

In the final stages of stellar evolution low- to intermediate-mass stars lose their envelope in increasingly massive stellar winds. Such winds affect the interstellar medium and the galactic chemical evolution as well as the circumstellar envelope where planetary nebulae form subsequently. Characteristics of this mass loss depend on both stellar properties and properties of gas and dust in the wind formation region. In this paper we present an approach towards studies of mass loss using both observations and models, focussing on the stage where the stellar envelope is nearly empty of mass. In a recent study we measure the mass-loss evolution, and other properties, of four planetary nebulae in the Galactic disk. Specifically we use the method of integral field spectroscopy on faint halos, which are found outside the much brighter central parts of a planetary nebula. We begin with a brief comparison between our and other observational methods to determine mass-loss rates in order to illustrate how they differ and complement each other. An advantage of our method is that it measures the gas component directly requiring no assumptions of properties of dust in the wind. Thereafter we present our observational approach in more detail in terms of its validity and its assumptions. In the second part of this paper we discuss capabilities and assumptions of current models of stellar winds. We propose and discuss improvements to such models that will allow meaningful comparisons with our observations. Currently the physically most complete models include too little mass in the model domain to permit a formation of winds with as high mass-loss rates as our observations show.

The interaction of planetary nebulae (PNe) with the interstellar medium (ISM) as they move through it is now acknowledged to be a major shaping effect not just for ancient and large PNe, but also for relatively young PNe with high-speed central stars. The most common effect is a rebrightening as the PN shell interacts with a pre-existing bow-shock structure formed during the previous evolutionary phase of the central star. In this review we consider this rebrightening in detail for the first time and discuss its origins, highlighting some observed examples. We go on to discuss the AGB progenitor stars, reviewing the evidence for bow-shock structures, and consider the progeny of rebrightened PNe — strongly disrupted objects which bear very little resemblance to typical PNe. Sh 2-68 is inferred to be perhaps the only documented case so far of such a PN.

The determination of heavy element abundances from planetary nebula (PN) spectra provides an exciting opportunity to study the nucleosynthesis occurring in the progenitor asymptotic giant branch (AGB) star. We perform post-processing calculations on AGB models of a large range of mass and metallicity to obtain predictions for the production of neutron-capture elements up to the first s-process peak at strontium. We find that solar metallicity intermediate-mass AGB models provide a reasonable match to the heavy element composition of Type I PNe. Likewise, many of the Se and Kr enriched PNe are well fitted by lower mass models with solar or close-to-solar metallicities. However, those objects most enriched in Krand those PN with sub-solar Se/O ratios are difficult to explain with AGB-nucleosynthesis models. Furthermore, we compute s-process abundance predictions for low-mass AGB models of very low metallicity ([Fe/H]≈−2.3) using both scaled solar and an α-enhanced initial composition. For these models, O is dredged to the surface, which means that abundance ratios measured relative to this element (e.g. X/O) do not provide a reliable measure of initial abundance ratios, or of production within the star owing to internal nucleosynthesis.