CSIRO PUBLISHING www.publish.csiro.au/journals/pasa Publications of the Astronomical Society of Australia, 2010, 27, 129–148 Planetary Nebulae: Observational Properties, Mimics and Diagnostics David J. Frew A,C,D and Quentin A. Parker A,B A Department of Physics, Macquarie University, North Ryde, NSW 2109 B Anglo–Australian Observatory, Epping, NSW 1710 C Perth Observatory, Bickley, WA 6076 D Corresponding author. Email: dfrew@science.mq.edu.au Received 2009 July 19, accepted 2010 January 21 Abstract: 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. Keywords: catalogs — planetary nebulae: general — stars: AGB and post-AGB — stars: fundamental parameters — surveys — techniques: photometric, spectroscopic 1 Introduction A classical planetary nebula (PN) is a shell of ionized gas, ejected from a star of low to intermediate mass (∼1 to 8 M ⊙ ) towards the end of its life. A PN is produced at the end of the asymptotic giant branch (AGB) phase, as the red giant ejects its outer envelope in a final stage of copious mass loss termed the ‘superwind’. After the envelope ejection, the remnant core of the star increases in temperature before the nuclear burning ceases and the star quickly fades, becoming a white dwarf (WD). The high temperature of the central star (CS) causes the previously ejected material to be ionized, which becomes visible as a PN, its shape sculpted by the interaction between the old red giant envelope and a tenous, fast wind from the hot CS (Kwok, Purton & Fitzgerald 1978). PN 1 are an important, albeit brief (≤10 5 yr), evolution- ary phase in the lifetimes of a significant fraction of Milky Way stars. They are an important tool in our understanding of the physics of mass loss for intermediate-mass stars, the chemical enrichment of our Galaxy, and in turn, its star formation history. PN are ideal test particles to probe the dynamics of the Milky Way and are amongst the best kinematic tracers in external galaxies. The PN luminosity function is also a powerful extragalactic distance indicator (e.g. Ciardullo 2010, this issue). 1 For simplicity, we use PN as an abbreviation for both singular and plural forms. The total number of currently known Galactic PN is nearly 3000, which includes all known confirmed PN from the catalogues of Acker et al. (1992, 1996) and Kohoutek (2001), published PN found since 2000, including ∼1250 from the recent MASH catalogues (Parker et al. 2006; Miszalski et al. 2008), additional discoveries from the IPHAS survey (Viironen et al. 2009a, 2009b), the new faint PN found as part of the ongoing Deep Sky Hunters project (Kronberger et al. 2006; Jacoby et al. 2010, this issue), plus a modest number of additional PN gleaned from the literature. It is beyond the scope of this paper to give an exhaustive summary of our current state of knowledge concerning PN. For reviews on different aspects of the PN pheno- menon, the reader is referred to the works of Pottasch (1992), Balick & Frank (2002) and De Marco (2009), and the monograph by Kwok (2000). 2 Planetary Nebulae: a Working Definition There has been considerable controversy in the past over the working definition of a PN, based in part on the differing morphological, spectroscopic, and physical or evolutionary criteria used for classification, which have changed over the two centuries since their discovery. It is commonly argued that the PN moniker has a dis- tinct physical or evolutionary meaning, and should be restricted to the ionized gaseous shell ejected at the end of the AGB phase, either by a single star, or as part of a common-envelope ejection (De Marco 2009, and © Astronomical Society of Australia 2010 10.1071/AS09040 1323-3580/10/02129