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Density gradients in Galactic planetary nebulae
Certain hydrodynamic models of planetary nebulae (PNe) suggest thattheir shells possess appreciable radial density gradients. However, theobservational evidence for such gradients is far from clear. On the onehand, Taylor et al. claim to find evidence for radio spectral indices0.6 < α < 1.8, a trend which is taken to imply a variationne ~ r-2 in most of their sample of PNe. On theother hand, Siódmiak & Tylenda find no evidence for any suchvariations in density; shell inhomogeneities, where they occur, areprimarily attributable to `blobs or condensations'.It will be suggested that both of these analyses are unreliable, andshould be treated with a considerable degree of caution. A new analysiswithin the log(F(5GHz)/F(1.4GHz))-log(TB(5GHz)) plane will beused to show that at least 10-20 per cent of PNe are associated withstrong density gradients. We shall also show that the ratioF(5GHz)/F(1.4GHz) varies with nebular radius; an evolution that can beinterpreted in terms of varying shell masses, and declining electrondensities.

s-Process Abundances in Planetary Nebulae
The s-process should occur in all but the lower mass progenitor stars ofplanetary nebulae, and this should be reflected in the chemicalcomposition of the gas that is expelled to create the current planetarynebula shell. Weak forbidden emission lines are expected from severals-process elements in these shells and have been searched for and insome cases detected in previous investigations. Here we extend thesestudies by combining very high signal-to-noise ratio echelle spectra ofa sample of PNe with a critical analysis of the identification of theemission lines of Z>30 ions. Emission lines of Br, Kr, Xe, Rb, Ba,and Pb are detected with a reasonable degree of certainty in at leastsome of the objects studied here, and we also tentatively identify linesfrom Te and I, each in one object. The strengths of these lines indicateenhancement of s-process elements in the central star progenitors, andwe determine the abundances of Br, Kr, and Xe, elements for which atomicdata relevant for abundance determination have recently becomeavailable. As representative elements of the ``light'' and ``heavy''s-process peaks, Kr and Xe exhibit similar enhancements over solarvalues, suggesting that PN progenitors experience substantial neutronexposure.

Planetary nebulae abundances and stellar evolution
A summary is given of planetary nebulae abundances from ISOmeasurements. It is shown that these nebulae show abundance gradients(with galactocentric distance), which in the case of neon, argon, sulfurand oxygen (with four exceptions) are the same as HII regions and earlytype star abundance gradients. The abundance of these elements predictedfrom these gradients at the distance of the Sun from the center areexactly the solar abundance. Sulfur is the exception to this; the reasonfor this is discussed. The higher solar neon abundance is confirmed;this is discussed in terms of the results of helioseismology. Evidenceis presented for oxygen destruction via ON cycling having occurred inthe progenitors of four planetary nebulae with bilobal structure. Theseprogenitor stars had a high mass, probably greater than 5 Mȯ. Thisis deduced from the high values of He/H and N/H found in these nebulae.Formation of nitrogen, helium and carbon are discussed. The high massprogenitors which showed oxygen destruction are shown to have probablydestroyed carbon as well. This is probably the result of hot bottomburning.

The structure of planetary nebulae: theory vs. practice
Context.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.

Chemical abundances in seven galactic planetary nebulae
An observational study of chemical abundances in the galactic planetarynebulae NGC 1535, NGC 2438,NGC 2440, NGC 3132, NGC3242, NGC 6302, and NGC7009 based on long-slit spectra of high signal-to-noise ratioin the 3100 to 6900 Å range is presented. We determined the N, O,Ne, S, and Cl abundances from collisionally excited lines and the He andO++ abundances from recombination lines. TheO++/H+ estimates derived from recombination linesare about a factor of four and two higher than those derived fromforbidden lines for NGC 7009 and NGC3242, respectively. Spatial profiles ofO++/H+ abundance from O II permitted lines andfrom [O III] forbidden lines were obtained for the planetary nebulaNGC 7009. The differences betweenO++/H+ derived from recombination and fromforbidden lines present smooth variations along the nebular surface ofNGC 7009, with the differences decreasing from thecenter to the edges of the nebula. If these abundance differences areexplained by the presence of electron temperature fluctuations,quantified by the parameter t2, a value of aboutt2=0.09 is required for NGC 3242 andNGC 7009.

Discovery of Multiple Coaxial Rings in the Quadrupolar Planetary Nebula NGC 6881
We report the discovery of multiple two-dimensional rings in thequadrupolar planetary nebula NGC 6881. As many as four pairs of ringsare seen in the bipolar lobes, and three rings are seen in the centraltorus. While the rings in the lobes have the same axis as one pair ofthe bipolar lobes, the inner rings are aligned with the other pair. Thetwo pairs of bipolar lobes are likely to be carved out by two separatehigh-velocity outflows from the circumstellar material left over fromthe asymptotic giant branch (AGB) wind. The two-dimensional rings couldbe the results of dynamical instabilities or the consequence of a fastoutflow interacting with remnants of discrete AGB circumstellar shells.

Electron temperature fluctuations in planetary nebulae
An observational study of the spatial variation of the electrontemperature and density in 10 galactic planetary nebulae is presented.The data consist of long-slit spectra of high signal-to-noise ratio inthe 3100 to 6900 Å range. Electron temperatures were determinedfrom the [O III](λ 4959 + λ 5007)/λ 4363 and [NII](λ 6548 + λ 6583)/λ 5755 ratios and from theBalmer discontinuity. Electron densities were estimated from the [SII]λ 6716/λ 6731, [Cl III]λ 5517/λ 5537, and[Ar IV]λ 4711/λ 4740 ratios. Electron temperaturevariations of low amplitude were found across the nebular surface in theplanetary nebulae studied. The temperature distribution across eachnebula presents a variance relative to the mean corresponding to 0.0003≤ t2s(Bal) ≤ 0.0078, 0.0003 ≤t2s(N II) ≤ 0.0097, and 0.0011 ≤t2s(O III) ≤ 0.0050. A systematic spatialvariation of electron density has been detected in most of objects(NGC 1535, NGC 2438, NGC2440, NGC 3132, NGC3242, NGC 6302, NGC6563, and NGC 7009). The remaining objects(NGC 6781 and NGC 6853) have notshown any significant electron density dependence on position.NGC 2438, NGC 6563, NGC6781, and NGC 6853 are in general the mostdiffuse and probably evolved objects studied here, with low meandensities in the range Ne(S II) ≈ 95-158~cm-3.An anti-correlation between temperature and density was found forNGC 2438 and NGC 3132, with theelectron temperature increasing with the decrease of electron densityand a correlation between temperature and density was found forNGC 2440, NGC 3242, NGC6302, and NGC 7009, with the electrontemperature increasing with the increase of electron density. Theserelationships seem to be associated with the structure of the nebula.The nebulae in which the correlation between temperature and density ispresent are ring shaped. The anti-correlation between temperature anddensity is found in bipolar planetary nebulae that are denser in thecentre of the nebula.

The Hubble-Type Outflows from the High-Excitation, Polypolar Planetary Nebula NGC 6302
Spatially resolved profiles of the Hα and [N II] lines have beenobtained at unprecedented signal-to-noise ratios over the outflowinglobes of the high-excitation, polypolar planetary nebula NGC 6302. Adeep image in the light of [N II] λ6584 was also obtained of theextremities of the prominent northwestern lobe. The Manchester EchelleSpectrometer combined with the 2.1 m San Pedro Martir telescope (Mexico)was used for these observations. First, an accurate value of thesystemic heliocentric radial velocity of Vsys=-29.8+/-1 kms-1 has been established. Also, from ``velocity ellipses''across its diameter from previous observations, the parallel-sidednorthwestern lobe is shown to have a circular section with a tilt of itsaxis to the plane of the sky of 12.8d. With this starting point theposition-velocity arrays of profiles have been very closely simulated,using the SHAPE code, with Hubble-type outflows. The faint extremitiesof the northwestern outflow are shown to be expanding at >=600 kms-1. The prominent lobes of NGC 6302 have then been generatedin an eruptive event with a dynamical age of 1900 yr for an expansionproper-motion distance of 1.04+/-0.16 kpc, as measured here by comparinga 1956 image with one taken in 2002. Kinematical evidence of ahigh-speed ``skirt'' around the nebular core, expanding nearlyorthogonally to the lobes, is also presented, as are the unusual motionsat the western extremities of the northwestern lobe.

Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio
Airborne 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 Multitude of Molecular Hydrogen Knots in the Helix Nebula
We present Hubble Space Telescope NICMOS imaging of the H22.12 μm emission in five fields in the Helix Nebula ranging in radialdistance from 250" to 450" from the central star. The images revealarcuate structures with their apexes pointing toward the central star.These molecular hydrogen knots are most highly structured in the fieldsclosest to the central star and become increasingly less structured withincreasing radius. Comparison of these images with ground-based imagesof comparable resolution reveals that the molecular gas is more highlyclumped than the ionized gas line tracers. From our images, we determinean average number density of knots in the molecular gas ranging from 162knots arcmin-2 in the denser regions to 18 knotsarcmin-2 in the lower density outer regions. The decreasingnumber density of H2 knots in the outer regions creates alower filling factor of neutral and molecular gas emission in the radioobservations of CO and H I and may explain why these outer regions,where we clearly detect H2 2.12 μm, fall below thedetection limit of the radio observations. Using this new numberdensity, we estimate the total number of knots in the Helix to be~23,000, which is a factor of 6.5 larger than previous estimates. Thetotal neutral gas mass in the Helix is 0.35 Msolar assuming amass of ~1.5×10-5 Msolar for the individualknots. The H2 emission structure of the entire Helix Nebulasupports the recent interpretation of the Helix as a nearly pole-onpolypolar planetary nebula (PN). The H2 intensity,(5-9)×10-5 ergs s-1 cm-2sr-1, remains relatively constant with projected distancefrom the central star, suggesting a heating mechanism for the moleculargas that is distributed almost uniformly in the knots throughout thenebula. The temperature and H2 2.12 μm intensity of theknots can be approximately explained by photodissociation regions (PDRs)in the individual knots; however, theoretical PDR models of PNsunderpredict the intensities of some knots by a factor of 10. Thebrightest H2 emission (~3×10-4 ergss-1 cm-2 sr-1) may be enhanced by alarger than unity area filling factor of H2 knots or may bean individual H2 knot exposed to direct starlight, causingrapid photoevaporation compared with the more embedded knots of thedisk.Based in part on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.

Fluorine Abundances in Planetary Nebulae
We have determined fluorine abundances from the [F II] λ4789 and[F IV] λ4060 nebular emission lines for a sample of planetarynebulae (PNe). Our results show that fluorine is generally overabundantin PNe, thus providing new evidence for the synthesis of fluorine inasymptotic giant branch (AGB) stars. [F/O] is found to be positivelycorrelated with the C/O abundance ratio, in agreement with thepredictions of theoretical models of fluorine production in thermallypulsing AGB stars. A large enhancement of fluorine is observed in theWolf-Rayet PN NGC 40, suggesting that high mass-loss rates probablyfavor the survival of fluorine.

The mean properties of planetary nebulae as a function of Peimbert class
Planetary nebulae are known to possess a broad range of abundances, andthese (with other characteristics) have been used to define five classesof outflow. Peimbert Type I sources, for instance, possess high N and Heabundances, filamentary structures, and low mean scaleheights above theGalactic plane, whilst those of Type III have much lower abundances,high peculiar velocities, and belong to the Galactic thick disc. Apartfrom some rather ill-defined indications, however, very little is knownconcerning their mean physical, spatial, structural, kinematic andthermal characteristics.We have performed a comprehensive study of all of these properties, andfind evidence for strong variations between the various Peimbertclasses. Certain of these differences are consistent with Type I sourceshaving the highest progenitor masses, although it seems that thesenebulae also possess the lowest rms densities and 5-GHz brightnesstemperatures. The latter results are in conflict with a range of recentmodelling.

The creation of the Helix planetary nebula (NGC 7293) by multiple events
A deep continuum-subtracted image of NGC 7293 has been obtained in thelight of the Hα+[N II] emission lines. New images of twofilamentary halo structures have been obtained, and a possible detectionof a collimated outflow made.Spatially resolved long-slit profiles of the Hα+[N II] lines havebeen observed across several of these features with the Manchesterechelle spectrometer combined with the San Pedro Martir 2.1-m telescope;these are compared with the [NII] 6584Å, [OIII] 5007Å, HeII6560Åand Hα profiles obtained over the nebular core.The central HeII emission originates in a ~0.34-pc diameter sphericalvolume expanding at <=12 km s-1, which is surrounded by,and partially coincident with, an [OIII] 5007 Åemitting innershell expanding at 12 km s-1. The bright helical structuresurrounding this inner region is modelled as a bipolar nebula with lobeexpansions of 25 km s-1 whose axis is tilted at 37° tothe sightline but with a toroidal waist itself expanding at 14 kms-1.These observations are compared with the expectations of the interactingtwo-winds model for the formation of planetary nebulae. Only after thefast wind has switched off could this global velocity structure begenerated. Ablated flows must complicate any interpretation.It is suggested that the clumpy nature of much of the material couldplay a part in creating the radial `spokes' shown here to be apparentlypresent close to the central star. These `spokes' could in fact be thepersistent tails of cometary globules whose heads have now completelyphotoevaporated.A halo arc projecting from the north-east of the bright core has acounterpart to the south-east. Anomolies in the position-velocity arraysof line profiles could suggest that these are part of an expanding discnot aligned with the central helical structure, although bipolar lobesexpanding along a tilted axis are not ruled out.

G313.3+00.3: A New Planetary Nebula Discovered by the Australia Telescope Compact Array and the Spitzer Space Telescope
We present a new planetary nebula, first identified in images from theAustralia Telescope Compact Array, although not recognized at that time.Recent observations with the Spitzer Space Telescope during the GLIMPSELegacy program have rediscovered the object. The high-resolution radioand infrared images enable the identification of the central star or itswind, the recognition of the radio emission as thermal, and the probablepresence of polycyclic aromatic hydrocarbons in and around the source.These lead to the conclusion that G313.3+00.3 is a planetary nebula.This object is of particular interest because it was discovered solelythrough radio and mid-infrared imaging, without any optical (ornear-infrared) confirmation, and acts as a proof of concept for thediscovery of many more highly extinguished planetary nebulae.G313.3+00.3 is well resolved by both the instruments with which it wasidentified and suffers extreme reddening due to its location in theScutum-Crux spiral arm.

The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edge
We 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.

Recombination Line versus Forbidden Line Abundances in Planetary Nebulae
Recombination 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].

Evolution from AGB to planetary nebula in the MSX survey
We investigate the evolution of oxygen- and carbon-rich AGB stars,post-AGB objects, and planetary nebulae using data collected mainly fromthe MSX catalogue. Magnitudes and colour indices are compared with thosecalculated from a grid of synthetic spectra that describe the post-AGBevolution beginning at the onset of the superwind. We find that carbonstars and OH/IR objects form two distinct sequences in the (K-[8.3])×([8.3]-[14.7]) MSX colour diagram. OH/IR objects are distributedin two groups: the bluest ones are crowded near [14.7]-[21.3]≃ 1and [8.3]-[14.7]≃ 2, and a second, redder group is spread over alarge area in the diagram, where post-AGB objects and planetary nebulaeare also found. High mass-loss rate OH/IR objects, post-AGB stars, andplanetary nebulae share the same region in the (K-[8.3])×([8.3]-[14.7]) and [14.7]-[21.3]×([8.3]-[14.7]) colour-colourdiagrams. This region in the diagram is clearly separated from a bluerone where most OH/IR stars are found. We use a grid of models ofpost-AGB evolution, which are compared with the data. The gap in thecolour-colour diagrams is interpreted as the result of the rapidtrajectory in the diagram of the stars that have just left the AGB.Based on results obtained by the MSX survey.Tables 1 to 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/565

Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418
We 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.

Unraveling the Helix Nebula: Its Structure and Knots
Through Hubble Space Telescope (HST) imaging of the inner part of themain ring of the Helix Nebula, together with CTIO 4 m images of thefainter outer parts, we have a view of unprecedented quality of thenearest bright planetary nebula. These images have allowed us todetermine that the main ring of the nebula is composed of an inner diskof about 499" diameter (0.52 pc) surrounded by an outer ring (in realitya torus) of 742" diameter (0.77 pc) whose plane is highly inclined tothe plane of the disk. This outer ring is surrounded by an outermostring of 1500" (1.76 pc) diameter, which is flattened on the sidecolliding with the ambient interstellar medium. The inner disk has anextended distribution of low-density gas along its rotational axis ofsymmetry, and the disk is optically thick to ionizing radiation, as isthe outer ring. Published radial velocities of the knots provide supportfor the two-component structure of the main ring of the nebula and forthe idea that the knots found there are expanding along with the nebularmaterial from which they recently originated. These velocities indicatea spatial expansion velocity of the inner disk of 40 and 32 kms-1 for the outer ring, which yields expansion ages of 6560and 12,100 yr, respectively. The outermost ring may be partially ionizedthrough scattered recombination continuum from the inner parts of thenebula, but shocks certainly are occurring in it. This outermost ringprobably represents a third period of mass loss by the central star.There is one compact, outer object that is unexplained, showing shockstructures indicating a different orientation of the gas flow from thatof the nebula. There is a change in the morphology of the knots as afunction of the distance from the local ionization front. This supportsa scenario in which the knots are formed in or near the ionization frontand are then sculpted by the stellar radiation from the central star asthe ionization front advances beyond them.Based in part on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.Based in part on observations obtained at the Cerro TololoInter-American Observatory, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under a CooperativeAgreement with the National Science Foundation.

A deep survey of heavy element lines in planetary nebulae - II. Recombination-line abundances and evidence for cold plasma
In 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 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.

Infrared Array Camera (IRAC) Observations of Planetary Nebulae
We present the initial results from the Infrared Array Camera (IRAC)imaging survey of planetary nebulae (PNs). The IRAC colors of PNs arered, especially in the 8.0 μm band. Emission in this band is likelydue to contributions from two strong H2 lines and a [Ar III]line in that bandpass. IRAC is sensitive to the emission in the halos aswell as in the ionized regions that are optically bright. In NGC 246, wehave observed an unexpected ring of emission in the 5.8 and 8.0 μmIRAC bands not seen previously at other wavelengths. In NGC 650 and NGC3132, the 8.0 μm emission is at larger distances from the centralstar compared to the optical and other IRAC bands, possibly related tothe H2 emission in that band and the tendency for themolecular material to exist outside of the ionized zones. In theflocculi of the outer halo of NGC 6543, however, this trend is reversed,with the 8.0 μm emission bright on the inner edges of the structures.This may be related to the emission mechanism, where the H2is possibly excited in shocks in the NGC 6543 halo, whereasH2 emission is likely fluorescently excited in the UV fieldsnear the central star.

On the O II Ground Configuration Energy Levels
The most accurate way to measure the energy levels for the O II2p3 ground configuration has been from the forbidden lines inplanetary nebulae. We present an analysis of modern planetary nebuladata that nicely constrain the splitting within the 2D termand the separation of this term from the ground4S3/2 level. We extend this method to H II regionsusing high-resolution spectroscopy of the Orion Nebula, covering all sixvisible transitions within the ground configuration. These data confirmthe splitting of the 2D term while additionally constrainingthe splitting of the 2P term. The energies of the2P and 2D terms relative to the ground(4S) term are constrained by requiring that all six linesgive the same radial velocity, consistent with independent limits placedon the motion of the O+ gas and the planetary nebula data.

The distances of Type I planetary nebulae
The 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.

Flux Ratio [Nev] 14.3/24.3 as a Test of Collision Strengths
From ISO [Nev] 14.3/24.3 μm line flux ratios, we find that 10 out of20 planetary nebulae (PNs) have measured ratios below the low-electrondensity (Ne) theoretical predicted limit. Such astronomicaldata serve to provide important tests of atomic data, collisionstrengths in this case. In principle, well-calibrated measurements ofthe [Nev] 14.3/24.3 flux ratio could improve upon the existing atomicdata.

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.

12C/13C Ratio in Planetary Nebulae from the IUE Archives
We investigated the abundance ratio of 12C/13C inplanetary nebulae by examining emission lines arising from C III2s2p3Po2,1,0-->2s21S0.Spectra were retrieved from the International Ultraviolet Explorerarchives, and multiple spectra of the same object were co-added toachieve improved signal-to-noise ratio. The 13C hyperfinestructure line at 1909.6 Å was detected in NGC 2440. The12C/13C ratio was found to be ~4.4+/-1.2. In allother objects, we provide an upper limit for the flux of the 1910Å line. For 23 of these sources, a lower limit for the12C/13C ratio was established. The impact on ourcurrent understanding of stellar evolution is discussed. The resultinghigh-signal-to-noise ratio C III spectrum helps constrain the atomicphysics of the line formation process. Some objects have the measured1907/1909 Å flux ratio outside the low-electron densitytheoretical limit for 12C. A mixture of 13C with12C helps to close the gap somewhat. Nevertheless, someobserved 1907/1909 Å flux ratios still appear too high to conformto the currently predicted limits. It is shown that this limit, as wellas the 1910/1909 Å flux ratio, are predominantly influenced byusing the standard partitioning among the collision strengths for themultiplet1S0-3PoJaccording to the statistical weights. A detailed calculation for thefine-structure collision strengths between these individual levels wouldbe valuable.

Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IV. Synthesis and the Sulfur Anomaly
We have compiled a large sample of O, Ne, S, Cl, and Ar abundances thathave been determined for 85 Galactic planetary nebulae in a consistentand homogeneous manner using spectra extending from 3600 to 9600Å. Sulfur abundances have been computed using the near-IR lines of[S III] λλ9069, 9532 along with [S III] temperatures. Wefind average values, expressed logarithmically with a standarddeviation, of log(S/O)=-1.91+/-0.24, log(Cl/O)=-3.52+/-0.16, andlog(Ar/O)=-2.29+/-0.18, numbers consistent with previous studies of bothplanetary nebulae and H II regions. We also find a strong correlationbetween [O III] and [S III] temperatures among planetary nebulae. Inanalyzing abundances of Ne, S, Cl, and Ar with respect to O, we find atight correlation for Ne-O, and loose correlations for Cl-O and Ar-O.All three trends appear to be colinear with observed correlations for HII regions. S and O also show a correlation, but there is a definiteoffset from the behavior exhibited by H II regions and stars. We suggestthat this S anomaly is most easily explained by the existence ofS+3, whose abundance must be inferred indirectly when onlyoptical spectra are available, in amounts in excess of what is predictedby model-derived ionization correction factors in PNe. Finally for thedisk PNe, abundances of O, Ne, S, Cl, and Ar all show gradients whenplotted against Galactocentric distance. The slopes are statisticallyindistinguishable from one another, a result which is consistent withthe notion that the cosmic abundances of these elements evolve inlockstep.

J320 (PN G190.3-17.7) as a poly-polar planetary nebula surrounded by point-symmetric knots
Spatially resolved, long-slit spectra obtained with the Manchesterechelle spectrometer of the planetary nebula J320 have been comparedwith Hubble Space Telescope and ground-based images. The morphology andkinematics of the nebular core have been convincingly modelled using theXSHAPE code.Two and possibly three bipolar lobes, expanding at ~ 46 kms-1 but with different axial orientations, have been found toconstitute the bright central nebula. These are found to be surroundedby two sets of point-symmetric, high-speed knots reminiscent of thosethat could be generated by a bipolar, rotating, episodic jet. Anotherisolated knot exists but without a counterpart.The sequence and nature of the periodic ejections that have given riseto this complex structure are considered in view of the most recenttheories of the shaping and evolution of planetary nebulae.

The 3-D ionization structure and evolution of NGC 7009 (Saturn Nebula)
Tomographic and 3-D analyses for extended, emission-line objects areapplied to long-slit ESO NTT + EMMI high-resolution spectra of theintriguing planetary nebula NGC 7009, covered at twelve position angles.We derive the gas expansion law, the diagnostics and ionic radialprofiles, the distance and the central star parameters, the nebularphoto-ionization model and the spatial recovery of the plasma structureand evolution. The Saturn Nebula (distance≃1.4 kpc, age≃6000yr, ionized mass≃0.18 Mȯ) consists of severalinterconnected components, characterized by different morphology,physical conditions, excitation and kinematics. We identify four``large-scale'', mean-to-high excitation sub-systems (the internalshell, the main shell, the outer shell and the halo), and as many``small-scale'' ones: the caps (strings of low-excitation knots withinthe outer shell), the ansae (polar, low-excitation, likely shockedlayers), the streams (high-excitation polar regions connecting the mainshell with the ansae), and an equatorial, medium-to-low excitationpseudo-ring within the outer shell. The internal shell, the main shell,the streams and the ansae expand at Vexp≃4.0 × Rarcsec km s-1, the outer shell, the caps and the equatorialpseudo-ring at Vexp≃3.15 × R arcsec kms-1, and the halo at Vexp≃10 kms-1. We compare the radial distribution of the physicalconditions and the line fluxes observed in the eight sub-systems withthe theoretical profiles coming from the photo-ionization code CLOUDY,inferring that all the spectral characteristics of NGC 7009 areexplainable in terms of photo-ionization by the central star, a hot (log T* ≃4.95) and luminous ( logL*/Lȯ≃3.70) 0.60-0.61Mȯ post-AGB star in the hydrogen-shell nuclear burningphase. The 3-D shaping of the Saturn Nebula is discussed within anevolutionary scenario dominated by photo-ionization and supported by thefast stellar wind: it begins with the superwind ejection (firstisotropic, then polar deficient), passes through the neutral, transitionphase ({lasting} ≃3000 yr), the ionization start (occurred≃2000 yr ago), and the full ionization of the main shell(≃1000 yr ago), at last reaching the present days: the wholenebula is optically thin to the UV stellar flux, except the caps (meanlatitude condensations in the outer shell, shadowed by the main shell)and the ansae (supersonic ionization fronts along the major axis).Based on observations made with: ESO Telescopes at the La SillaObservatories (program ID 65.I-0524), and the NASA/ESA Hubble SpaceTelescope, obtained from the data archive at the Space TelescopeInstitute. Observing programs: GO 6117 (P.I. Bruce Balick), GO 6119(P.I. Howard Bond) and GO 8390 (P.I. Arsen Hajian). STScI is operated bythe association of Universities for Research in Astronomy, Inc. underthe NASA contract NAS 5-26555. We extensively apply the photo-ionizationcode CLOUDY, developed at the Institute of Astronomy of the CambridgeUniversity (Ferland et al. 1998).

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Right ascension:07h41m54.91s
Apparent magnitude:11

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