Progenitors Of Planetary Nebulae Research Articles

Whither or Wither the Sulfur Anomaly in Planetary Nebulae?

We present a thorough investigation of the long-standing sulfur anomaly enigma. Our analysis uses chemical abundances from the most extensive data set available for 126 planetary nebulae (PNs) with improved accuracy and reduced uncertainties from a 10° × 10° Galactic bulge region. By using argon as a superior PN metallicity indicator, the anomaly is significantly reduced and better constrained. For the first time in PNs we show sulfur α-element lockstep with both oxygen and argon. We dispel hypotheses that the anomaly originates from underestimation of higher sulfur ionization stages. Using a machine-learning approach, we show that earlier ionization correction factor schemes contributed significantly to the anomaly. We find a correlation between the sulfur anomaly and the age/mass of PN progenitors, with the anomaly either absent or significantly reduced in PNs with young progenitors. Despite inherent challenges and uncertainties, we link this to PN dust chemistry, noting those with carbon-dust chemistry show a more pronounced anomaly. By integrating these findings, we provide a plausible explanation for the residual, reduced sulfur anomaly and propose its potential as an indicator of relative galaxy age compositions based on PNs.

Point-symmetry in SNR G1.9+0.3: A Supernova that Destroyed its Planetary Nebula Progenitor

I analyze a new X-ray image of the youngest supernova remnant (SNR) in the Galaxy, which is the type Ia SNR G1.9+0.3, and reveal a very clear point-symmetrical structure. Since explosion models of type Ia supernovae (SNe Ia) do not form such morphologies, the point-symmetrical morphology must come from the circumstellar material (CSM) into which the ejecta expands. The large-scale point-symmetry that I identify and the known substantial deceleration of the ejecta of SNR G1.9+0.3 suggest a relatively massive CSM of ≳1M ⊙. I argue that the most likely explanation is the explosion of this SN Ia into a planetary nebula. The scenario that predicts a large fraction of SN Ia inside PNe (SNIPs) is the core degenerate scenario. Other SN Ia scenarios might lead to only a very small fraction of SNIPs or none at all.

Morphologies and Central Stars of Planetary Nebulae in the Galactic bulge from VLT, HST and Pan-STARRS imaging

ABSTRACT This is the first in a series of papers that present sets of different results for 136 compact, known planetary nebulae within a 10 × 10 degree region of the Galactic bulge. We use a large, previously unpublished sample of our own extensive ESO 8-m VLT deep imaging and spectroscopic data. This is combined with archival deep Hubble Space Telescope imaging where available to provide a detailed morphological classification and study. The influence of angular resolution and sensitivity when assigning a morphology is discussed. A large fraction (68 per cent) of the sample are shown to be bipolars and the implications for this in the context of planetary nebulae progenitors are explored. Four new planetary nebula central stars are also identified which are not in Gaia. This is based on both Very Large Telescope (VLT) and deep archival Pan-STARRS broad-band imagery. Some 11 putative central stars previously reported, based on Gaia positions, are also not the true central star. In other cases, the Gaia central stars reported in the literature are actually based on the overall centroid position of a very compact planetary nebula rather than the actual central star within it. Gaia parallax distances and kinematic ages for PNe in this sample are provided where possible based on fresh angular size measures from the new VLT imagery and Gaia distances and literature expansion velocities when available. All these results are discussed within the context of the overall characteristics of the Galactic bulge and its planetary nebula population.

N/O abundance ratio and the progenitor mass for the most luminous planetary nebulae of M 31

Context. Plasma diagnostics form the basis of investigation into the physico-chemical properties of line-emitting gaseous systems. Aims. To perform plasma diagnostics properly, it is essential to accurately correct the input spectrum for extinction because determining the degree of extinction is dependent on the physical properties of the line-emitting gas. Hence, both extinction correction and plasma diagnostics have to be performed simultaneously and self-consistently. Methods. By comparing the results of analyses performed for a sample of nine bright planetary nebulae in M 31, both with and without the fully iterative self-consistent simultaneous extinction correction and plasma diagnostics, we demonstrate how a seemingly benign initial assumption of the physical conditions of the line-emitting gas in extinction correction could compromise the results of the entire analyses in terms of the extinction, electron density and temperature, and ionic and elemental abundances. Results. While the electron density and temperature are relatively immune to the imposed inconsistent assumptions, the compromised extinction would cause systematic offsets in the extinction-corrected spectrum, and hence, line strengths, which consequently would impose adverse effects on the resulting ionic and elemental abundances, and other inferences made from the incorrect results. Conclusions. We find that this M 31 PN sample simply represents those around the high-mass end of the mass range for low-mass planetary nebula progenitor stars as expected from the existing theoretical models. It appears that the suspicion raised in the previous study – that these PNe could be anomalously nitrogen-overabundant for the expected progenitor mass range – is simply caused by the apparent underestimation in extinction that originates from the imposed inconsistent assumptions in extinction correction. In a larger context, the results of plasma diagnostics in the literature that are done without seeking simultaneous self-consistency with extinction corrections have to be handled with caution. Ideally, such previous results should be re-evaluated by always seeking simultaneous self-consistency.

On the most luminous planetary nebulae of M 31

Context.The planetary nebula luminosity function (PNLF) is a standard candle that comprises a key rung on the extragalactic distance ladder. The method is based on the empirical evidence that the luminosity function of planetary nebulae (PNe) in the [O III]λ5007 nebular emission line reaches a maximum value that is approximately invariant with population age, metallicity, or host galaxy type. However, the presence of bright PNe in old stellar populations is not easily explained by single-star evolutionary models.Aims.To gain information about the progenitors of PNe at the tip of the PNLF, we obtained the deepest existing spectra of a sample of PNe in the galaxy M 31 to determine their physico-chemical properties and infer the post-asymptotic giant branch (AGB) masses of their central stars (CSs). Precise chemical abundances allow us to confront the theoretical yields for AGB stellar masses and metallicities expected at the bright end of the PNLF. Central star masses of the sampled PNe provide direct information on the controversial origin of the universal cutoff of the PNLF.Methods.Using the OSIRIS instrument at the 10.4 m Gran Telescopio Canarias (GTC), optical spectra of nine bright M 31 PNe were obtained: four of them at the tip of the PNLF, and the other five some 0.5 mag fainter. A control sample of 21 PNe with previous GTC spectra from the literature is also included. We analyze their physical properties and chemical abundances (He, N, O, Ar, Ne, and S), searching for relevant differences between bright PNe and the control samples. The CS masses are estimated with Cloudy modeling using the most recent evolutionary tracks.Results.The studied PNe show a remarkable uniformity in all their nebular properties, and the brightest PNe show relatively large electron densities. Stellar characteristics also span a narrow range: ⟨L*/L⊙⟩ = 4300 ± 310, ⟨Teff⟩ = 122 000 ± 10 600 K for the CSs of the four brightest PNe, and ⟨L*/L⊙⟩ = 3300 ± 370, ⟨Teff⟩ = 135 000 ± 26 000 K for those in the control set. This groups all the brightest PNe at the location of maximum temperature in the post-AGB tracks for stars with initial massesMi = 1.5M⊙.Conclusions.These figures provide robust observational constraints for the stellar progenitors that produce the PNLF cutoff in a star-forming galaxy such as M 31, where a large range of initial masses is in principle available. Inconsistency is found, however, in the computed N/O abundance ratios of five nebulae, which are 1.5 to 3 times larger than predicted by the existing nucleosynthesis models for stars of these masses.

Shaping of Planetary Nebulae by Exoplanets

(1) Background: We investigate the hypothesis that exoplanet engulfment can help explain the observed non-spherical planetary nebula population, as a complementary shaping mechanism to the binary hypothesis. The aim is to investigate the extent to which massive planets can explain the population of non-spherical planetary nebulae; (2) Methods: This research utilises a new tool to calculate the planet-fraction of planetary nebulae progenitor stars called simsplash; (3) Results: we conclude that ∼15–30% of non-spherical planetary nebulae around single stars will have a history in which they engulfed a massive planet on the AGB; and (4) Conclusions: Engulfment of massive exoplanets may contribute significantly to the formation of non-spherical planetary nebulae around single stars, yet appears to be insufficient to explain them all.

WORKPLANS: Workshop on Planetary Nebula Observations

This workshop is the second of the WORKPLANS series, which we started in 2016. The main goal of WORKPLANS is to build up a network of planetary nebulae (PNe) experts to address the main open questions in the field of PNe research. The specific aims of the WORKPLANS workshop series are (i) to discuss and prioritize the most important topics to be investigated by the PN community in the following years; (ii) to establish a network of excellent researchers with complementary expertise; (iii) to formulate ambitious observing proposals for the most advanced telescopes and instrumentation presently available (ALMA, SOFIA, VLT, GTC, HST, etc.), addressing those topics; and (iv) to develop strategies for major proposals to future observatories (JWST, ELT, SPICA, Athena, etc.). To achieve these goals, WORKPLANS II brought together experts in all key sub-areas of the PNe research field, namely: analysis and interpretation of PNe observational data; theoretical modeling of gas and dust emission; evolution from Asymptotic Giant Branch stars (PNe progenitors) to PNe; and the instrumentation and technical characteristics of the relevant observatories.

Physical properties of the fluorine and neutron-capture element-rich PN Jonckheere 900

ABSTRACT We performed detailed spectroscopic analyses of a young C-rich planetary nebula (PN) Jonckheere 900 (J900) in order to characterize the properties of the central star and nebula. Of the derived 17 elemental abundances, we present the first determination of eight elemental abundances. We present the first detection of the [F iv] 4059.9 Å, [F v] 13.4 μm, and [Rb iv] 5759.6 Å lines in J900. J900 exhibits a large enhancement of F and neutron-capture elements Se, Kr, Rb, and Xe. We investigated the physical conditions of the H2 zone using the newly detected mid-IR H2 lines while also using the previously measured near-IR H2 lines, which indicate warm (∼670 K) and hot (∼3200 K) temperature regions. We built the spectral energy distribution (SED) model to be consistent with all the observed quantities. We found that about 67 per cent of all dust and gas components (4.5 × 10−4 M⊙ and 0.83 M⊙, respectively) exists beyond the ionization front, indicating the critical importance of photodissociation regions in understanding stellar mass loss. The best-fitting SED model indicates that the progenitor evolved from an initially ∼2.0 M⊙ star that had been in the course of the He-burning shell phase. Indeed, the derived elemental abundance pattern is consistent with that predicted by an asymptotic giant branch star nucleosynthesis model for a 2.0 M⊙ star with Z = 0.003 and partial mixing zone mass of 6.0 × 10−3 M⊙. Our study demonstrates how accurately determined abundances of C/F/Ne/neutron-capture elements and gas/dust masses help us understand the origin and internal evolution of the PN progenitors.

Semiregular Variable Stars

The studies of semiregular variables of stars by different authors are considered, and the main theoretical and observational problems associated with these stars are reviewed. Their evolutionary status and possible connection with long-period variables such as Mira Ceti are discussed. Individual objects belonging to different types of semiregular variables are described in detail. After leaving the main sequence, the stars pass through the region of instability of Cepheids, turning into radially pulsating variables of type \delta Cephei. These stars can be associated with semiregular variables giants and supergiants of spectral classes F-K, which are usually denoted by the symbol SRd. In the process of further evolution of the variables of high luminosity fall in the region of red supergiants, becoming the type variables SRc (or Lc), and the variables lower luminosity turn into a semiregular variables SRab (or wrong Lb) of late spectral classes. Variables of the RV Tau type are a class of low-mass (with masses of the order of one solar) pulsating F-K-supergiants, which may be at the short-term evolutionary stage of transition from the red giant to the protoplanetary nebula, which explains the small number of stars of this type of variability. Shklovsky (1956) was the first to point to stars of this type as the progenitors of planetary nebulae.

The evolution of Galactic planetary nebula progenitors through the comparison of their nebular abundances with AGB yields

We study the chemical abundances of a wide sample of 142 Galactic planetary nebulae (PNe) with good quality observations, for which the abundances have been derived more or less homogeneously, thus allowing a reasonable comparison with stellar models. The goal is the determination of mass, chemical composition and formation epoch of their progenitors, through comparison of the data with results from AGB evolution. The dust properties of PNe, when available, were also used to further support our interpretation. We find that the majority ($\sim60\%$) of the Galactic PNe studied has nearly solar chemical composition, while $\sim40\%$ of the sources investigated have sub-solar metallicities. About half of the PNe have carbon star progenitors, in the $1.5~M_{\odot} < M < 3~M_{\odot}$ mass range, which have formed between 300 Myr and 2 Gyr ago. The remaining PNe are almost equally distributed among PNe enriched in nitrogen, which we interpret as the progeny of $M > 3.5~M_{\odot}$ stars, younger than 250 Myr, and a group of oxygen-rich PNe, descending from old ($> 2$ Gyr) low-mass ($M < 1.5~M_{\odot}$) stars that never became C-stars. This analysis confirms the existence of an upper limit to the amount of carbon which can be accumulated at the surface of carbon stars, probably due to the acceleration of mass loss in the late AGB phases. The chemical composition of the present sample suggests that in massive AGB stars of solar (or slightly sub-solar) metallicity, the effects of third dredge up combine with hot bottom burning, resulting in nitrogen-rich - but not severely carbon depleted - gaseous material to be ejected.

Identification of Near-infrared [Se iii] and [Kr vi] Emission Lines in Planetary Nebulae∗

Abstract We identify [Se iii] 1.0994 μm in the planetary nebula (PN) NGC 5315 and [Kr vi] 1.2330 μm in three PNe from spectra obtained with the Folded-Port InfraRed Echellette (FIRE) spectrometer on the 6.5 m Baade Telescope. Se and Kr are the two most widely detected neutron-capture elements in astrophysical nebulae, and can be enriched by s-process nucleosynthesis in PN progenitor stars. The detection of [Se iii] 1.0994 μm is particularly valuable when paired with observations of [Se iv] 2.2864 μm, as it can be used to improve the accuracy of nebular Se abundance determinations, and allows Se ionization correction factor (ICF) schemes to be empirically tested for the first time. We present new effective collision strength calculations for Se2+ and Kr5+, which we use to compute ionic abundances. In NGC 5315, we find that the Se abundance computed from Se3+/H+ is lower than that determined with ICFs that incorporate Se2+/H+. We compute new Kr ICFs that take Kr5+/H+ into account, by fitting correlations found in grids of Cloudy models between Kr ionic fractions and those of more abundant elements, and use these to derive Kr abundances in four PNe. Observations of [Se iii] and [Kr vi] in a larger sample of PNe, with a range of excitation levels, are needed to rigorously test the ICF prescriptions for Se and our new Kr ICFs.

Molecular studies of Planetary Nebulae

AbstractCircumstellar envelopes (CEs) around evolved stars are an active site for the production of molecules. After evolving through the Asymptotic Giant Branch (AGB), proto-planetary nebula (PPN), to planetary nebula (PN) phases, CEs ultimately merge with the interstellar medium (ISM). The study of molecules in PNe, therefore, is essential to understanding the transition from stellar to interstellar materials. So far, over 20 molecular species have been discovered in PNe. The molecular composition of PNe is rather different from those of AGB and PPNe, suggesting that the molecules synthesized in PN progenitors have been heavily processed by strong ultraviolet radiation from the central star. Intriguingly, fullerenes and complex organic compounds having aromatic and aliphatic structures can be rapidly formed and largely survive during the PPN/PN evolution. The similar molecular compositions in PNe and diffuse clouds as well as the detection of C60+ in the ISM reinforce the view that the mass-loss from PNe can significantly enrich the ISM with molecular species, some of which may be responsible for the diffuse interstellar bands. In this contribution, I briefly summarize some recent observations of molecules in PNe, with emphasis on their implications on circumstellar chemistry.

The kinematics of the nebular shells around low mass progenitors of PNe with low metallicity

AbstractIn the past few years we provided strong observational support for theoretical studies regarding the internal kinematics of Planetary Nebulae (PNe). A total of 257 objects segregated by different galactic populations were analized. Based upon spatially-resolved, long-slit, echelle spectroscopy drawn from the San Pedro Mártir Kinematic Catalogue of PNe†, we characterized the kinematics of PNe shells measuring their global expansion velocities. We present here a brief summary of these observational results, with a focus on our most recent study of about 26 PNe with low metallicity that appear to derive from progenitor stars of the lowest masses (including the halo PNe population). Low expansion velocities were found for these nebulae, less than 20 km s−1, which are most likely associated with a weak central star wind driving the kinematics of the nebular shell in this particular population.

Understanding Galactic planetary nebulae with precise/reliable nebular abundances

AbstractWe compare recent precise/reliable nebular abundances - as derived from high-quality optical spectra and the most recent ICFs - in a sample of Galactic planetary nebulae (PNe) with nucleosynthesis predictions (HeCNOCl) from asymptotic giant branch (AGB) ATON models in the metallicity range Z⊙/4 &lt; Z &lt; 2Z⊙. According to the infrared dust features, the sample is divided among carbon-, oxygen-, and double-dust chemistry (CC, OC, and DC, respectively), providing an independent proxy for the nature of the PNe progenitors. Our AGB models, with diffusive overshooting from all the convective borders, nicely reproduce the O overabundances observed in CC PNe, indicating that they evolve from low-Z low-mass (∼1 −3 M⊙) AGB stars. This indicates that O is not always a good indicator of the original ISM metallicity and that the O production by low-mass stars should be considered in galactic-evolution models. The lowest metallicity OC PNe evolve from low-mass (∼1 M⊙) O-rich AGBs, while the higher metallicity ones (all with uncertain dust classifications) display a chemical pattern similar to the DC PNe. In agreement with the recent literature, the DC PNe mostly descend from high-mass (M &gt; 3.5 M⊙) solar/supersolar metallicity AGBs that experience hot bottom burning (HBB), but other formation channels in low-mass AGBs like extra mixing, stellar rotation, binary interaction, or He pre-enrichment cannot be disregarded until more accurate C/O ratios can be obtained. Two DC PNe show the imprint of advanced CNO processing and deep second dredge-up, suggesting progenitors masses close to the limit to evolve as core collapse supernovae (above 6 M⊙). Their actual C/O ratios, if confirmed, indicate contamination from the third dredge-up, rejecting the hypothesis that the chemical composition of such high-metallicity massive AGBs is modified exclusively by HBB.

Planetary nebula progenitors that swallow binary systems

I propose that some irregular `messy' planetary nebulae owe their morphologies to triple-stellar evolution where tight binary systems evolve inside and/or on the outskirts the envelope of asymptotic giant branch (AGB) stars. In some cases the tight binary system can survive, in other it is destroyed. The tight binary system might breakup with one star leaving the system. In an alternative evolution, one of the stars of the brook-up tight binary system falls toward the AGB envelope with low specific angular momentum, and drowns in the envelope. In a different type of destruction process the drag inside the AGB envelope causes the tight binary system to merge. This releases gravitational energy within the AGB envelope, leading to a very asymmetrical envelope ejection, with an irregular and `messy' planetary nebula as a descendant. The evolution of the triple-stellar system can be in a full common envelope evolution (CEE) or in a grazing envelope evolution (GEE). Both before and after destruction (if take place) the system might lunch pairs of opposite jets. One pronounced signature of triple-stellar evolution might be a large departure from axisymmetrical morphology of the descendant planetary nebula. I estimate that about one in eight non-spherical PNe is shaped by one of these triple-stellar evolutionary routes.

Oxygen enrichment in carbon-rich planetary nebulae

We study the relation between the chemical composition and the type of dust present in a group of 20 Galactic planetary nebulae (PNe) that have high quality optical and infrared spectra. The optical spectra are used, together with the best available ionization correction factors, to calculate the abundances of Ar, C, Cl, He, N, Ne, and O relative to H. The infrared spectra are used to classify the PNe in two groups depending on whether the observed dust features are representative of oxygen-rich or carbon-rich environments. The sample contains one object from the halo, eight from the bulge, and eleven from the local disc. We compare their chemical abundances with nucleosynthesis model predictions and with the ones obtained in seven Galactic H II regions of the solar neighbourhood. We find evidence of O enrichment (by $\sim$ 0.3 dex) in all but one of the PNe with carbon-rich dust (CRD). Our analysis shows that Ar, and especially Cl, are the best metallicity indicators of the progenitors of PNe. There is a tight correlation between the abundances of Ar and Cl in all the objects, in agreement with a lockstep evolution of both elements. The range of metallicities implied by the Cl abundances covers one order of magnitude and we find significant differences in the initial masses and metallicities of the PNe with CRD and oxygen-rich dust (ORD). The PNe with CRD tend to have intermediate masses and low metallicities, whereas most of the PNe with ORD show higher enrichments in N and He, suggesting that they had high-mass progenitors.

The light element abundance distribution in NGC 5128 from planetary nebulae

The light element abundance pattern from many planetary nebulae (PNe) covering the upper 4 mag. of the [O III] luminosity function was observed with ESO VLT FORS1 multi-slit. Spectra of 51 PNe over the wavelength range 3500-7500 Angstrom were obtained in three fields at 4, 8 and 17 kpc, for a distance of 3.8 Mpc. Emission line ratios are entirely typical of PN such as in the Milky Way. The temperature sensitive [O III]4363A line was weakly detected in 10 PNe, both [O II] and [O III] lines were detected in 30 PNe, and only the bright [O III]5007A line in 7 PN. Cloudy photoionization models were run to match the spectra by a spherical, constant density nebula ionized by a black body central star. He, N, O and Ne abundances with respect to H were determined and, for brighter PNe, S and Ar; central star luminosities and temperatures are also derived. For 40 PNe with Cloudy models, from the upper 2 mag. of the luminosity function, the most reliably estimated element, oxygen, has a mean 12+log(O/H) of 8.52. No obvious radial gradient is apparent in O/H over a range 2-20 kpc. Comparison of the PN abundances with the stellar population, from the spectra of the integrated starlight on the multi-slits and photometric studies, suggests [Fe/H]=-0.4 and [O/Fe]=0.25. The masses of the PN central stars in NGC 5128 from model tracks imply an epoch of formation more recent than for the minority young population from colour-magnitude studies. The PNe progenitors may belong to the young tail of a recent, minor, star formation episode or derive from other evolutionary channels.[Abridged]

Low-Resolution Spectroscopy of Hot Post-AGB Candidates II. LS, LSS, LSE Stars and Additional IRAS Sources

Hot (OB) post-asymptotic giant branch (post-AGB) stars are immediate progenitors of planetary nebulae (PNe). Very few hot post-AGB stars are known. Detecting new hot post-AGB candidates and follow-up multiwavelength studies will enable us to further understand the processes during the post-AGB evolution that lead to the formation of PNe. Case-Hamburg OB star surveys and their extension (LS, LSS, and LSE catalogues) and IRAS (point source) catalogues are good sources for detecting new hot post-AGB candidates from low-resolution spectroscopy. Spectral types are determined from low-resolution optical spectra of 44 stars selected from the LS, LSS, and LSE catalogues. Unlike the stars mentioned in the first paper, the stars discussed in this paper were selected using criteria other than positional coincidence with an IRAS source with far-IR (IRAS) colours similar to post-AGB supergiants and planetary nebulae. These included high galactic latitude, spectral types of O, B, A supergiants, emission lines in the spectrum and known spectral peculiarity. From the present study we find that LSS 1179, LSS 1222, LSS 1256, LSS 1276, LSS 1341, LSS 1394, LSS 2241, LSS 2429, LSS 4560, LSE 3, LSE 16, LSE 42, and LSE 67 to be new hot post-AGB candidates. Further studies of these candidates are needed.

Understanding the immediate progenitors of planetary nebulae

AbstractPre-Planetary Nebulae (PPNe) are believed to represent a relatively short, intermediate evolutionary phase in the evolution of AGB stars to Planetary Nebulae (PNe). Our unbiased, high-resolution imaging surveys with HST of young PNe and PPNe show very strong morphological similarities between these classes, enabling us to extend our morphological scheme for PPN classification to young PNe, preserving virtually all of the primary and secondary descriptors, and adding a few new ones. These morphological surveys show that the primary shaping of PNe begins during the PPNe and/or late-AGB phase, and the key to understanding the shaping process lies in the study of these PNe progenitors. Here we present results from two recent studies of PNe progenitors: (i) Hα emission with very broad wings and P-Cygni profiles as probes of the region where the fast post-AGB outflows that do the shaping are most likely launched from, and (ii) equatorial waists with large-sized dust grains and large masses.

YOUNG PLANETARY NEBULAE:HUBBLE SPACE TELESCOPEIMAGING AND A NEW MORPHOLOGICAL CLASSIFICATION SYSTEM

Using Hubble Space Telescope images of 119 young planetary nebulae, most of which have not previously been published, we have devised a comprehensive morphological classification system for these objects. This system generalizes a recently devised system for pre-planetary nebulae, which are the immediate progenitors of planetary nebulae (PNs). Unlike previous classification studies, we have focussed primarily on young PNs rather than all PNs, because the former best show the influences or symmetries imposed on them by the dominant physical processes operating at the first and primary stage of the shaping process. Older PNs develop instabilities, interact with the ambient interstellar medium, and are subject to the passage of photoionization fronts, all of which obscure the underlying symmetries and geometries imposed early on. Our classification system is designed to suffer minimal prejudice regarding the underlying physical causes of the different shapes and structures seen in our PN sample, however, in many cases, physical causes are readily suggested by the geometry, along with the kinematics that have been measured in some systems. Secondary characteristics in our system such as ansae indicate the impact of a jet upon a slower-moving, prior wind; a waist is the signature of a strong equatorial concentration of matter, whether it be outflowing or in a bound Keplerian disk, and point symmetry indicates a secular trend, presumably precession, in the orientation of the central driver of a rapid, collimated outflow.