Revista Mexicana de Astronom´ıa y Astrof´ısica, 50, 203–223 (2014) THE PRESENT AND FUTURE OF PLANETARY NEBULA RESEARCH. A WHITE PAPER BY THE IAU PLANETARY NEBULA WORKING GROUP K. B. Kwitter1, R. H. M´endez2, M. Pe˜na3, L. Stanghellini4, R. L. M. Corradi5,6, O. De Marco7, X. Fang8,9, R. B. C. Henry10, A. I. Karakas11, X.-W. Liu8,9, J. A. L´opez12, A. Manchado5,6, and Q. A. Parker7 Received 2014 January 22; accepted 2014 March 21 RESUMEN Se presenta un resumen del estado actual de la investigaci´on sobre las nebulosas planetarias y sus estrellas centrales, y temas relacionados como los procesos at´omicos en nebulosas ionizadas, y la evoluci´on de las estrellas de la rama gigante asint´otica y post-gigante asint´otica. Se discuten los avances futuros que ser´an necesarios para incrementar sustancialmente nuestro conocimiento en este campo. ABSTRACT We present a summary of current research on planetary nebulae and their central stars, and related subjects such as atomic processes in ionized nebulae, AGB and post- AGB evolution. Future advances are discussed that will be essential to substantial improvements in our knowledge in the field. Key Words: ISM: abundances — planetary nebulae: general — stars: AGB and post- AGB — stars: evolution 1. INTRODUCTION and easy to identify; their emission lines are very strong and can be used to derive C, N, O abundances that in The field of planetary nebula (PN) research has turn characterize the stellar progenitor by comparison continued to mature, incorporating a variety of new of the spectra with the yields from stellar evolution the- and ingenious approaches that leverage the value of PN ory. PNe can evolve in binary systems and their mor- observations to maximize astrophysical insight. PNe phology and chemical history reflect this type of evolu- are the gaseous relics of the evolution of low- and tion. They can survive in the intracluster medium, and intermediate-mass stars, and as such are ubiquitous in are rare probes of this interesting environment. Extra- the Galaxy and beyond. They are probes of stellar evo- galactic PNe have been studied to disclose a charac- lution, populations, gas dynamics, dust and molecules. teristic luminosity function whose high-luminosity cut- They are also extragalactic probes of metallicity and off appears invariant (or almost invariant) across PN dynamics in spiral and elliptical galaxies and in the in- populations, providing a good standard candle. After tracluster medium. Spectra of PNe are characteristic decades of searching in a variety of celestial objects, re- 1 searchers have observed fullerenes in PNe, the first en- Department of Astronomy, Williams College, Williamstown, MA, USA. vironment of stellar origin where these molecules have 2Institute for Astronomy, University of Hawaii, Honolulu, been observed. USA. 3Instituto de Astronom´ıa, Universidad Nacional Aut´onoma de M´exico, Mexico. 4National Optical Astronomy Observatory, Tucson AZ, USA. As this brief list reveals, the modern study of PNe is 5Instituto de Astrof´ısica de Canarias, La Laguna, Tenerife, extremely fruitful, with many connections to adjacent Spain. fields of research, including stellar structure and evolu- 6 Departamento de Astrof´ısica, Universidad de La Laguna, La tion, binary stars, stellar populations, radial metallic- Laguna, Tenerife, Spain. 7Department of Physics & Astronomy, Macquarie University, ity gradients in spiral galaxies and their evolution, as Sydney, Australia. well as galaxy rotation, evolution, merging, and cos- 8Department of Astronomy, School of Physics, Peking Uni- mology. This White Paper by the IAU PN Working versity, Beijing, China. Group, represents a summary of our science activities, 9Kavli Institute for Astronomy and Astrophysics, Peking Uni- versity, Beijing, China. and is an attempt to set the stage for future develop- 10H.L. Dodge Department of Physics and Astronomy, Univer- ments in the field. Its aims are to raise interest and sity of Oklahoma, USA. 11 to spark discussion within the international PN com- Research School of Astronomy & Astrophysics, Mt. Stromlo munity, as well as across other interested communities, Observatory, Weston Creek, Australia. 12Instituto de Astronom´ıa, Universidad Nacional Aut´onoma and finally, to serve as preparation for the next PN de M´exico, Ensenada, B.C., Mexico. Symposium several years hence. 203 204 KWITTER ET AL. 2. PERSPECTIVE ON PLANETARY NEBULA identification techniques now available are briefly de- DETECTION AT NON-OPTICAL scribed below. WAVELENGTHS 2.1.1. Eliminating non-PN contaminants 2.1. The dawn of true multi-wavelength imaging of Recently Frew & Parker (2010) tested and de- PNe: Discovery, refinement and characterisation veloped criteria to more effectively eliminate con- Over the last decade Galactic PN discoveries have taminants using the online availability of new multi- entered a golden age due to the emergence of high wavelength surveys combined with emission line ratios sensitivity, high resolution narrow-band surveys of the from follow-up spectroscopy. Galactic plane (e.g., Parker et al. 2005; Drew et al. Applying these criteria to mid-IR samples of pre- 2005). These have been coupled with access to comple- viously known optically detected Galactic PNe seen in 14 ◦ mentary, deep, multi-wavelength surveys across near- GLIMPSE and that overlap with the SHS (|b| ≤1 ◦ ◦ ◦ IR, mid-IR and radio regimes, in particular from both and from 210 (through 360 ) to 40 in Galactic longi- ground-based and space-based telescopes and have pro- tude) showed that 45% of previously known pre-MASH vided additional new powerful diagnostic and discovery PNe in this zone are in fact H II region contaminants capabilities. (Cohen et al. 2011). Independently applying these cri- The total number of Galactic PNe known is cur- teria to the MASH PNe revealed a contaminant frac- rently ≈3500, more than double what it was a decade tion of 5% in the same zone, largely because similar ago. This is largely thanks to the ≈1200 significant discrimination techniques had already been applied to PN discoveries uncovered by the two MASH13 surveys MASH. Furthermore, the external filaments, extended (Parker et al. 2006; Miszalski et al. 2008) based on structures and/or amorphous halos that are seen in scrutiny of the SuperCOSMOS AAO/UKST Hα sur- the mid-IR associated with apparently discrete emis- vey of the Southern Galactic plane (SHS). sion sources in the optical generally indicate that the Note that significant numbers of true PNe are at the object is an H II region. This is an important mid-IR very faint, highly evolved end as shown to exist in the discriminatory diagnostic for resolved mid-IR sources. local volume sample of Frew (2008), but they rapidly It has also been recently demonstrated that PN 15 become undetectable at distances greater than a few mid-IR/radio flux ratios and the Spitzer IRAC colour kpc. There are also serious problems with obtaining indices are robust attributes, invariant among PN truly representative samples of PNe across the galaxy types including those in nearby external galaxies of dif- due to variable extinction. ferent metallicity such as the Large Magellanic Cloud It is also clear that a significant population of (LMC) (Cohen et al. 2011). The median PN mid- Galactic PNe must be lurking behind the extensive IR/radio ratio is 4.7±1.1 and does not vary signifi- clouds of gas and dust that obscure large regions of cantly with PN evolutionary phase, allowing clear sep- our view across the optical regime. Indeed, it is the aration from their major H II region contaminants, re- extension of previous PN discovery techniques away gardless of whether they are diffuse or ultra-compact. from the optically dominant [O III] PN emission line These mid-infrared colours and radio fluxes are at in un-reddened spectra to the longer wavelength Hα wavelengths minimally affected by dust obscuration. emission line (that can peer at least partially through The recent advent of the near-infrared Vista Vari- 16 the dust), that has led to the major discoveries of the ables in the Via Lactea (VVV) and UKIRT In- previous decade. frared Deep Sky surveys of the Galactic plane have Consequently, as part of the mission to improve the ≈4 mag of improved depth and far better resolution inventory of Galactic PNe, extension of PN identifica- compared to 2MASS (Two Micron All Sky Survey) tion techniques to longer, more favourable wavelengths and these, too, offer improved prospects for PN stud- would be advantageous. This is now possible with the ies over these wavelengths. The judicious combination advent of powerful, new, multi-wavelength sky-surveys of the J,H,Ks bands into pseudo-colour images can of high sensitivity and resolution. Such surveys now offer a powerful visual aid as to nature when taken allow us to both find and accurately characterise the together with the individual near-infrared band pho- properties of PNe across a broader range evolution- tometry. The importance and prospects of the new ary state and across a wider range of the electromag- mid-IR sky surveys to PN studies are summarised in netic spectrum than ever before, opening up new win- the following section. dows into their physical characteristics and dust prop- 2.1.2. New mid-IR sky surveys and their application erties in particular. Such broader perspectives have to PN science also given us the capability not only to uncover more PNe can be quite strong mid-IR emitting objects PNe but also to refine identification of the many mimics because of PAH (polycyclic aromatic hydrocarbon) that still lurk in existing PN catalogues. The improved 14Galactic Legacy Infrared Mid-Plane Survey Extraordinaire. 13Macquarie/AAO/Strasbourg Hα Planetary Galactic Cata- 15InfraRed Array Camera. log. 16United Kingdom Infrared Telescope. PRESENT AND FUTURE OF PLANETARY NEBULA RESEARCH 205 emission, fine structure lines, high excitation mid-IR distributions across a broad wavelength range from all lines like [O IV] 25.89 µm, H2 molecular lines (e.g.