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Planetary nebula distances re-examined: an improved statistical scale
The distances of planetary nebulae (PNe) are still quite uncertain.Although observational estimates are available for a small proportion ofPNe, based on statistical parallax and the like, such distances are verypoorly determined for the majority of galactic PNe. In particular,estimates of so-called `statistical' distance appear to differ byfactors of ~2.7.We point out that there is a well-defined correlation between the 5-GHzluminosity of the sources, L5, and their brightnesstemperatures, TB. This represents a different trend to thoseinvestigated in previous statistical analyses, and permits us todetermine independent distances to a further 449 outflows. Thesedistances are shown to be closely comparable to those determined using aTB-R correlation, providing that the latter trend is taken tobe non-linear.This non-linearity in the TB-R plane has not been noted inprevious analyses, and is likely responsible for the broad (andconflicting) ranges of distance that have previously been published.Finally, we point out that there is a close accord between observedtrends within the L5-TB and TB-Rplanes, and the variation predicted through nebular evolutionarymodelling. This is used to suggest that observational biases areprobably modest, and that our revised distance scale is reasonablytrustworthy.
| The relation between Zanstra temperature and morphology in planetary nebulae
We have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology.
| Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinates
We have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}
| The Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic Sample
Northern Galactic planetary nebulae (PNs) are studied to disclosepossible correlations between the morphology of the nebulae and theevolution of the central stars (CSs). To this end, we have built thebest database available to date, accounting for homogeneity andcompleteness. We use updated statistical distances and an updatedmorphological classification scheme, and we calculate Zanstratemperatures for a large sample of PNs. With our study we confirm thatround, elliptical, and bipolar PNs have different spatial distributionswithin the Galaxy, with average absolute distances to the Galactic planeof 0.73, 0.38, and 0.21 kpc, respectively. We also find evidence thatthe distributions of the CS masses are different across thesemorphological groups, although we do not find that CSs hosted by bipolarPNs are hotter, on average, than CSs within round and elliptical PNs.Our results are in broad agreement with previous analyses, indicatingthat round, elliptical, and bipolar PNs evolve from progenitors indifferent mass ranges and might belong to different stellar populations,as also indicated by the helium and nitrogen abundances of PNs ofdifferent morphology.
| 3-D ionization structure (in stereoscopic view) of planetary nebulae: the case of NGC 1501.
Long-slit echellograms of the high excitation planetary nebula NGC 1501,reduced according to the methodology developed by Sabbadin et al.(2000a,b), allowed us to obtain the "true" distribution of the ionizedgas in the eight nebular slices covered by the spectroscopic slit. A 3-Drendering procedure is described and applied, which assembles thetomographic maps and rebuilds the spatial structure. The images of NGC1501, as seen in 12 directions separated by 15o, form a series ofstereoscopic pairs giving surprising 3-D views in as many directions.The main nebula consists of an almost oblate ellipsoid of moderateellipticity (a ~= 44 arcsec, a/b ~= 1.02, a/c ~= 1.11), brighter in theequatorial belt, deformed by several bumps, and embedded in a quitehomogeneous, inwards extended cocoon. Some reliability tests are appliedto the rebuilt nebula; the radial matter profile, the small scaledensity fluctuations and the 2-D (morphology) -3-D (structure)correlation are presented and analysed. The wide applications of the 3-Dreconstruction to the morphology, physical conditions, ionizationparameters and evolutionary status of expanding nebulae in general(planetary nebulae, nova and supernova remnants, shells aroundPopulation I Wolf-Rayet stars, nebulae ejected by symbiotic stars,bubbles surrounding early spectral type main sequence stars etc.) areintroduced.
| Electron densities in planetary nebulae, and the unusual characteristics of the [S BT II] emission zone} ] densities in planetary nebulae
We investigate the radial variation of electron densities in planetarynebulae, using values of ne deriving from the [S ii]<~mbda6717/<~mbda6730 line ratio. As a result, we are able to showthat there is a sharp discontinuity in densities of order 1.4 dex closeto nebular radii R=0.1 pc. It is proposed, as a consequence, that mostnebulae contain two primary [S ii] emission zones, with densitiesdiffering by a factor ~ 10(2) . The intensity of emission from thedenser component increases by an order of magnitude where nebulae passfrom radiation to density-bound expansion regimes, resulting in acorresponding discontinuous jump in [S ii]/Hβ line ratios. Theorigins of these changes are not entirely clear, although one mechanismis investigated whereby the superwind outflows shock interact withexterior AGB envelopes. Finally, the derived trends in ne(R)are used to determine distances for a further 262 nebulae. The resultingdistance scale appears to be comparable to that of Daub (1982) and Cahnet al. (1992).
| On The Kinematics of Multiple-Shell Planetary Nebulae. I. Data and Expansion Velocities
We present spatially resolved echelle spectroscopy, obtained at highspectral resolution, for 15 multiple-shell planetary nebulae. Mostexhibit faint detached halos (IC 1295, MA 3, M 2-2, M 2-40, NGC 6804,NGC 6826, NGC 6884, NGC 6891, NGC 7662, PM 1-295, and Vy 2-3).Furthermore, we have included some with attached shells (IC 1454, K1-20, K 3-73, and PM 1-276) to allow comparison of the kinematicproperties of the two subclasses of multiple-shell planetary nebulae. Inaddition, some of the nebulae in our sample show a triple-shellstructure, composed of the bright main nebula and a combination of twoattached shells (PB 9), one attached shell and one detached halo (NGC6826, NGC 6891, NGC 7662, and Vy 2-3), or two detached shells (NGC6804). A new method for computing the expansion velocities of thoseshells that do not show line splitting has been developed. This methodassumes a thick-shell model and uses the observed Hα emissionbrightness profile to compute the volume emissivity dependence,ɛ(r), with the distance from the center of the nebula. Theexpansion velocity is then worked out by modeling how much the width ofa the Hα line decreases with the radius of the shell. The radialvelocity, expansion velocities of each shell, and turbulencecontribution to the line width are presented. The expansion velocity ofthe detached halos spans from 12 to 30 km s^-1. It is worth noting thatthe expansion velocities obtained by this method are greater than ifthey were computed with a thin-shell model, as has previously been done.In relation to the attached shells, their expansion velocities span from10 to 30 km s^-1. When the expansion velocities of the outer attachedshells are related to the ellipticity of the inner shells, a trendtoward faster expansion of the outer than the inner shells at higherellipticities is found. The turbulent contribution to the line width hasalso been established. It is smaller for halos (0 km s^-1 <=sigma_tur <= 6 km s^-1) than for attached shells (0 km s^-1 <=sigma_tur <= 15 km s^-1). This suggests that large-scale hydrodynamicprocesses are more important in attached shells than in detached halos.We have also studied the kinematics of the detached halos whosemorphology is perturbed from a round shape to a dipole asymmetry,indicating its interaction with the surrounding interstellar medium. Wefound systematic differences between the kinematical behavior of theenhanced edge of the halo and the opposite side in these cases, thusrevealing the kinematic effect of the interaction of the halos with theinterstellar medium.
| Planetary Nebulae in the NRAO VLA Sky Survey
The 1.4 GHz NRAO VLA Sky Survey (NVSS) images and source catalog wereused to detect radio emission from the 885 planetary nebulae north ofJ2000 declination delta = -40 deg in the Strasbourg-ESO Catalogue ofGalactic Planetary Nebulae. We identified 680 radio sources brighterthan about S = 2.5 mJy beam-1 (equivalent to T ~ 0.8 K in the 45" FWHMNVSS beam) with planetary nebulae by coincidence with accurate opticalpositions measured from Digitized Sky Survey (DSS) images. Totalextinction coefficients c at lambda = 4861 Angstroms were calculated forthe 429 planetary nebulae with available H beta fluxes and low free-freeoptical depths at 1.4 GHz. The variation of c with Galactic latitude andlongitude is consistent with the extinction being primarily interstellarand not intrinsic.
| A Morphological Study of Planetary Nebulae
We have produced simulated images of 110 planetary nebulae using theellipsoidal shell model. This process has allowed us to remove theprojection effects from the morphological classification of planetarynebulae and has provided quantitative measures of the intrinsicasymmetries of the nebulae. It is shown that the morphology of mostplanetary nebulae can be reproduced with pole-to-equator density ratiosof 0.1-1. Many planetary nebulae also show a modest departure from axialsymmetry. Contrary to previous findings by Khromov & Kohoutek, thesky orientation of planetary nebulae in this sample is consistent with apurely random distribution. Extremely bipolar nebulae (e.g., those ofbutterfly shape) point to a steep density profile in the AGB envelopeand are more likely to be type I (high helium and/or nitrogen abundance)nebulae. We found evidence that these nebulae are likely to have moremassive progenitors and are at a more advanced stage of dynamicalevolution.
| Properties That Cannot Be Explained by the Progenitors of Planetary Nebulae
I classify a large number of planetary nebulae (458) according to theprocess that caused their progenitors to blow axisymmetrical winds. Theclassification is based primarily on the morphologies of the differentplanetary nebulae, assuming that binary companions, stellar orsubstellar, are necessary in order to have axisymmetrical mass loss onthe asymptotic giant branch. I propose four evolutionary classes,according to the binary-model hypothesis: (1) Progenitors of planetarynebula that did not interact with any companion. These amount to ~10% ofall planetary nebulae. (2) Progenitors that interact with stellarcompanions that avoided a common envelope, 11^{+2}_{-3}% of all nebulae.(3) Progenitors that interact with stellar companions via a commonenvelope phase, 23^{+11}_{-5}% of all nebulae. (4) Progenitors thatinteract with substellar (i.e., planets and brown dwarfs) companions viaa common envelope phase, 56^{+5}_{-8}% of all nebulae. In order todefine and build the different classes, I start with clarifying somerelevant terms and processes related to binary evolution. I then discusskinematical and morphological properties of planetary nebulae thatappear to require the interaction of the planetary nebula progenitorsand/or their winds with companions, stellar or substellar.
| Radiation gasdynamics of planetary nebulae - VI. The evolution of aspherical planetary nebulae
This paper reports the results of the numerical study of the formationof aspherical planetary nebulae through the generalized interactingwinds model, taking into account the effects caused by the evolvingcentral star and fast wind. The results show for the first time thataspherical nebulae do form within the required time-scale. Considerationof the development of the nebula shows that in the early stages it isthe ionization of the aspherical AGB wind that contributes considerablyto the shaping of the nebula. Furthermore, the passing through of theionization front may modify the density distribution in the slow wind,leading to the formation of a surrounding envelope, and sometimes adifferent morphology for the nebula from that to be expected from theinitial conditions. I consider how the different phases of ionizationfronts and wind swept bubbles can be observationally distinguished.
| A statistical distance scale for Galactic planetary nebulae
A statistical distance scale is proposed. It is based on the correlationbetween the ionized mass and the radius and the correlation between theradio continuum surface brightness temperature and the nebular radius.The proposed statistical distance scale is an average of the twodistances obtained while using the correlation. These correlations,calibrated based on the 1`32 planetary nebulae with well-determinedindividual distances by Zhang, can reproduce not only the averagedistance of a sample of Galactic Bulge planetary nebulae exactly at thedistance to the Galactic center, but also the expected Gaussiandistribution of their distances around the Galactic center. This newdistance scale is applied to 647 Galactic planetary nebulae. It isestimated that this distance scale can be accurate on average to35%-50%. Our statistical distance scale is in good agreement with theone recently proposed by Van de Steene and Zijlstra. The correlationsfound in this study can be attributed to the fact that the core mass ofthe central stars has a very sharp distribution, strongly peaked atapprox. 0.6 solar mass. We stress that the scatter seen in thestatistical distance scale is likely to be real. The scatter is causedby the fact that the core mass distribution, although narrow andstrongly peaked, has a finite width.
| The gasdynamic evolution of spherical planetary nebulae. Radiation-gasdynamics of PNe III.
Using a radiation-gasdynamics code the evolution of spherical planetarynebulae is followed, while taking into account the evolution of centralstar and the fast wind. These models show the importance of ionizationfronts for the structure of planetary nebulae, especially for the socalled multiple shell nebulae (MSPN). It is shown that the outer shellis formed by the ionization front while the inner shell is swept-up bythe fast wind. These models explain the emission profiles of the outershells as well as their various kinematic properties. Because they areshaped by the ionization front these outer shells only give indirectinformation on the AGB mass loss history. The models indicate thattypical MSPN structures point to mass loss variations during the AGBphase. The ionization also leads to a stalling of the expansion of thenebula, leading to nebulae with expansion ages lower than theirevolutionary age. Values for ionized mass and Zanstra temperatures arederived from the models.
| A catalogue HeII 4686 line intensities in Galactic planetary nebulae.
We have compiled the intensities of the HeII 4686 lines measured inGalactic planetary nebulae. We present a few observational diagramsrelated to this parameter, and discuss them with the help of theoreticaldiagrams obtained from simple model planetary nebulae surroundingevolving central stars of various masses. We determine the hydrogen andhelium Zanstra temperature for all the objects with accurate enoughdata. We argue that, for Galactic planetary nebulae as a whole, the maincause for the Zanstra discrepancy is leakage of stellar ionizing photonsfrom the nebulae.
| Planetary nebulae: A modern view
Our current understanding of the origin and evolution of planetarynebulae is reviewed. We now recognized that a planetary nebularepresents a dynamical system in which evolution is tightly coupled tothe evolution of the central star. Not only is the ionization structureof the nebula controlled by the radiative output from the central star,the dynamics and morphology of the nebula are heavily influenced by themechanical energy output from the central star. Since the time scale ofevolution of the central star is strongly dependent on its mass, theextent of the radiative and mechanical interactions between the star andthe nebula not only vary with time, but also vary with stellar mass. Thephysical properties of planetary nebulae are discussed in this context.
| The features of chemical abundances in Galactic planetary nebulae
The chemical composition of 217 Galactic planetary nebulae isinvestigated; 203 of them are subdivided into four classes according tothe masses of nebulae and progenitor stars. The values of localabundances, Galactic abundance gradients and Galactic electrontemperature gradients are found for each class of nebula. Thecorrelations between the abundances of pairs of elements are alsocalculated for each class of nebula. The results are compared withtheoretical predictions. In particular, it is concluded that CN cyclingcan play a role in progenitor stars for all classes of planetary nebula.
| Astrophysical gasdynamics confronts reality - The shaping of planetary nebulae
We present two-dimensional numerical simulations, which use techniquesof radiation gasdynamics to simulate the structures of planetary nebulae(PNs). Our model incorporates realistic volume emissivities in order tofully account for the conversion of mechanical and thermal energy intoradiation. The model also produces detailed predictions of observablessuch as projected structure (e.g., H-alpha and forbidden N II images)and kinematic patterns. Virtually the full range of PN morphologies areeasily reproduced, as are the basic kinematics, ionization structures,and temperatures.
| Comparison of two methods for determining the interstellar extinction of planetary nebulae
A comparison of the planetary nebulae as derived from the Balmerdecrement and from the ratio of radio and H-beta fluxes is presented onthe basis of a compilation of all the relevant radio and opticalmeasurements and a serious selection of the best data. It is shown thatthe extinction determined from the Balmer decrement is systematicallylarger than the one derived from the radio data, the slope of the (Copt,Crad) relation being about 1.2. It is argued that, for most distantplanetary nebulae, the total to selection extinction is significantlylower than 3, the value corresponding to the standard extinction law forthe interstellar medium.
| The extinction constants for galactic planetary nebulae
The extinction constants are determined from Balmer decrementmeasurements for over 900 planetary nebulae. Comparison with publishedextinction constants shows that the results from ESO are fairlyreliable. An analysis of the extinction constants derived from theBalmer decrement and from the radio/Hβ flux ratio indicates thatthe latter tends to be systematically smaller than the former forincreasing extinction. We suggest that the radio measurements ofPottasch's group probably underestimate the radio fluxes, at least forsome (faintest) objects.
| A catalogue of absolute fluxes and distances of planetary nebulae
The paper presents a complete list of averaged recalibrated absoluteH-beta fluxes, global (where possible) relative He II lambda 4686fluxes, 5 GHz radio flux densities, and H-alpha/H-beta interstellarextinction constants for 778 Galactic planetary nebulae. The catalogprovides much of the fundamental data required to generate Zanstratemperatures. When data with the lowest errors are selected, the opticaland radio/optical extinctions show a peculiar correlation, with theradio values slightly high at low extinction and notably low at highextinction. The data are used, along with the best estimates of angulardiameters, to calculate Shklovsky distances according to the Daub schemeon the scale used earlier by Cahn and Kaler (1971). Use of this distancescale shows approximate equality of the death rates of optically thickand optically thin planetary nebulae. The method gives the correctdistances to the Magellanic Clouds.
| Stellar wind paleontology. II - Faint halos and historical mass ejection in planetary nebulae
The large, faint, generally circular, and limb-brightened nebularstructures (called "halos") surrounding some planetary nebulae (PN) areexplored using deep CCD images of NGC 40, 650-1, 1535, 2392, 6210, 6543,6720, 6803, 6804, 6826, 6853, 6891, 6894, 7009, 7662, IC 1454, 3568,4593, Abell 1, 2, 3, and BD +30 deg 3639. New halos have been discoveredin a few objects (IC 1454, 4593, and possibly NGC 40, 6210, and 6803),and known halos have been mapped in detail in several PN (e.g., NGC6543, 6720, 6826, 6853 and 7662). The present deep search does notreveal similar large and faint halos in NGC 1535, 2392, 6894, 7009, andIC 3568-PN whose inner regions are morphologically similar to otherswith easily observable halos.
| Strasbourg - ESO catalogue of galactic planetary nebulae. Part 1; Part 2
Not Available
| Hydrodynamical models of aspherical planetary nebulae
The emergence of these nebulae is accomplished with the 'Roe-solver'characteristic-based modeling technique extended by Eulderink (1990).The technique treats the Riemann problem with a numerical approximationthat linearizes the problem. The correct velocity is thereby found ifthe initial discontinuity is a pure contact discontinuity or a pureshock. The aspherical planetary nebulae can be modeled in terms ofaspherical bubbles by utilizing a density contrast in the remnants ofthe AGB wind. The analytical results of Icke (1988) are generallyconfirmed including the division of the shocks into spherical and'protruding' parts. A cusp is described that forms the transitionbetween these two parts and relates to a high degree of collimation inthe high-density flow. A wide range of morphologies can be formed in thenebulae if it is assumed that intermediate-mass stars lose mass in twosuccessive hydrodynamical stages.
| Chemical abundances in planetary nebulae - Basic data and correlations between elements
To form a critical compilation of the abundances of He, C, N, O and Ne,all the individual determinations of chemical abundances in PN obtainedwith linear detectors have been considered. The data cover allobservations of the Galactic PN made in the 20 years up to the end of1989. Average chemicql abundances have been evaluated separately fornebulae of type I and of types II-III of Peimbert. For the first, theyare He/H = 0.137, C/H = 8.72, N/H = 8.60, O/H = 8.66, and Ne/H = 8.05.For the second they are He/H = 0.103, C/H= 8.82, N/H = 8.07, O/H = 8.66,and Ne/H = 7.99, in the usual units. The data analysis suggests that theenrichment of nitrogen in type I PNs is due mostly to the ON cycle,while that in PNs of type II-III is due mostly to the CN cycle.
| Stellar wind paleontology - Shells and halos of planetary nebulae
The structures of PN shells and halos are studied in order to find cluesto the history of mass ejection in the AGB stage. Data are presentedwhich show that the emission measure profiles of shells of PN shareseveral common observational properties. Constraints are placed on theform of the mass ejection that formed the shell, the external pressurethat forces its postejection hydrodynamic evolution to the present, andthe evolutionary timescales. Hydrodynamical simulations conducted tomodel the emission measure distribution of the shells and halos of PNshow that mass is ejected in pulses whose duration is short compared tothe AGB-PN evolutionary timescale, and that the linear form of theemission measure occurs about 5000 yr after the superwind ejectionceases.
| Large planetary nebulae and their significance to the late stages of stellar evolution
Spectrophotometry of 75 large PNe with Shklovsky radii greater than 0.15pc is presented and used to calculate nebular parameters andcompositions, stellar Zanstra temperatures and luminosities, and coremasses. Nine new Peimbert type I nebulae are identified. About 40percent of the stars that are on cooling tracks are above 0.7 solarmass, and over 15 percent are above 0.8 solar mass. The largeplanetaries demonstrate a clear positive correlation between nitrogenenrichment and core mass. N/O is anticorrelated with O/H. The radii ofthe nebulae whose stars lie along specific cooling tracks increasemonotonically with decreasing central star temperature. For a givencentral temperature, the nebular radii also increase with increasingcore mass, showing that in this part of the log L-log T plane the highermass cores evolve more slowly in agreement with theoretical prediction.However, theoretical evolutionary rates for the large nebulae starsappear to be much too slow.
| A catalogue of VLA radio continuum observations of planetary nebulae with the Very Large Array
An extended VLA survey of about 300 planetary nebulae, ranging in sizefrom 4 arcsec to 8 arcmin, has been completed. The maps and the measuredparameters, such as radio flux density, diameter, and position, arepresented. The brightness temperatures, total far-infrared flux, andinfrared excess are derived. Newly measured optical positions are givenfor a few sources. A compilation of all published VLA data of planetarynebulae is also included. High-resolution radio synthesis observationsare now available for approximately 400 nebulae.
| A catalogue of expansion velocities of Galactic planetary nebulae
Published observational data on 288 Galactic PN are compiled in tables,graphs, and sketches based on spatiokinematical models and brieflycharacterized. The criteria used in selecting the data are discussed,and particular attention is given to the accuracies of the distanceestimates and their implications for theoretical models of PN or stellarevolution.
| The shapes and shaping of planetary nebulae.
Not Available
| The evolution of planetary nebulae. I - Structures, ionizations, and morphological sequences
An atlas of CCD pictures of fifty-one planetary nebulae (PNs) taken inthe light of low-, moderate-, and high-ionization emission lines ispresented. The shapes of many of the PNs can be organized into anempirical morphological sequence of round, elliptical, bipolar, andbutterfly classes. Many PNs, especially the ones of high surfacebrightness such as NGC 2392, 3242, 6543, 6826, and 7662, show evidencefor thin inclusions of anomalously low ionization in a high-ionizationsubstrate. The morphological sequence of PNs is shown to be consistentwith the precepts of the shaping of PNs by interacting winds. Thisconsistency places the concepts of ongoing hydrodynamic shaping of PNson firm grounds, and justifies further observations that can lead to abetter understanding of the physical processes. PNs are proposed as anideal astrophysical laboratory for studying the interactions of winds ona structured environment.
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