Planetary Nebulae: Observational Properties, Mimics, and Diagnostics

TL;DR

This paper reviews the recent increase in known planetary nebulae due to wide-field imaging surveys, discusses methods for distinguishing true nebulae from mimics, and explores the heterogeneity and multiple origins of PNe.

Contribution

It provides an updated overview of PN discovery techniques, improved diagnostic criteria, and insights into the diverse evolutionary scenarios of planetary nebulae.

Findings

Nearly 3000 PNe now known in the Milky Way, doubling previous counts.

Improved discrimination between true PNe and mimics using multiwavelength data.

Evidence suggests PNe are heterogeneous with multiple formation pathways.

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.