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# NGC 6781

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 Tiny bat vs mighty Eagle.Not Available Two new evolved bipolar planetary nebulae in the solar neighbourhoodWe present AAO/UKST Hα+[N II] narrow-band imagery and low- andmedium-resolution optical spectroscopy of RCW24 and RCW69. These nebulaewere previously classified as HII regions, but we now show them to betwo of the largest and nearest bipolar Type I PNe yet discovered.Distances were estimated using extinction-distance and kinematicmethods, and via a new Hα surface brightness-radius relation. Theadopted distances are 1.0 +/- 0.3kpc for RCW24 and 1.3 +/- 0.2kpc forRCW69. Both objects have enhanced nitrogen abundances, withlogN/O~=+0.44 for RCW24, and logN/O=+0.33 for RCW 69. Systemicvelocities and |z| distances are VLSR = +5 km s-1and |z| ~ 23pc for RCW 24, and VLSR = -33 km s-1and only |z| ~ 7pc for RCW 69. Both PNe originated from massiveprogenitors (>2.0-2.5Msolar), as deduced from theirchemical abundances, large ionized masses, small |z| distances, lowpeculiar velocities and relatively hot central stars. These two objectsform an important addition to the small sample of evolved bipolar PNe inthe solar neighbourhood. Three-Dimensional Photoionization Structure and Distances of Planetary Nebulae. III. NGC 6781Continuing our series of papers on the three-dimensional (3D) structuresof and accurate distances to planetary nebulae (PNe), we present ourstudy of the planetary nebula NGC 6781. For this object, we construct a3D photoionization model and, using the constraints provided byobservational data from the literature, we determine the detailed 3Dstructure of the nebula, the physical parameters of the ionizing source,and the first precise distance. The procedure consists of simultaneouslyfitting all the observed emission-line morphologies, integratedintensities, and the two-dimensional (2D) density map from the [S II]line ratios to the parameters generated by the model, and in aniterative way obtaining the best fit for the central star parameters andthe distance to NGC 6781, obtaining values of 950+/-143 pc and 385Lsolar for the distance and luminosity of the central star,respectively. Using theoretical evolutionary tracks of intermediate andlow-mass stars, we derive the mass of the central star of NGC 6781 andits progenitor to be 0.60+/-0.03 Msolar and 1.5+/-0.5Msolar, respectively. The 3-D View of Planetary NebulaeBy taking a 3-D view of Planetary Nebulae (PNe) we show that the largeuncertainties associated with classical methods of modeling andobserving PNe to obtain their 3-D structures, distances and physicalparameters are significantly reduced. Using long slit or Integral FieldUnit spectrophotometry to constrain modern 3-D photoionization modelsfor PNe, we determine detailed 3-D structures, central star parameters,and distances accurate to 10-20% by eliminating the uncertainties in thedensity and filling factor. Traditional 1-D methods gave distances totypically a factor of 3 or more with the associated very largeuncertainties on the physical parameters of the objects. Conclusions of the workshop on the interferometric mode of OSIRISWe present the conclusions of the workshop organized to define thescientific drivers and the derived main characteristics of high-orderscanning Fabry-Perot interferometers proposed to be coupled to OSIRIS atthe GTC. Electron temperature fluctuations in planetary nebulaeAn 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. Molecular hydrogen kinematics in the ring-like planetary nebula NGC 6781This paper presents the radial velocity mapping of the molecularhydrogen in the ring-like planetary nebula NGC 6781. Observations of theH2 v=1{-}0 S(1) emission line (2.122 μm, Δ λ=0.02 μm)were obtained by Fabry-Pérot spectroscopy. These observationsprovide a very detailed map of the kinematic structure of the molecularhydrogen envelope in NGC 6781. The kinematic structure is explained witha model consisting of a thin hollow cylinder whose axis is tilted withrespect to the line of sight, and gas expanding radially outward with avelocity proportional to the distance to the central star. Molecularhydrogen emission was detected at LSR velocities ranging from -6.2 to+57.1 km s-1, with peak intensities between +15.7 and +25.5km s-1. There is evidence that the molecular hydrogenenvelope is excited by shocks from the ionization front. The mass ofmolecular gas is estimated to be of the order of 0.2 M_ȯ. Theemergent picture from the H2 kinematic structure supports the scenariowhere the structure of NGC 6781 was formed from the ionization anddestruction of an ellipsoidal molecular envelope that began with thedestruction and ionization of the least dense polar caps and willcontinue until the densest molecular material at nebular waist is fullyionized. 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]). Chemical abundances of planetary nebulae from optical recombination lines - II. Abundances derived from collisionally excited lines and optical recombination linesIn Paper I, we presented spectrophotometric measurements of emissionlines from the ultraviolet (UV) to the far-infrared for 12 Galacticplanetary nebulae (PNe) and derived nebular thermal and densitystructures using a variety of plasma diagnostics. The measurements andplasma diagnostic results are used in the current paper to determineelemental abundances in these nebulae. Abundance analyses are carriedout using both strong collisionally excited lines (CELs) and weakoptical recombination lines (ORLs) from heavy element ions.Assuming electron temperatures and densities derived from HIrecombination spectra (line and continuum), we are able to determine theORL C abundance relative to hydrogen for all the PNe in our sample, Nand O abundances for 11 of them and Ne abundances for nine of them. Inall cases, ORL abundances are found to be systematically higher than thecorresponding values deduced from CELs. In NGC 40, the discrepancybetween the abundances derived from the two types of emission linereaches a factor of 17 for oxygen. For the other 10 PNe, thediscrepancies for oxygen vary from 1.6 to 3.1. In general, collisionallyexcited infrared fine-structure lines, which have excitation energiesless than 103 K and consequently emissivities that areinsensitive to electron temperature and temperature fluctuations, yieldionic abundances comparable to those derived from optical/UV CELs. For agiven nebula, the discrepancies between the ORL and CEL abundances areof similar magnitude for different elements. In other words, relativeabundance ratios such as C/O, N/O and Ne/O deduced from the traditionalmethod based on strong CELs are comparable to those yielded by ORLs, fora wide range of ORL to CEL oxygen abundance ratios, varying from nearunity to over a factor of 20.We have also determined ORL abundances relative to hydrogen for thethird-row element magnesium for 11 nebulae in our sample. In strongcontrast to the cases for second-row elements, Mg abundances derivedfrom the MgII 3d-4f λ4481 ORL are nearly constant for all the PNeanalysed so far and agree within the uncertainties with the solarphotospheric value.In accordance with results from previous studies, the ORL to CELabundance ratio is correlated with the difference between the electrontemperatures derived from the [OIII] forbidden-line ratio, on the onehand, and from the hydrogen recombination Balmer discontinuity, on theother. We find that the discrepancy between the ORL and CEL abundancesis correlated with nebular absolute diameter, surface brightness, theelectron density derived from [SII] CELs, and excitation class. Theresults confirm that the dichotomy of temperatures and heavy elementalabundances determined from the two types of emission line, which hasbeen widely observed in PNe, is a strong function of nebular evolution,as first pointed out by Garnett and Dinerstein.Our analyses show that temperature fluctuations and/or densityinhomogeneities are incapable of explaining the large discrepanciesbetween the heavy elemental abundances and electron temperaturesdetermined from the two types of emission line. Our analyses support thebi-abundance model of Liu et al., who have proposed that PNe containanother previously unseen component of ionized gas which, highlyenriched in heavy elements, has an electron temperature of<~103 K and emits strongly in recombination lines but notin CELs. Our determinations of low average emission temperatures fromthe observed line intensity ratios of HeI and OII ORLs lend furthersupport to this scenario. Chemical abundances of planetary nebulae from optical recombination lines - I. Observations and plasma diagnosticsWe have obtained deep optical spectra of medium resolution for a sampleof 12 Galactic planetary nebulae (PNe). Optical recombination lines(ORLs) from carbon, nitrogen and oxygen have been detected in 11 of themand neon ORLs in nine of them. All spectra were obtained by scanning along slit across the nebular surface, yielding relative line intensitiesfor the entire nebula that are suitable for comparison with integratedline fluxes measured in other wavelength regions using space-bornefacilities, such as the Infrared Space Observatory (ISO) and theInternational Ultraviolet Explorer (IUE). For 11 PNe, ISO infraredspectra between 2.4 and 197 μm are available, most of them taken byourselves, plus a Kuiper Airborne Observatory (KAO) infrared spectrum ofNGC 6210. IUE ultraviolet (UV) spectra are available for all nebulaeexcept one in our sample. The UV, optical and infrared spectra have beencombined to study nebular thermal and density structures and todetermine elemental abundances.We have determined UV to optical extinction curves towards these PNe byexamining observed fluxes of HI and HeII recombination lines, radiofree-free continuum flux density, and UV to optical nebular continua.For 11 PNe in our sample, the derived optical reddening curves are foundto be consistent with the standard Galactic extinction law for atotal-to-selective extinction ratio, R≡A(V)/EB-V= 3.1.However, the optical extinction curve towards Hu 1-2 yields R= 2.0. TheUV extinction towards Hu 1-2 and NGC 6572 is also found to be muchsteeper than the standard Galactic reddening law. In contrast, the UVextinction curve along the sight lines towards NGC 6210 is found to bemuch shallower, although in the latter case the uncertainties involvedare quite large.Electron temperatures and densities have been derived using a variety ofdiagnostic ratios of collisionally excited lines (CELs) in the UV,optical and infrared. The results show clear stratifications, both intemperature and density. Lines emitted by ions formed in regions ofhigher ionization degree yield higher temperatures than lines arisingfrom regions of lower ionization degree, while densities deduced fromratios of infrared diagnostic CELs of low critical densities, such asthe [OIII] 88-μm/52-μm ratio, are systematically lower than thosederived from UV and optical diagnostic lines, which in general have muchhigher critical densities than the infrared fine-structure lines.Electron temperatures have also been derived from the ratio of thenebular continuum Balmer discontinuity to H 11 for 11 PNe. For four ofthese, the Balmer jump temperatures are more than 1000 K lower thanvalues derived from the [OIII] optical collisionally excited diagnosticline ratio. With a difference of 3580 K, NGC 40 has the lowest Balmerjump temperature relative to the [OIII] optical forbidden-linetemperature. High-order Balmer line decrements have been used todetermine electron densities. The results are consistent with valuesderived from forbidden-line density-diagnostics. 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. Planetary Nebulae with H[2] EmissionWe review the emission of molecular hydrogen (H[2]) in planetary nebulae(PNe) and we discuss the association between this emission and thebipolar morphology of the objects. We describe the energy levels of thehydrogen molecule, the main excitation mechanisms (shocks andfluorescence) and the ways of discrimination between the differentexcitation mechanisms. We propose another way of identification of theexcitation mechanism based on H[2] kinematical studies. We presentpreliminary results of the H[2] kinematics we are conducting on a sampleof five bipolar PNe. By means of this study we are able of identifyingshocks as the main excitation mechanism of the H[2] emission lines inthese objects. We have also estimated the masses of the H[2] gas inthese PNe and the result implies that the progenitor's masses of theseobjects are larger than those of typical PNe progenitors. 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} Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical propertiesWe present a comprehensive observational study of haloes aroundplanetary nebulae (PNe). Deep Hα+[NII] and/or [OIII] narrow-bandimages have been obtained for 35 PNe, and faint extended haloes havebeen newly discovered in the following 10 objects: Cn 1-5, IC 2165, IC2553, NGC 2792, NGC 2867, NGC 3918, NGC 5979, NGC 6578, PB 4, andpossibly IC 1747. New deep images have also been obtained of other knownor suspected haloes, including the huge extended emission around NGC3242 and Sh 2-200. In addition, the literature was searched, andtogether with the new observations an improved data base containing some50 PN haloes has been compiled.The halo sample is illustrated in an image atlas contained in thispaper, and the original images are made available for use by thescientific community at http://www.ing.iac.es/~rcorradi/HALOES/.The haloes have been classified following the predictions of modernradiation-hydrodynamical simulations that describe the formation andevolution of ionized multiple shells and haloes around PNe. According tothe models, the observed haloes have been divided into the followinggroups: (i) circular or slightly elliptical asymptotic giant branch(AGB) haloes, which contain the signature of the last thermal pulse onthe AGB; (ii) highly asymmetrical AGB haloes; (iii) candidaterecombination haloes, i.e. limb-brightened extended shells that areexpected to be produced by recombination during the late post-AGBevolution, when the luminosity of the central star drops rapidly by asignificant factor; (iv) uncertain cases which deserve further study fora reliable classification; (v) non-detections, i.e. PNe in which no halois found to a level of <~10-3 the peak surface brightnessof the inner nebulae.We discuss the properties of the haloes: detection rate, morphology,location of the central stars in the Hertzsprung-Russell diagram, sizes,surface brightness profiles, and kinematical ages. Among the mostnotable results, we find that, as predicted by models, ionized AGBhaloes are a quite common phenomenon in PNe, having been found in 60 percent of elliptical PNe for which adequately deep images exist. Another10 per cent show possible recombination haloes. In addition, using thekinematical ages of the haloes and inner nebulae, we conclude that mostof the PNe with observed AGB haloes have left the AGB far from a thermalpulse, at a phase when hydrogen burning is the dominant energy source.We find no significant differences between the AGB haloes ofhydrogen-poor and hydrogen-rich central stars. 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). NIR Fabry-Perot Imaging Spectroscopy of PNeNot Available Photochemistry and Molecular Formation in Planetary Nebulae (invited review)Not Available The Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic SampleNorthern Galactic planetary nebulae (PNs) are studied to disclosepossible correlations between the morphology of the nebulae and theevolution of the central stars (CSs). To this end, we have built thebest database available to date, accounting for homogeneity andcompleteness. We use updated statistical distances and an updatedmorphological classification scheme, and we calculate Zanstratemperatures for a large sample of PNs. With our study we confirm thatround, elliptical, and bipolar PNs have different spatial distributionswithin the Galaxy, with average absolute distances to the Galactic planeof 0.73, 0.38, and 0.21 kpc, respectively. We also find evidence thatthe distributions of the CS masses are different across thesemorphological groups, although we do not find that CSs hosted by bipolarPNs are hotter, on average, than CSs within round and elliptical PNs.Our results are in broad agreement with previous analyses, indicatingthat round, elliptical, and bipolar PNs evolve from progenitors indifferent mass ranges and might belong to different stellar populations,as also indicated by the helium and nitrogen abundances of PNs ofdifferent morphology. The 3-D ionization structure of the planetary nebula NGC 6565A detailed study of the planetary nebula NGC 6565 has been carried outon long-slit echellograms (lambda /Delta lambda =60 000, spectral range= lambda lambda 3900-7750 Å) at six, equally spaced positionangles. The expansion velocity field, the c(Hβ ) distribution andthe radial profile of the physical conditions (electron temperature anddensity) are obtained. The distance, radius, mass and filling factor ofthe nebula and the temperature and luminosity of the central star arederived. The radial ionization structure is analyzed using both theclassical method and the photo-ionization code CLOUDY. Moreover, wepresent the spatial structure in a series of images from differentdirections, allowing the reader to see'' the nebula in 3-D. NGC 6565results to be a young (2000-2500 years), patchy, optically thicktriaxial ellipsoid (a=10.1 arcsec, a/b=1.4, a/c=1.7) projected almostpole-on. The matter close to major axis was swept-up by someaccelerating agent (fast wind? ionization? magnetic fields?), formingtwo faint and asymmetric polar cups. A large cocoon of almost neutralgas completely embeds the ionized nebula. NGC 6565 is in a recombinationphase, because of the luminosity drop of the massive powering star,which is reaching the white dwarf domain (log T* =~ 5.08 K;log L*/Lsun =~ 2.0). The stellar decline startedabout 1000 years ago, but the main nebula remained optically thin forother 600 years before the recombination phase occurred. In the nearfuture the ionization front will re-grow, since the dilution factor dueto the expansion will prevail on the slower and slower stellar decline.NGC 6565 is at a distance of 2.0 (+/-0.5) kpc and can be divided intothree radial zones: the fully ionized'' one, extending up to0.029-0.035 pc at the equator (0.050 pc at the poles), thetransition'' one, up to 0.048-0.054 pc (0.080 pc), the halo'',detectable up to 0.110 pc. The ionized mass ( =~ 0.03 Msun)is only a fraction of the total mass (>= 0.15 Msun), whichhas been ejected by an equatorial enhanced superwind of 4 (+/-2) x10-5 Msun yr-1 lasted for 4 (+/-2) x103 years. Based on observations made with ESO Telescopes atthe La Silla Observatories, under programme ID 65.I-0524, and onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive at the Space Telescope Institute (observingprogram GO 7501; P.I. Arsen Hajian). STScI is operated by theassociation of Universities for Research in Astronomy, Inc. under theNASA contract NAS 5-26555. We have applied the photoionization codeCLOUDY, developed at the Institute of Astronomy of the CambridgeUniversity. Kinematics of the Planetary Nebula NGC6781We report the radial velocity field of the ionized and molecularhydrogen gas in NGC 6781, obtained from scanning Fabry-Perotinterferometer observations at the optical [N II] line and thenear-infrared vibrationally excited line H[2] S(1) 1-0 at 2.122 mu m. 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. Gas-phase models for the evolved planetary nebulae NGC 6781, M4-9 and NGC 7293We have studied the chemistry of the molecular gas in evolved planetarynebulae. Three pseudo-time-dependent gas-phase models have beenconstructed for dense (104-105cm-3) andcool (T~15K) clumpy envelopes of the evolved nebulae NGC 6781, M4-9 andNGC 7293. The three nebulae are modelled as carbon-rich stars evolvedfrom the asymptotic giant branch to the late planetary nebula phase. Theclumpy neutral envelopes are subjected to ultraviolet radiation from thecentral star and X-rays that enhance the rate of ionization in theclumps. With the ionization rate enhanced by four orders of magnitudeover that of the ISM, we find that resultant abundances of the speciesHCN, HNC, HC3N and SiC2 are in good agreement withobservations, while those of CN, HCO+, CS and SiO are inrough agreement. The results indicate that molecular species such as CH,CH2, CH2+, HCl, OH and H2O are anticipated to behighly abundant in these objects. The distance scale of planetary nebulaeBy collecting distances from the literature, a set of 73 planetarynebulae with mean distances of high accuracy is derived. This sample isused for recalibration of the mass-radius relationship, used by manystatistical distance methods. An attempt to correct for a statisticalpeculiarity, where errors in the distances influences the mass-radiusrelationship by increasing its slope, has been made for the first time.Distances to PNe in the Galactic Bulge, derived by this new method aswell as other statistical methods from the last decade, are then usedfor the evaluation of these methods as distance indicators. In order ofachieving a Bulge sample that is free from outliers we derive newcriteria for Bulge membership. These criteria are much more stringentthan those used hitherto, in the sense that they also discriminateagainst background objects. By splitting our Bulge sample in two, onewith optically thick (small) PNe and one with optically thin (large)PNe, we find that our calibration is of higher accuracy than most othercalibrations. Differences between the two subsamples, we believe, aredue to the incompleteness of the Bulge sample, as well as the dominanceof optical diameters in the thin'' sample and radio diameters in thethick'' sample. Our final conclusion is that statistical methods givedistances that are at least as accurate as the ones obtained from manyindividual methods. Also, the `long'' distance scale of Galactic PNe isconfirmed. The physical structure of the planetary nebula NGC 6781The planetary nebula NGC 6781 was imaged in major optical emissionlines. These lines allow us to construct maps of the projected, twodimensional Balmer decrement, electron density, electron temperature,ionization and abundance structure. The average electron density,determined from the [S Ii] lines, is ~500 cm-3, while theelectron temperature distribution, determined from the [N Ii] lines, isflat at ~10 000 K. The Balmer decrement map shows that there arevariations in extinction between the north and south areas of theplanetary nebula. The higher extinction observed to the north of thecentral star is probably caused by dust spatially associated with COemission at blue-shifted velocities. The [N Ii] image reveals the knownoptical halo, at a flux level of ~0.2% of the strong shell emission inthe east, but now the angular extent of 216 arcsec x 190 arcsec is muchlarger than previous measurements. The halo is also present in [O Iii],where we measure an extent of 190 arcsec x 162 arcsec. The ionizationmaps indicate substantial ionization along the caps of the ellipsoid aswell as in the halo. The maps also show a sharp decrease in ionizationalong the outer edge of the shell in the west and the east, south-east.The typical log abundances measured for He, N, O and S are 10.97, 8.14,8.72 and 6.90, respectively. An analysis of the observed radio emission from planetary nebulaeWe have analysed the radio fluxes for 264 planetary nebulae for whichreliable measurements of fluxes at 1.4 and 5 GHz, and of nebulardiameters are available. For many of the investigated nebulae, theoptical thickness is important, especially at 1.4 GHz. Simple modelslike the one specified only by a single optical thickness or spherical,constant density shells do not account satisfactorily for theobservations. Also an r-2 density distribution is ruled out.A reasonable representation of the observations can be obtained by atwo-component model having regions of two different values of opticalthickness. We show that the nebular diameters smaller than 10arcsec areuncertain, particularly if they come from photographic plates orGaussian fitting to the radio profile. While determining theinterstellar extinction from an optical to radio flux ratio, cautionshould be paid regarding optical thickness effects in the radio. We havedeveloped a method for estimating the value of self absorption. At 1.4GHz self absorption of the flux is usually important and can exceed afactor of 10. At 5 GHz self absorption is negligible for most of theobjects, although in some cases it can reach a factor of 2. The Galacticbulge planetary nebulae when used to calibrate the Shklovsky method givea mean nebular mass of 0.14 Msun. The statistical uncertaintyof the Shklovsky distances is smaller than a factor of 1.5. Table 1 isonly available in electronic form at http://www.edpsciences.org. ISO LWS observations of planetary nebula fine-structure linesWe have obtained 43-198μm far-infrared (IR) spectra for a sample of51 Galactic planetary nebulae (PN) and protoplanetary nebulae (PPN),using the Long Wavelength Spectrometer (LWS) on board the Infrared SpaceObservatory (ISO). Spectra were also obtained of the former PN candidateLo 14. The spectra yield fluxes for the fine-structure lines [Nii]122μm, [Niii] 57μm and [Oiii] 52 and 88μm emitted in theionized regions and the [Oi] 63- and 146-μm and [Cii] 158-μm linesfrom the photodissociation regions (PDRs), which have been used todetermine electron densities and ionic abundances for the ionizedregions and densities, temperatures and gas masses for the PDRs. Thestrong [Niii] and [Oiii] emission lines detected in the LWS spectrumtaken centred on Lo 14 could be associated with the nearby strong radioand infrared source G 331.5-0.1. We find that the electron densitiesyielded by the [Oiii] 88μm/52μm doublet ratio are systematicallylower than those derived from the optical [Ariv]λ4740/λ4711 and [Cliii] λ5537/λ5517 doubletratios, which have much higher critical densities than the 52- and88-μm lines, suggesting the presence of density inhomogeneities inthe nebulae. Ionic abundances, N+/H+,N2+/H+ and O2+/H+, as wellas the N2+/O2+ abundance ratio, which provides agood approximation to the N/O elemental abundance ratio, are derived.Although ionic abundances relative to H+ deduced from thefar-IR fine-structure lines are sensitive to the adopted electrondensity and the presence of density inhomogeneities, the strongdependence on the nebular physical conditions is largely cancelled outwhen N2+/O2+ is calculated from the57μm/(52μm+88μm) flux ratio, owing to the similarity of thecritical densities of the lines involved. The temperatures and densitiesof the PDRs around 24 PN have been determined from the observed [Oi] and[Cii] line intensity ratios. Except for a few objects, the deducedtemperatures fall between 200 and 500K, peaking around 250K. Thedensities of the PDRs vary from104-105cm-3, reaching3×105cm-3 in some young compact PN. With aderived temperature of 1600K and a density of105cm-3, the PDR of NGC 7027 is one of the warmestand at the same time one of the densest amongst the nebulae studied. Formost of the PN studied, the [Cii]-emitting regions contain only modestamounts of material, with gas masses <~0.1Msolar.Exceptional large PDR masses are found for a few nebulae, including NGC7027, the bipolar nebulae M2-9 and NGC 6302, the young dense planetarynebulae BD+30°3639, IC 418 and NGC 5315, and the old, probablyrecombining, nebulae IC 4406 and NGC 6072. Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.Not Available

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