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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

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.

The abundance distributions of Galactic bulge and disc planetary nebulae
We present an abundance analysis of a sample of Galactic bulge planetarynebulae (GBPNe). The observational data set consists of spectra of 88nebulae obtained with the FLAIR II multiobject spectrograph on the UKSchmidt Telescope, together with spectra of 42 nebulae obtained with theRGO Spectrograph on the Anglo-Australian Telescope. After selectingthose for which reliable nebular electron temperatures could be derived,collisionally excited line abundances were derived for 45 GBPNe.These were then compared with similarly derived abundances for 54Galactic disc PNe (GDPNe), taken from the work of Kingsburgh &Barlow. We find that within the errors the abundances have the sameaverage values, essentially the same distributions - including that forthe mass-sensitive N/O ratio - and show the same relationships. Thewidth of the nitrogen distribution exceeds that arising from errors,which could be a consequence of the range of precursor masses. The ratioof Type-I to non-Type-I PNe in the bulge and disc samples is similar, 18and 25 per cent respectively. For the GDPNe, we find larger N/H and N/Oratios for the small number of those with He/H > 0.14, compared withthose with He/H < 0.14.For neither disc nor bulge sample is there any strong evidence for adepletion of oxygen for the higher-mass precursor stars (Type I PNe). Wefind no correlation between O/H and N/O or He/H. On the N/O to He/Hplane, the bulge and disc PNe show a distribution whereby the low N/Ovalues only occur for low He/H values, but at N/O > 0.25 the wholerange of He/H values were sampled. The theoretical tracks to which wecompare our data do not explain the PNe with low He/H abundance and highN/O ratio.Realistic uncertainties in collisionally excited lines (CEL) abundancesfor individual PNe are quite large, of the order of 40 per cent foroxygen. Large samples are therefore required to get good statisticalaccuracy. This is usually achieved by combining many studies, and so wehave compared the results of a number of published studies with our own,to search for any systematic differences. The average abundances arefound to be the same within the errors except for cases where theabundance derivation methods are dissimilar, where systematicdifferences can occur. The N/O ratio is especially sensitive to thedetails of the abundance derivations.Our bulge PN sample shows no evidence for either very-low-metallicityobjects or for super-metal-rich objects - the implied mass and agedistributions of the bulge PN precursor stars are indistinguishablewithin the observational errors from those in the local Galactic disc.

The relation between Zanstra temperature and morphology in planetary nebulae
We have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology.

Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinates
We have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}

Angular dimensions of planetary nebulae
We 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

New Planetary Nebulae towards the galactic bulge. II. Objects surrounding the central area
In the second part of the objective-prism survey of PNe towards thegalactic centre we present the discovery of further 44 new planetarynebulae found in six ESO fields surrounding the central area and give arough description of these objects. Besides, we give coordinates of manyknown objects in the area of the survey. The discussion of the angulardiameters leads to the belief that the majority of our PNe is veryprobably located in or close to the galactic bulge.

An analysis of the observed radio emission from planetary nebulae
We 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.

Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.
Not Available

Distances of Galactic Planetary Nebulae Based on a Relationship Between the Central Star Mass and the N/O Abundance
In this paper, we propose a method to determine distances of Galacticplanetary nebulae on the basis of a relationship between the centralstar mass and the nebular N/O abundance ratio. This relationship is usedin combination with some basic parameters of the central stars, such asthe lambda 5480 flux, surface gravity and visual magnitude in order toobtain distances to a sample of a hundred Galactic planetary nebulae.

Gas Dynamics in Planetary Nebulae: From Macro-structures to FLIERs
Purpose of this paper is to clarify how Planetary Nebulae (PNe) are veryinteresting laboratories to study cosmic gas dynamics. I first recallthe history of PNe which are generated from low and intermediate massstars through successive mass loss processes starting in the Reg Giantphase of evolution and continuing also after the termination of thepulsed AGB phase, where most of the nebular mass is believed to beejected. The correponding stellar winds are the ingredients of thenebula. Their initial properties and subsequent mutual interactions,under the action of the evolving stellar radiation field, areresponsible for the properties of the nebula. The observed structures ofPNe are considered in detail. Larger scale macroscopic structures (MACS)are examined separately from quite smaller scale microscopic structures(MICS). The formation of MACS, at least in cases of round to moderatelyelliptical PNe, is shown to be reasonably well understood in terms ofexisting hydrodynamical models. Considering the kinematical behaviour,MICS can be separated into FLIERs (Fast Low Ionization Emitting Regions)and SLOWERs (slowly moving). Attention is focussed on FLIERs and on theproposed mechanisms to interpret them. Recent observations with theHubble Space Telescope have provided us with a wealth of detailed(subarcsec) information on the nebular structures. The inner structureof FLIERs is here illustrated to consist of substructures of variousshapes with an high degree of individually from object to object, alsowithin the same PN. These new data call for deeper thoretical efforts tosolve the problems of cosmic gas dynamics, posed by their observedproperties. An ample account is given of the most relevant originalworks, in an effort to allow the non specialist reader to quickly becomeacquainted with the status of art in the various aspects of the subject.

The dust content of planetary nebulae: a reappraisal
We have performed a statistical analysis using broad band IRAS data onabout 500 planetary nebulae with the aim of characterizing their dustcontent. Our approach is different from previous studies in that it usesan extensive grid of photoionization models to test the methods forderiving the dust temperature, the dust-to-gas mass ratio and theaverage grain size. In addition, we use only distance independentdiagrams. With our models, we show the effect of contamination by atomiclines in the broad band IRAS fluxes during planetary nebula evolution.We find that planetary nebulae with very different dust-to-gas massratios exist, so that the dust content is a primordial parameter for theinterpretation of far infrared data of planetary nebulae. In contrastwith previous studies, we find no evidence for a decrease in thedust-to-gas mass ratio as the planetary nebulae evolve. We also showthat the decrease in grain size advocated by Natta & Panagia(\cite{NattaPanagia}) and Lenzuni et al. (\cite{Lenzuni}) is an artefactof their method of analysis. Our results suggest that the timescale fordestruction of dust grains in planetary nebulae is larger than theirlifetime. Table~1 is only accessible in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

Infrared Planetary Nebulae in the NRAO VLA Sky Survey
In order to construct a sample of planetary nebulae (PNe) unbiased bydust extinction, we first selected the 1358 sources in the IRAS PointSource Catalog north of J2000 declination delta=-40^deg having measuredS(25 μm)>=1 Jy and colors characteristic of PNe: detections orupper limits consistent with both S(12 μm)<=0.35S(25 μm) andS(25 μm)>=0.35S(60 μm). The majority are radio-quietcontaminating sources such as asymptotic giant branch stars. Free-freeemission from genuine PNe should make them radio sources. The 1.4 GHzNRAO VLA Sky Survey (NVSS) images and source catalog were used to rejectradio-quiet mid-infrared sources. We identified 454 IRAS sources withradio sources brighter than S~2.5 mJy beam^-1 (equivalent to T~0.8 K inthe 45" FHWM NVSS beam) by positional coincidence. They comprise 332known PNe in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulaeand 122 candidate PNe, most of which lie at very low Galactic latitudes.Exploratory optical spectroscopic observations suggest that most ofthese candidates are indeed PNe optically dimmed by dust extinction,although some contamination remains from H II regions, Seyfert galaxies,etc. Furthermore, the NVSS failed to detect only 4% of the known PNe inour infrared sample. Thus it appears that radio selection can greatlyimprove the reliability of PN candidate samples withoutsacrificingcompleteness.

Electron densities in planetary nebulae, and the unusual characteristics of the [S BT II] emission zone} ] densities in planetary nebulae
We investigate the radial variation of electron densities in planetarynebulae, using values of ne deriving from the [S ii]<~mbda6717/<~mbda6730 line ratio. As a result, we are able to showthat there is a sharp discontinuity in densities of order 1.4 dex closeto nebular radii R=0.1 pc. It is proposed, as a consequence, that mostnebulae contain two primary [S ii] emission zones, with densitiesdiffering by a factor ~ 10(2) . The intensity of emission from thedenser component increases by an order of magnitude where nebulae passfrom radiation to density-bound expansion regimes, resulting in acorresponding discontinuous jump in [S ii]/Hβ line ratios. Theorigins of these changes are not entirely clear, although one mechanismis investigated whereby the superwind outflows shock interact withexterior AGB envelopes. Finally, the derived trends in ne(R)are used to determine distances for a further 262 nebulae. The resultingdistance scale appears to be comparable to that of Daub (1982) and Cahnet al. (1992).

The kinematics of 867 galactic planetary nebulae
We present a compilation of radial velocities of 867 galactic planetarynebulae. Almost 900 new measurements are included. Previously publishedkinematical data are compared with the new high-resolution data toassess their accuracies. One of the largest samples in the literatureshows evidence for a systematic velocity offset. We calculate weightedaverages between all available data. Of the final values in thecatalogue, 90% have accuracies better than 20 km s(-1) . We use thiscompilation to derive kinematical parameters of the galacticdifferential rotation obtained from least-square fitting and toestablish the Disk rotation curve; we find no significal trend for thepresence of an increasing external rotation curve. We examine also therotation of the bulge; the derived curve is consistent with a linearlyincreasing rotation velocity with l: we find V_b,r=(9.9+/-1.3)l -(6.7+/-8.5) km s(-1) . A possible steeper gradient in the innermostregion is indicated. Table 2 is available in electronic form only, viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

The planetary nebulae populations in five galaxies: abundance patterns and evolution
We have collected photometric and spectroscopic data on planetarynebulae (PNe) in 5 galaxies: the Milky Way (bulge), M 31 (bulge), M 32,the LMC and the SMC. We have computed the abundances of O, Ne and N andcompared them from one galaxy to another. In each Galaxy, thedistribution of oxygen abundances has a large dispersion. The averageO/H ratio is larger in the M 31 and the Galactic bulge PNe than in thosein the Magellanic Clouds. In a given galaxy, it is also larger for PNewith [O III] luminosities greater than 100 L_ȯ, which are likely toprobe more recent epochs in the galaxy history. We find that the M 31and the Galactic bulge PNe extend the very tight Ne/H-O/H correlationobserved in the Galactic disk and Magellanic Clouds PNe towards highermetallicities. We note that the anticorrelation between N/O and O/H thatwas known to occur in the Magellanic Clouds and in the disk PNe is alsomarginally found in the PNe of the Galactic bulge. Furthermore, we findthat high N/O ratios are higher for less luminous PNe. In M 32, all PNehave a large N/O ratio, indicating that the stellar nitrogen abundanceis enhanced in this galaxy. We have also compared the PN evolution inthe different galactic systems by constructing diagrams that areindependent of abundances, and have found strikingly differentbehaviours of the various samples. In order to help in theinterpretation of these data, we have constructed a grid of expanding,PN photoionization models in which the central stars evolve according tothe evolutionary tracks of Bl{öcker (1995). These models show thatthe apparent spectroscopic properties of PNe are extremely dependent,not only on the central stars, but also on the masses and expansionvelocities of the nebular envelopes. The main conclusion of theconfrontation of the observed samples with the model grids is that thePN populations are indeed not the same in the various parent galaxies.Both stars and nebulae are different. In particular, the central starsof the Magellanic Clouds PNe are shown to evolve differently from thehydrogen burning stellar evolutionary models of Bl{öcker (1995). Inthe Galactic bulge, on the other hand, the behaviour of the observed PNeis roughly compatible with the theoretical stellar evolutionary tracks.The case of M 31 is not quite clear, and additional observations arenecessary. It seems that the central star mass distribution is narrowerfor the M 31 PNe than for the Galactic bulge PNe. We show thatspectroscopy of complete samples of PNe down to a factor 100 below themaximum luminosity would help to better characterize the PN central starmass distribution. Tables 1 and 2 are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( orvia http: //cdsweb.u-strasbg.fr/Abstract.html

Planetary Nebulae in the NRAO VLA Sky Survey
The 1.4 GHz NRAO VLA Sky Survey (NVSS) images and source catalog wereused to detect radio emission from the 885 planetary nebulae north ofJ2000 declination delta = -40 deg in the Strasbourg-ESO Catalogue ofGalactic Planetary Nebulae. We identified 680 radio sources brighterthan about S = 2.5 mJy beam-1 (equivalent to T ~ 0.8 K in the 45" FWHMNVSS beam) with planetary nebulae by coincidence with accurate opticalpositions measured from Digitized Sky Survey (DSS) images. Totalextinction coefficients c at lambda = 4861 Angstroms were calculated forthe 429 planetary nebulae with available H beta fluxes and low free-freeoptical depths at 1.4 GHz. The variation of c with Galactic latitude andlongitude is consistent with the extinction being primarily interstellarand not intrinsic.

Orientation of planetary nebulae within the Galaxy
Narrow-band CCD images of 209 axially symmetrical planetary nebulae(PNe) have been examined in order to determine the orientation of theiraxes within the disc of the Galaxy. The nebulae have been divided intothe bipolar (B) and elliptical (E) PNe morphological types, according tothe scheme of Corradi & Schwarz. In both classes, contrary to theresults of Melnick & Harwit and Phillips we do not find any strongevidence for non-random orientations of the nebulae in the Galaxy.Compared with previous work in this field, the present study takesadvantage of the use of larger and morphologically more homogeneoussamples and offers a more rigorous statistical analysis.

A self-consistent determination of distances, physical parameters, and chemical composition for a large sample of galactic planetary nebulae: chemical composition
The relative abundances of He, C, N, O, Ne, Mg, Si, S, and Ar arepresented for, respectively, 185, 65, 212, 221, 180, 13, 41, 197, and205 Galactic planetary nebulae. The observed stages of ionization weretaken into account using the relations between the relative abundancesof different ions derived from a grid of photoionization models for thenebular emission. The chemical compositions of all the planetary nebulaewere determined using the same method and the same atomic data, so thatthe results have a high degree of uniformity; this is the first timethis has been done for such a large sample of Galactic planetary nebulae(221 objects).

Abundances in planetary nebulae near the galactic centre. I. Abundance determinations
Abundance determinations of about 110 planetary nebulae, which arelikely to be in the Galactic Bulge are presented. Plasma diagnosticshave been performed by making use of the available forbidden line ratioscombined with radio continuum measurements. Chemical abundances of He,O, N, Ne, S, Ar, and Cl are then derived by employing theoreticalnebular models as interpolation devices in establishing the ionizationcorrection factors (ICFs) used to estimate the distribution of atomsamong unobserved ionization stages. The overall agreement between theresults derived by using the model-ICFs and those obtained from thetheoretical models is reasonably good. The uncertainties related to thetotal abundances show a clear dependence on the level of excitation. Inmost cases, the abundances of chlorine can be derived only in objectswith a relatively high Cl-abundance. Contrary to the conclusionpreviously drawn by \cite[Webster (1988)]{we88}, we found the excitationclasses are not uniformly distributed. A clear peak at about classes 5and 6 is noticed. The distribution is shifted toward a lower excitationrange with respect to that of the nearby nebulae, reflecting thedifference in the central star temperature distribution between the twosamples.

A self-consistent determination of the distances, physical parameters, and chemical composition for a large sample of galactic planetary nebulae: The distances and parameters of central stars and the optical depths of envelopes
The distances and parameters of the central stars and the optical depthsof the envelopes in the Lyman limits of neutral hydrogen and neutralhelium were determined in a self-consistent way for 170 Galacticplanetary nebulae (PNe). The distance to each PN was so chosen that thetheoretically calculated evolutionary age of its nucleus was equal tothe dynamical age of its expanding envelope. The effective temperatureof the central star and its related parameters were determined either bythe generalized energy-balance method or, where appropriate, byZanstra's method. The derived distance estimates lend support to a`long' distance scale for PNe and are generally in agreement withcurrent individual and statistical estimates of the distances to PNeavailable in the literature. The mean distance to the bulge PNe is 7.9+/- 0.3 kpc, in agreement with the distance to the Galactic center. Themasses of the central stars of PNe corresponding to the deriveddistances are closely correlated with the nebular nitrogen-to-oxygenabundance ratio.

Properties That Cannot Be Explained by the Progenitors of Planetary Nebulae
I classify a large number of planetary nebulae (458) according to theprocess that caused their progenitors to blow axisymmetrical winds. Theclassification is based primarily on the morphologies of the differentplanetary nebulae, assuming that binary companions, stellar orsubstellar, are necessary in order to have axisymmetrical mass loss onthe asymptotic giant branch. I propose four evolutionary classes,according to the binary-model hypothesis: (1) Progenitors of planetarynebula that did not interact with any companion. These amount to ~10% ofall planetary nebulae. (2) Progenitors that interact with stellarcompanions that avoided a common envelope, 11^{+2}_{-3}% of all nebulae.(3) Progenitors that interact with stellar companions via a commonenvelope phase, 23^{+11}_{-5}% of all nebulae. (4) Progenitors thatinteract with substellar (i.e., planets and brown dwarfs) companions viaa common envelope phase, 56^{+5}_{-8}% of all nebulae. In order todefine and build the different classes, I start with clarifying somerelevant terms and processes related to binary evolution. I then discusskinematical and morphological properties of planetary nebulae thatappear to require the interaction of the planetary nebula progenitorsand/or their winds with companions, stellar or substellar.

Unveiling low-ionization microstructures in planetary nebulae.
(Hα+[NII])/[OIII] ratio images have been used to search forlow-ionization small-scale structures in a sample of 258 planetarynebulae. Radially symmetrical knots or jet-like structures, which areenhanced in the ratio images compared to the neighbouring regions of thenebulae, have been identified in 23 objects. Some of these features arebarely detected or invisible in the Hα+[NII] and [OIII] images.Empirical evidence, as well as some simple modelling, demonstrate thatthe selected features are characterized by having a lower ionizationthan the surrounding gas. For the above properties, most of themresemble the low-ionization microstructures discussed recently by Balicket al. (1993, 1994), and named FLIERs (Fast Low-Ionization EmissionRegions). The present work shows that the image-division technique is avery useful tool to look for ionization structures in planetary nebulaeand other HII regions. Its main advantage is that the division removesthe morphological components which are pure density enhancements, sothat the output image remains mostly sensitive to excitation andabundance variations through the nebulae.

Classification of planetary nebulae by cluster analysis and artificial neural networks.
According to the chemical composition, a sample of 192 Planetary Nebulaeof different types has been re-classified, and 41 others have beenclassified for the first time, by means of two methods not employed sofar in this field: hierarchical cluster analysis and supervisedartificial neural network. The cluster analysis reveals itself as a goodfirst guess for grouping Planetary Nebulae, while an artificial neuralnetwork provides reliable automated classification of this kind ofobjects.

The Distances of Planetary Nebulae and the Galactic Bulge
We describe an improved method for determining the distances ofplanetary nebulae (PNs) based on a theoretical/empirical relationshipbetween their radii and radio surface brightness. Like the Shklovsky(constant mass) distance method, our relationship requires only radioflux density and angular size measurements, which are widely availablein the literature. Based on models matching the overall Galacticdistribution of PNs, we determine how PNs observed in the direction ofthe Galactic center are actually distributed relative to the bulge inorder to establish the usefulness of these PNs for distance studies. Wethen use the bulge PNs along with PNs with independent distances toestablish, calibrate, and test the accuracy of the method. When comparedto the best available data our distance method appears to yield distanceerrors consistent with a scatter of 25% (1 (j). And, based on our modelsscaled to local PNs, we find a mean Galactic center distance of 8.3 +2.6 kpc for the bulge PNs. The relationship that PNs exhibit betweenradius and surface brightness is in excellent agreement with oursimulated nebulae from Paper I (Buckley & Schneider 1995). We findthat no simple power law can describe the changing mass and radius of aPN as it ages; however, our empirical relationship has a limitingbehavior that is almost indistinguishable from the assumption made inShklovsky's distance method that PNs have a constant ionized mass. Wereexamine the dispute about the validity of the Shklovsky's distancemethod as applied to Galactic center PNs in light of these results, andwe argue that the Shklovsky method does predict the distances of large,low surface brightness PNs well, but it increasingly overestimates thedistance of smaller PNs.

A statistical distance scale for Galactic planetary nebulae
A statistical distance scale is proposed. It is based on the correlationbetween the ionized mass and the radius and the correlation between theradio continuum surface brightness temperature and the nebular radius.The proposed statistical distance scale is an average of the twodistances obtained while using the correlation. These correlations,calibrated based on the 1`32 planetary nebulae with well-determinedindividual distances by Zhang, can reproduce not only the averagedistance of a sample of Galactic Bulge planetary nebulae exactly at thedistance to the Galactic center, but also the expected Gaussiandistribution of their distances around the Galactic center. This newdistance scale is applied to 647 Galactic planetary nebulae. It isestimated that this distance scale can be accurate on average to35%-50%. Our statistical distance scale is in good agreement with theone recently proposed by Van de Steene and Zijlstra. The correlationsfound in this study can be attributed to the fact that the core mass ofthe central stars has a very sharp distribution, strongly peaked atapprox. 0.6 solar mass. We stress that the scatter seen in thestatistical distance scale is likely to be real. The scatter is causedby the fact that the core mass distribution, although narrow andstrongly peaked, has a finite width.

On an alternative statistical distance scale for planetary nebulae. Catalog with statistical distances to planetary nebulae.
We have proposed a statistical method to determine distances toplanetary nebulae. The method is based on an empirical correlationbetween the radio-continuum brightness temperature and radius. Here wepresent a catalog of distance determinations calculated using thismethod.

Elemental abundances for a sample of southern galactic planetary nebulae.
Abstract image available at:http://adsabs.harvard.edu/abs/1994MNRAS.271..257K

Abundances and radial gradients from disk planetary nebulae: He, N, C, and CL
Chemical abundances of the elements He, N, C, and Cl are presented fordisk planetary nebulae, comprising Peimbert types I, II, and III.Average abundances for these classes are determined and compared withthe remaining abundances available. The presence of abundance gradientsrelative to hydrogen for disk nebulae is investigated in a region ofabout 8 kpc centered in the solar system. It can be concluded that thegradients of the ratios N/H, Cl/H, and probably C/H are similar to theO/H gradient, especially for type II objects.

A catalogue HeII 4686 line intensities in Galactic planetary nebulae.
We have compiled the intensities of the HeII 4686 lines measured inGalactic planetary nebulae. We present a few observational diagramsrelated to this parameter, and discuss them with the help of theoreticaldiagrams obtained from simple model planetary nebulae surroundingevolving central stars of various masses. We determine the hydrogen andhelium Zanstra temperature for all the objects with accurate enoughdata. We argue that, for Galactic planetary nebulae as a whole, the maincause for the Zanstra discrepancy is leakage of stellar ionizing photonsfrom the nebulae.

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