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 The structure of planetary nebulae: theory vs. practiceContext.This paper is the first in a short series dedicated to thelong-standing astronomical problem of de-projecting the bi-dimensional,apparent morphology of a three-dimensional mass of gas. Aims.Wefocus on the density distribution in real planetary nebulae (and alltypes of expanding nebulae). Methods. We introduce some basictheoretical notions, discuss the observational methodology, and developan accurate procedure for determining the matter radial profile withinthe sharp portion of nebula in the plane of the sky identified by thezero-velocity-pixel-column (zvpc) of high-resolution spectral images.Results. The general and specific applications of the method (andsome caveats) are discussed. Moreover, we present a series of evolutivesnapshots, combining illustrative examples of both model and trueplanetary nebulae. Conclusions. The zvpc radial-densityreconstruction - added to tomography and 3D recovery developed at theAstronomical Observatory of Padua (Italy) - constitutes a very usefultool for looking more closely at the spatio-kinematics, physicalconditions, ionic structure, and evolution of expanding nebulae. The Use of K_S Band Photometric Excesses to Investigate H(2) Emission in Planetary NebulaeWe have determined the distribution of H(2) emission in 14 planetarynebulae (PNe), using imaging and photometry published by the 2MASSinfrared survey. This technique is only applicable under certainstringent conditions, and requires precise broad band photometry, andaccurate spatial registration between the K_S and H band images. It is,in addition, only applicable to certain sources, and excludes outflowsin which central star and grain thermal excesses are appreciable. Ourresults for NGC 3132, NGC 6720, IC 4406 and M 2-9 are closely similar tothose of previous narrow band imaging, and confirm that H(2) emissionis confined to narrow, highly fragmented shells. Similar results areobtained for M 1-7, M 1-8, and M 3-5. Our spatial profiles also confirmthat the emission extends outside of the primary ionised shells. Whereenvelopes are large, and the PNe are more evolved, then the fractionalextensions Deltatheta/$theta appear to be at their smallest. They arealso similar to the radial widths predicted for H(2) abundanceprofiles, and to the values DeltaR/R determined throughmagnetohydrodynamic modelling of shocks. There appears, finally, to beevidence for an evolution in this parameter, such that Deltatheta/$thetavaries with increasing envelope size d(H) as Deltatheta/\$theta ~d(H)(-2.2) . Hot dust haloes in planetary nebulaeWe point out that many planetary nebulae (PNe) have large infraredindices (H-KS), and that this is likely to result from thepresence of hot grains, and/or H2 S(1) line emission atλ= 2.122μm. We are able to identify two groups of sourcesassociated with each of these mechanisms, both of which appear topossess distinct physical characteristics. One difference between thesegroups concerns the near-infrared dimensions of the sources. It appearsthat hot dust outflows frequently have sizes θ(KS)> θ(H) > θ(J). Four of the sources are particularlyextreme in this regard, and show emission extending well outside of theprimary shells. We propose that this is likely to arise fromhigh-temperature grains located in low-density haloes. The location ofsuch grains at large distances from the central stars represents achallenge for any mechanism purporting to explain this phenomenon. Themost likely explanation appears to be in terms of photon heating of verysmall grains. Oxygen Recombination Line Abundances in Gaseous NebulaeThe determination of the heavy element abundances from giantextragalactic H II regions has been generally based on collisionallyexcited lines. We will discuss the reasons to study the characteristicsof recombination lines, and then use these lines to determine chemicalabundances. Of these lines the oxygen (specifically the O II) lines arethe most important; and, of them, the lines of multiplet 1 of O II arethe most accessible. It has often been assumed that by measuring theintensity of a single line within a multiplet the intensities of all thelines in the multiplet can be determined; in recent studies we havefound that the intensity ratios of lines within a multiplet can dependon density; we will present empirical density-intensity relationshipsfor multiplet 1 based on recent observations of H II regions andplanetary nebulae. From observations of H II regions we find that thecritical density for collisional redistribution of the multiplet 1 O IIrecombination lines amounts to 2800+/-500 cm-3. We point out that theO/H recombination abundances of H II regions in the solar vicinity arein excellent agreement with the O/H solar value, while the abundancesderived from collisionally excited lines are not. We present acalibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8range based on O recombination lines. Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratioAirborne and space-based low-resolution spectroscopy in the 1980sdiscovered tantalizing quantitative relationships between the gas phaseC/O abundance ratio in planetary nebulae (PNe) and the fractions oftotal far-infrared (FIR) luminosity radiated by the 7.7- and 11.3-μmbands (the C = C stretch and C-H bend, respectively), of polycyclicaromatic hydrocarbons (PAHs). Only a very small sample of nebulae wasstudied in this context, limited by airborne observations of the7.7-μm band, or the existence of adequate IRAS Low ResolutionSpectrometer data for the 11.3-μm band. To investigate these trendsfurther, we have expanded the sample of planetaries available for thisstudy using Infrared Space Observatory (ISO) low-resolution spectrasecured with the Short Wavelength Spectrometer and the Long WavelengthSpectrometer. The new sample of 43 PNe, of which 17 are detected in PAHemission, addresses the range from C/O = 0.2-13 with the objective oftrying to delineate the pathways by which carbon dust grains might haveformed in planetaries. For the 7.7-μm and 11.3-μm bands, weconfirm that the ratio of band strength to total infrared (IR)luminosity is correlated with the nebular C/O ratio. Expressed inequivalent width terms, the cut-on C/O ratio for the 7.7-μm band isfound to be 0.6+0.2-0.4, in good accord with thatfound from sensitive ground-based measurements of the 3.3-μ band. The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edgeWe infer the gas kinematics, diagnostics and ionic radial profiles,distance and central star parameters, nebular photo-ionization model,spatial structure and evolutionary phase of the Planetary Nebula NGC6741 by means of long-slit ESO NTT+EMMI high-resolution spectra at nineposition angles, reduced and analysed according to the tomographic and3-D methodologies developed at the Astronomical Observatory of Padua(Italy). NGC 6741 (distance≃2.0 kpc, age≃ 1400 yr, ionizedmass Mion≃ 0.06 Mȯ) is a dense(electron density up to 12 000 cm-3), high-excitation,almost-prolate ellipsoid (0.036 pc × 0.020 pc × 0.018 pc,major, intermediate and minor semi-axes, respectively), surrounded by asharp low-excitation skin (the ionization front), and embedded in aspherical (radius≃ 0.080 pc), almost-neutral, high-density (n(HI)≃ 7 ×103 atoms cm-3) halo containinga large fraction of the nebular mass (Mhalo≥ 0.20Mȯ). The kinematics, physical conditions and ionicstructure indicate that NGC 6741 is in a deep recombination phase,started about 200 years ago, and caused by the rapid luminosity drop ofthe massive (M*=0.66{-}0.68 Mȯ), hot (logT* ≃ 5.23) and faint (logL*/Lȯ ≃ 2.75) post-AGB star, which hasexhausted the hydrogen-shell nuclear burning and is moving along thewhite dwarf cooling sequence. The general expansion law of the ionizedgas in NGC 6741, Vexp(km s-1)=13 × R arcsec,fails in the innermost, highest-excitation layers, which move slowerthan expected. The observed deceleration is ascribable to the luminositydrop of the central star (the decreasing pressure of the hot-bubble nolonger balances the pressure of the ionized gas), and appears instriking contrast to recent reports inferring that acceleration is acommon property of the Planetary Nebulae innermost layers. A detailedcomparative analysis proves that the "U"-shaped expansion velocity fieldis a spurious, incorrect result due to a combination of: (a) simplisticassumptions (spherical shell hypothesis for the nebula); (b) unfitreduction method (emission profiles integrated along the slit); and (c)inappropriate diagnostic choice (λ4686 Å of He II, i.e. athirteen fine-structure components recombination line). Some generalimplications for the shaping mechanisms of Planetary Nebulae arediscussed. The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their EnvelopesThe results of a study of the chemical compositions of Galacticplanetary nebulae taking into account two types of inhomogeneity in thenebular gas density in their envelopes are reported. New analyticalexpressions for the ionization correction factors have been derived andare used to determine the chemical compositions of the nebular gas inGalactic planetary nebulae. The abundances of He, N, O, Ne, S, and Arhave been found for 193 objects. The Y Z diagrams for various Heabundances are analyzed for type II planetary nebulae separately andjointly with HII regions. The primordial helium abundance Y p andenrichment ratio dY/dZ are determined, and the resulting values arecompared with the data of other authors. Radial abundance gradients inthe Galactic disk are studied using type II planetary nebulae. Helium recombination spectra as temperature diagnostics for planetary nebulaeElectron temperatures derived from the HeI recombination line ratios,designated Te(HeI), are presented for 48 planetary nebulae(PNe). We study the effect that temperature fluctuations inside nebulaehave on the Te(HeI) value. We show that a comparison betweenTe(HeI) and the electron temperature derived from the Balmerjump of the HI recombination spectrum, designated Te(HI),provides an opportunity to discriminate between the paradigms of achemically homogeneous plasma with temperature and density variations,and a two-abundance nebular model with hydrogen-deficient materialembedded in diffuse gas of a normal' chemical composition (i.e.~solar), as the possible causes of the dichotomy between the abundancesthat are deduced from collisionally excited lines and those deduced fromrecombination lines. We find that Te(HeI) values aresignificantly lower than Te(HI) values, with an averagedifference of = 4000 K. Theresult is consistent with the expectation of the two-abundance nebularmodel but is opposite to the prediction of the scenarios of temperaturefluctuations and/or density inhomogeneities. From the observeddifference between Te(HeI) and Te(HI), we estimatethat the filling factor of hydrogen-deficient components has a typicalvalue of 10-4. In spite of its small mass, the existence ofhydrogen-deficient inclusions may potentially have a profound effect inenhancing the intensities of HeI recombination lines and thereby lead toapparently overestimated helium abundances for PNe. Recombination Line versus Forbidden Line Abundances in Planetary NebulaeRecombination lines (RLs) of C II, N II, and O II in planetary nebulae(PNs) have been found to give abundances that are much larger in somecases than abundances from collisionally excited forbidden lines (CELs).The origins of this abundance discrepancy are highly debated. We presentnew spectroscopic observations of O II and C II recombination lines forsix planetary nebulae. With these data we compare the abundances derivedfrom the optical recombination lines with those determined fromcollisionally excited lines. Combining our new data with publishedresults on RLs in other PNs, we examine the discrepancy in abundancesderived from RLs and CELs. We find that there is a wide range in themeasured abundance discrepancyΔ(O+2)=logO+2(RL)-logO+2(CEL),ranging from approximately 0.1 dex (within the 1 σ measurementerrors) up to 1.4 dex. This tends to rule out errors in therecombination coefficients as a source of the discrepancy. Most RLsyield similar abundances, with the notable exception of O II multipletV15, known to arise primarily from dielectronic recombination, whichgives abundances averaging 0.6 dex higher than other O II RLs. Wecompare Δ(O+2) against a variety of physical propertiesof the PNs to look for clues as to the mechanism responsible for theabundance discrepancy. The strongest correlations are found with thenebula diameter and the Balmer surface brightness; high surfacebrightness, compact PNs show small values of Δ(O+2),while large low surface brightness PNs show the largest discrepancies.An inverse correlation of Δ(O+2) with nebular densityis also seen. A marginal correlation of Δ(O+2) is foundwith expansion velocity. No correlations are seen with electrontemperature, He+2/He+, central star effectivetemperature and luminosity, stellar mass-loss rate, or nebularmorphology. Similar results are found for carbon in comparing C II RLabundances with ultraviolet measurements of C III]. Physical conditions in Photo-Dissociation Regions around Planetary NebulaeWe present observations of the infrared fine-structure lines of [Si II](34.8 μm), [O I] (63.2 and 145.5 μm) and [C II] (157.7 μm)obtained with the ISO SWS and LWS spectrographs of nine PlanetaryNebulae (PNe). These lines originate in the Photo-Dissociation Regions(PDRs) associated with the nebulae and provide useful information on theevolution and excitation conditions of the ejected material in theseregions. In order to interpret the observations, the measured lineintensities have been compared with those predicted byphoto-dissociation models. This comparison has been done taking intoaccount the C/O content in the nebulae. The densities derived with thiscomparison show a large scatter for some nebulae, probably because thedensity is higher than the critical density. Therefore, they are nolonger sensitive to this parameter implying that transitions from otherspecies with higher critical density should be used. The possiblecontribution of shocks to the observed emission characteristics of thesePNe is briefly discussed and it is shown that the radiation field is themain driving force responsible for the atomic lines in the PNe that havebeen studied. In addition, data on the pure rotational lines ofH2 in three nebulae (NGC 7027, NGC 6302 and Hb 5) are alsopresented. Assuming local thermal equilibrium the rotational temperatureand densities have been derived. We have derived the mass of atomic gasin the PDR associated with these PNe and compared those to ionic massesderived from Hβ and molecular masses derived from low J COobservations. This comparison shows that for these nebulae, the PDR isthe main reservoir of gas surrounding these objects. A comparison of theresults of these evolved PNe with very young PNe from the literaturesuggests that as the nebula ages the relative amount of ionic gasincreases at the expense of the atomic and molecular mass.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http: / /cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/523 A reexamination of electron density diagnostics for ionized gaseous nebulaeWe present a comparison of electron densities derived from opticalforbidden line diagnostic ratios for a sample of over a hundred nebulae.We consider four density indicators, the [O II]λ3729/λ3726, [S II] λ6716/λ6731, [Cl III]λ5517/λ5537 and [Ar IV] λ4711/λ4740 doubletratios. Except for a few H II regions for which data from the literaturewere used, diagnostic line ratios were derived from our own high qualityspectra. For the [O II] λ3729/λ3726 doublet ratio, we findthat our default atomic data set, consisting of transition probabilitiesfrom Zeippen (\cite{zeippen1982}) and collision strengths from Pradhan(\cite{pradhan}), fit the observations well, although at high electrondensities, the [O II] doublet ratio yields densities systematicallylower than those given by the [S II] λ6716/λ6731 doubletratio, suggesting that the ratio of transition probabilities of the [OII] doublet, A(λ3729)/A(λ3726), given by Zeippen(\cite{zeippen1982}) may need to be revised upwards by approximately 6per cent. Our analysis also shows that the more recent calculations of[O II] transition probabilities by Zeippen (\cite{zeippen1987a}) andcollision strengths by McLaughlin & Bell (\cite{mclaughlin}) areinconsistent with the observations at the high and low density limits,respectively, and can therefore be ruled out. We confirm the earlierresult of Copetti & Writzl (\cite{copetti2002}) that the [O II]transition probabilities calculated by Wiese et al. (\cite{wiese}) yieldelectron densities systematically lower than those deduced from the [SII] λ6716/λ6731 doublet ratio and that the discrepancy ismost likely caused by errors in the transition probabilities calculatedby Wiese et al. (\cite{wiese}). Using our default atomic data set for [OII], we find that Ne([O II])  Ne([S II]) ≈Ne([Cl III])< Ne([Ar IV]). Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418We present high signal-to-noise ratio echelle spectra of the compacthigh surface brightness, low-ionization planetary nebula (PN) IC 418.These reveal 807 emission lines down to intensities less than10-5 that of Hβ for which we determine widths andrelative intensities. We show that line profiles are a valuableparameter for making line identifications and in constraining theexcitation mechanism of the lines. We present evidence that indicatesthat many supposed high-level recombination lines may in fact be excitedby a process other than recombination. We contend from the detection ofdielectronic recombination lines that their relatively low intensitiesargue against their making a significant contribution to levelpopulations of the heavy ions in this object. Following similar analysesof other PNe we find that IC 418 shows a small discrepancy in ionabundances derived from forbidden versus recombination lines of theheavy elements. A deep survey of heavy element lines in planetary nebulae - II. Recombination-line abundances and evidence for cold plasmaIn our Paper I, we presented deep optical observations of the spectra of12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe,carrying out an abundance analysis using the collisionally excitedforbidden lines. Here, we analyse the relative intensities of faintoptical recombination lines (ORLs) from ions of carbon, nitrogen andoxygen in order to derive the abundances of these ions relative tohydrogen. The relative intensities of four high-l CII recombinationlines with respect to the well-known 3d-4f λ4267 line are foundto be in excellent agreement with the predictions of recombinationtheory, removing uncertainties about whether the high C2+abundances derived from the λ4267 line could be due tonon-recombination enhancements of its intensity.We define an abundance discrepancy factor (ADF) as the ratio of theabundance derived for a heavy element ion from its recombination linesto that derived for the same ion from its ultraviolet, optical orinfrared collisionally excited lines (CELs). All of the PNe in oursample are found to have ADFs that exceed unity. Two of the PNe, NGC2022 and LMC N66, have O2+ ADFs of 16 and 11, respectively,while the remaining 13 PNe have a mean O2+ ADF of 2.6, withthe smallest value being 1.8.Garnett and Dinerstein found that for a sample of about 12 PNe themagnitude of the O2+ ADF was inversely correlated with thenebular Balmer line surface brightness. We have investigated this for alarger sample of 20 PNe, finding weak correlations with decreasingsurface brightness for the ADFs of O2+ and C2+.The C2+ ADFs are well correlated with the absolute radii ofthe nebulae, although no correlation is present for the O2+ADFs. We also find both the C2+ and O2+ ADFs to bestrongly correlated with the magnitude of the difference between thenebular [OIII] and Balmer jump electron temperatures (ΔT),corroborating a result of Liu et al. for the O2+ ADF.ΔT is found to be weakly correlated with decreasing nebularsurface brightness and increasing absolute nebular radius.There is no dependence of the magnitude of the ADF upon the excitationenergy of the ultraviolet, optical or infrared CEL transition used,indicating that classical nebular temperature fluctuations - i.e. in achemically homogeneous medium - are not the cause of the observedabundance discrepancies. Instead, we conclude that the main cause of thediscrepancy is enhanced ORL emission from cold ionized gas located inhydrogen-deficient clumps inside the main body of the nebulae, as firstpostulated by Liu et al. for the high-ADF PN, NGC 6153. We havedeveloped a new electron temperature diagnostic, based upon the relativeintensities of the OII 4f-3d λ4089 and 3p-3s λ4649recombination transitions. For six out of eight PNe for which bothtransitions are detected, we derive O2+ ORL electrontemperatures of <=300 K, very much less than the O2+forbidden-line and H+ Balmer jump temperatures derived forthe same nebulae. These results provide direct observational evidencefor the presence of cold plasma regions within the nebulae, consistentwith gas cooled largely by infrared fine-structure transitions; at suchlow temperatures, recombination transition intensities will besignificantly enhanced due to their inverse power-law temperaturedependence, while ultraviolet and optical CELs will be significantlysuppressed. Planetary nebula distances re-examined: an improved statistical scaleThe 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 distances of Type I planetary nebulaeThe distances D of planetary nebulae (PNe) are still extremelyuncertain. Although a variety of methods have been used to evaluate thisparameter, these are often in conflict, and subject to large random andsystematic errors. It is therefore important to evaluate D using as manyindependent procedures as possible. We outline here one further way inwhich this parameter may be assessed. It is noted that where the nebularmass range is narrow, then one might expect observed PNe radii to beroughly similar. This, where it occurs, would also result in acorrelation between their angular diameters Θ, and distances D.We find that just such a trend occurs for Type I nebulae, and we employthis to determine distances to a further 44 such outflows. Our meanvalues of D appear similar to those of Zhang [ApJS 98 (1995) 659],implying a relatively long PNe distance scale. A reanalysis of chemical abundances in galactic PNe and comparison with theoretical predictions New determinations of chemical abundances for He, N, O, Ne, Ar and Sare derived for all galactic planetary nebulae (PNe) so far observedwith a relatively high accuracy, in an effort to overcome differences inthese quantities obtained over the years by different authors usingdifferent procedures. These include: ways to correct for interstellarextinction, the atomic data used to interpret the observed line fluxes,the model nebula adopted to represent real objects and the ionizationcorrections for unseen ions. A unique good quality' classical-typeprocedure, i.e. making use of collisionally excited forbidden lines toderive ionic abundances of heavy ions, has been applied to allindividual sets of observed line fluxes in each specific position withineach PN. Only observational data obtained with linear detectors, andsatisfying some quality' criteria, have been considered. Suchobservations go from the mid-1970s up to the end of 2001. Theobservational errors associated with individual line fluxes have beenpropagated through the whole procedure to obtain an estimate of theaccuracy of final abundances independent of an author's prejudices'.Comparison of the final abundances with those obtained in relevantmulti-object studies on the one hand allowed us to assess the accuracyof the new abundances, and on the other hand proved the usefulness ofthe present work, the basic purpose of which was to take full advantageof the vast amount of observations done so far of galactic PNe, handlingthem in a proper homogeneous way. The number of resulting PNe that havedata of an adequate quality to pass the present selection amounts to131. We believe that the new derived abundances constitute a highlyhomogeneous chemical data set on galactic PNe, with realisticuncertainties, and form a good observational basis for comparison withthe growing number of predictions from stellar evolution theory. Owingto the known discrepancies between the ionic abundances of heavyelements derived from the strong collisonally excited forbidden linesand those derived from the weak, temperature-insensitive recombinationlines, it is recognized that only abundance ratios between heavyelements can be considered as satisfactorily accurate. A comparison withtheoretical predictions allowed us to assess the state of the art inthis topic in any case, providing some findings and suggestions forfurther theoretical and observational work to advance our understandingof the evolution of low- and intermediate-mass stars. A deep survey of heavy element lines in planetary nebulae - I. Observations and forbidden-line densities, temperatures and abundancesWe present deep optical spectrophotometry of 12 Galactic planetarynebulae (PNe) and three Magellanic Cloud PNe. Nine of the Galactic PNewere observed by scanning the slit of the spectrograph across thenebula, yielding relative line intensities for the entire nebula thatare suitable for comparison with integrated nebular fluxes measured inother wavelength regions. In this paper we use the fluxes ofcollisionally excited lines (CELs) from the nebulae to derive electrondensities and temperatures, and ionic abundances. We find that thenebular electron densities derived from optical CEL ratios aresystematically higher than those derived from the ratios of the infrared(IR) fine-structure (FS) lines of [OIII]. The latter have lower criticaldensities than the typical nebular electron densities derived fromoptical CELs, indicating the presence of significant density variationswithin the nebulae, with the IR CELs being biased towards lower densityregions.We find that for several nebulae the electron temperatures obtained from[OII] and [NII] optical CELs are significantly affected by recombinationexcitation of one or more of the CELs. When allowance is made forrecombination excitation, much better agreement is obtained with theelectron temperatures obtained from optical [OIII] lines. We alsocompare electron temperatures obtained from the ratio of optical nebularto auroral [OIII] lines with temperatures obtained from the ratio of[OIII] optical lines to [OIII] IR FS lines. We find that when the latterare derived using electron densities based on the [OIII]52 μm/88μm line ratio, they yield values that are significantly higher thanthe optical [OIII] electron temperatures. In contrast to this, [OIII]optical/IR temperatures derived using the higher electron densitiesobtained from optical [ClIII]λ5517/λ5537 ratios show muchcloser agreement with optical [OIII] electron temperatures, implyingthat the observed [OIII] optical/IR ratios are significantly weighted bydensities in excess of the critical densities of both [OIII] FS lines.Consistent with this, ionic abundances derived from [OIII] and [NIII] FSlines using electron densities from optical CELs show much betteragreement with abundances derived for the same ions from optical andultraviolet CELs than do abundances derived from the FS lines using thelower electron densities obtained from the observed [OIII]52 μm/88μm ratios. The behaviour of these electron temperatures, obtainedmaking use of the temperature-insensitive [OIII] IR FS lines, providesno support for significant temperature fluctuations within the nebulaebeing responsible for derived Balmer jump electron temperatures that arelower than temperatures obtained from the much more temperaturesensitive [OIII] optical lines. The relation between Zanstra temperature and morphology in planetary nebulaeWe 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 coordinatesWe 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} Angular dimensions of planetary nebulaeWe have measured angular dimensions of 312 planetary nebulae from theirimages obtained in Hα (or Hα + [NII]). We have appliedthree methods of measurements: direct measurements at the 10% level ofthe peak surface brightness, Gaussian deconvolution and second-momentdeconvolution. The results from the three methods are compared andanalysed. We propose a simple deconvolution of the 10% levelmeasurements which significantly improves the reliability of thesemeasurements for compact and partially resolved nebulae. Gaussiandeconvolution gives consistent but somewhat underestimated diameterscompared to the 10% measurements. Second-moment deconvolution givesresults in poor agreement with those from the other two methods,especially for poorly resolved nebulae. From the results of measurementsand using the conclusions of our analysis we derive the final nebulardiameters which should be free from systematic differences between small(partially resolved) and extended (well resolved) objects in our sample.Table 1 is only available in electronic form athttp://www.edpsciences.org The relation between elemental abundances and morphology in planetary nebulaeAn investigation of the variation of elemental abundances with planetarynebula morphology is of considerable interest, since it has a bearingupon how such sources are formed, and from which progenitors they areejected. Recent advances in morphological classification now enable usto assess such trends for a statistically significant number of sources.We find, as a result, that the distribution N[log(X/H)] of sources withrespect to elemental abundance (X/H) varies between the differingmorphologies. Circular sources tend to peak towards low abundancevalues, whilst bipolar nebulae (BPNe) peak towards somewhat highervalues. This applies for most elemental species, although it is perhapsleast apparent for oxygen. In contrast, elliptical sources appear todisplay much broader functions N[log(X/H)], which trespass upon thedomains of both circular and elliptical planetary nebulae (PNe).We take these trends to imply that circular sources derive fromlower-mass progenitors, bipolar sources from higher-mass stars, and thatelliptical nebulae derive from all masses of progenitor, high and low.Whilst such trends are also evident in values of mean abundance, they are much less clear. Only in the cases of He/H, N/H,Ne/H and perhaps Ar/H is there evidence for significant abundancedifferences.Certain BPNe appear to possess low abundance ratios He/H and Ar/H, andthis confirms that a few such outflows may arise from lower-massprogenitors. Similarly, we note that ratios are quite modestin elliptical planetary nebulae, and not much different from those forcircular and bipolar PNe; a result that conflicts with the expectationsof at least one model of shell formation. Knots in Planetary NebulaeWe have studied the closest bright planetary nebulae with the HubbleSpace Telescope's WFPC2 in order to characterize the dense knots alreadyknown to exist in NGC 7293. We find knots in all of the objects, arguingthat knots are common, simply not always observed because of distance.The knots appear to form early in the life cycle of the nebula, probablybeing formed by an instability mechanism operating at the nebula'sionization front. As the front passes through the knots they are exposedto the photoionizing radiation field of the central star, causing themto be modified in their appearance. This would then explain as evolutionthe difference of appearance like the lacy filaments seen only inextinction in IC 4406 on the one extreme and the highly symmetric`cometary" knots seen in NGC 7293. The intermediate form knots seen inNGC 2392, NGC 6720, and NGC 6853 would then represent intermediatephases of this evolution. Winds, Bubbles, and Outflows in Planetary NebulaeThe aim of this work is to highlight the contributions that John Dysonhas made to the study of the interstellar medium in general and, inparticular, to the field of planetary nebulae. I review a fewoutstanding problems regarding the formation and evolution of outflowsin planetary nebulae. The chemistry of compact planetary nebulaeWe report high-sensitivity millimetre observations of several molecularspecies (13CO, HCN, HNC, CN, HCO+ andN2H+) in a sample of compact planetary nebulae.Some species such as HCO+ and CN are particularly abundantcompared to envelopes around AGB stars or even interstellar clouds. Wehave estimated the following average values for the column densitiesratios: CN/HCN ~ 2.6, HCO+/HCN ~ 0.5, and HNC/HCN ~ 0.4.Thus, the chemical composition of the molecular envelopes in thesecompact PNe appears somewhat intermediate between the composition ofproto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe (such asthe Ring, M4-9, or the Helix). From observations of the CO isotopomers,we have estimated that the 12C/13C ratio is in therange 10 la 12C/13C <~ 40. These values arebelow those expected from standard asymptotic giant branch models andsuggest non-standard mixing processes. The observed molecular abundancesare compared to very recent modelling work, and we conclude that theobservations are well explained, in general terms, by time-dependentgas-phase chemical models in which the ionization rate is enhanced byseveral orders of magnitude with respect to the average interstellarvalue. Thus, our observations confirm that the chemistry in the neutralshells of PNe is essentially governed by the high energy radiation fromthe hot central stars. The complexity of the chemical processes isincreased by numerous factors linked to the properties of the centralstar and the geometry and degree of clumpiness of the envelope. Severalaspects of the PN chemistry that remains to be understood are discussedwithin the frame of the available chemical models. Based on observationscarried out with the IRAM 30m telescope. IRAM is supported by INSU/CNRS(France), MPG (Germany) and IGN (Spain). Photochemistry and Molecular Formation in Planetary Nebulae (invited review)Not Available Knots in Nearby Planetary NebulaeHST emission-line images of five of the arguably closest planetarynebulae have shown that there is a progression of characteristics oftheir knots. This progression begins with dark tangential structuresshowing no alignment with the central star and location near the mainionization front. At the end of the progression in the largest nebulae,the knots are located throughout much of the ionized zone, where theyare photoionized on the side facing the central star and accompanied bylong tails well aligned radially. This modification of characteristicsis what would be expected if the knots were formed near or outside themain ionization front, obtaining densities high enough to lead to theirbeing only partially ionized as they are fully illuminated by the Lymancontinuum (Lyc) radiation field. Their expansion velocities must belower than that of the main body of the nebular shell. Their forms arealtered by exposure to the radiation field from the star, although it isnot clear as to the relative role of radiation pressure acting on thedust component vis-à-vis ionization shadowing. The one objectthat does not fit into this sequence is NGC 2392, which is the mostcomplex nebula in our sample. In this case the inner part of the nebulais composed of a series of loops of material, some being ionizationbounded, which cover only a small fraction of the area illuminated bythe star. This complex structure may be what gives rise to the largevariations in electron temperature inferred from low spatial resolutionobservations. Cometary-form knots are seen in the outer part of thisobject, with these objects closely resembling those found in the largestnebula in our sample, NGC 7293. Based in part on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555. Classification of planetary nebulae by their departure from axisymmetryWe propose a scheme to classify planetary nebulae (PNe) according totheir departure from axisymmetric structure. We consider only departurealong and near the equatorial plane, i.e. between the two sidesperpendicular to the symmetry axis of the nebula. We consider six typesof departure from axisymmetry: (1) PNe where the central star is not atthe centre of the nebula; (2) PNe having one side brighter than theother; (3) PNe having unequal size or shape of the two sides; (4) PNewhere the symmetry axis is bent, e.g. the two lobes in a bipolar PN arebent toward the same side; (5) PNe where the main departure fromaxisymmetry is in the outer regions, e.g. an outer arc; and (6) PNe thatshow no departure from axisymmetry, i.e. any departure, if it exists, ison scales smaller than the scale of blobs, filaments and otherirregularities in the nebula. PNe that possess more than one type ofdeparture are classified by the most prominent type. We discuss theconnection between departure types and the physical mechanisms that maycause them, mainly resulting from the influence of a stellar binarycompanion. We find that ~50 per cent of all PNe in the analysed samplepossess large-scale departure from axisymmetry. This number is largerthan that expected from the influence of binary companions, namely~25-30 per cent. We argue that this discrepancy comes from many PNewhere the departure from axisymmetry, mainly unequal size, shape orintensity, results from the presence of long-lived and large (hot orcool) spots on the surface of their asymptotic giant branch progenitors.Such spots locally enhance the mass-loss rate, leading to a departurefrom axisymmetry, mainly near the equator, in the descendent PN. A possible observational measure of evolution in bipolar nebulaeBipolar planetary nebulae (BPNe) possess a broad range of shapes,ranging from narrow-waisted butterfly-like structures, through to thosewhich are more nearly cylindrical. We point out that these morphologiesappear to be correlated with radio surface brightness Tb, inthe sense that higher values of Tb are associated withenvelopes having narrower waists. If one interprets the variation inTb as arising from shell evolution, as is usually assumed forother planetaries, this would then imply that shell morphology varieswith time in a manner which appears not to be consistent withevolutionary models. It also remains possible, however, that differentBPNe morphologies arise as a result of differing mechanisms offormation, and that the pre-collimation of high velocity central windsgives rise to narrower waists, and higher surface brightness nuclei. Ourresults, if this is true, may then imply that central winds possess abroad range of collimations. First Detections of Molecular Gas Associated with the Wolf-Rayet Ring Nebula NGC 3199This paper presents the first observations of molecular gas associatedwith the Wolf-Rayet ring nebula NGC 3199 around the WR star WR 18. Thisincludes first observations of the molecules HCN, HCO+, CN,and HNC seen in any Wolf-Rayet ring nebula. Our observations immediatelysuggest the presence of high-density molecular gas (>104cm-3) in the nebula with significant amounts of associatedmolecular gas, which is in the form of clumpy ejecta and/or interstellarmaterial. Molecular CO gas was mapped across the optically brightportion of the nebula and out into the diffuse ionized component usingthe 12CO J=1-->0 line. CO gas is not seen within theoptically bright rim of NGC 3199 but adjacent to it. The opticalemission rim therefore appears to mark regions of photodissociation.Velocity components in the CO data are consistent with those seen inhigh-resolution optical spectra of the Hα line but extend beyondthe visible emission. A prior suggestion of the formation of the nebulavia a bow shock appears unlikely since Hipparcos measurements show theproper motion of WR 18 is almost at right angles to the directionrequired for the bow shock model. Instead, line splitting toward thenorth of the nebula suggests that a possible blowout of the Wolf-Rayetwind through surrounding ejecta may be responsible for some of thevelocity features observed. Preliminary estimates of molecularabundances in the nebula seen toward the central star are significantlyhigher than for the interstellar medium and are similar to those inplanetary nebulae, although CN is distinctly underabundant in comparisonto the very high values found in many planetary nebulae. The abundancesfound are consistent with the idea that at least a portion of themolecular material is associated with ejecta from the central star.Based on observations collected at the Swedish-ESO SubmillimetreTelescope (SEST) at the European Southern Observatory, La Silla, Chile.The Swedish-ESO Submillimetre Telescope is operated jointly by theEuropean Southern Observatory (ESO) and the Swedish National Facilityfor Radio Astronomy, Onsala Space Observatory, at Chalmers University ofTechnology. High Spectral and Spatial Resolution Observations of Shocked Molecular Hydrogen at the Galactic CenterThe presence of OH (1720 MHz) masers and the absence of counterparts at1665/1667 MHz has proved to be a clear diagnostic of shocked moleculargas associated with Galactic supernova remnants. This suggests thatshocked molecular gas should be associated with the OH (1720 MHz) masersthat have been detected in the circumnuclear disk (CND) and Sgr A Eastat the Galactic center. In order to test this hypothesis, we observedthe H2 1-0 S(1) and Brγ lines using NICMOS on theHubble Space Telescope (HST) and the University of New South WalesInfrared Fabry-Perot (UNSWIRF) etalon at the Anglo-Australian Telescope(AAT), near the regions where OH (1720 MHz) masers are detected in theCND and Sgr A East. We present the distribution of H2 in thenorth and south lobes of the CND and in Sgr A East. H2emission accompanies almost all the maser spots detected at the Galacticcenter. In particular, we find a striking filamentary structure near thenorthwest of the CND and evidence that shocked molecular gas isassociated with the 70 km s-1 molecular cloud at the Galacticcenter. We argue that the emission from the CND could arise in gasheated by the dissipation of the random motion of clumps by collisionsor the dissipation of turbulence in a more homogeneous medium. Inaddition, highly redshifted gas of up to 140 km s-1 close tothe eastern edge of the Sgr A East shell is detected. These observationscombined with OH (1720 MHz) results suggest that the H2 gasis shocked and accelerated by the expansion of Sgr A East into the 50and the 70 km s-1 clouds and into the lobes of the CND.
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