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Trigonometric Parallaxes of Central Stars of Planetary Nebulae
Trigonometric parallaxes of 16 nearby planetary nebulae are presented,including reduced errors for seven objects with previous initial resultsand results for six new objects. The median error in the parallax is0.42 mas, and 12 nebulae have parallax errors of less than 20%. Theparallax for PHL 932 is found here to be smaller than was measured byHipparcos, and this peculiar object is discussed. Comparisons are madewith other distance estimates. The distances determined from theseparallaxes tend to be intermediate between some short distance estimatesand other long estimates; they are somewhat smaller than those estimatedfrom spectra of the central stars. Proper motions and tangentialvelocities are presented. No astrometric perturbations from unresolvedclose companions are detected.

Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79northern Galactic planetary nebulae (PNe) whose morphological classeshave been uniformly determined. Ionic abundances and plasma diagnosticswere derived from selected optical line strengths in the literature, andelemental abundances were estimated with the ionization correctionfactor developed by Kingsbourgh & Barlow in 1994. We compare theelemental abundances to the final yields obtained from stellar evolutionmodels of low- and intermediate-mass stars, and we confirm that mostbipolar PNe have high nitrogen and helium abundance and are the likelyprogeny of stars with main-sequence mass greater than 3Msolar. We derive =0.27 and discuss the implication of such ahigh ratio in connection with the solar neon abundance. We determine theGalactic gradients of oxygen and neon and foundΔlog(O/H)/ΔR=-0.01 dex kpc-1 andΔlog(Ne/H)/ΔR=-0.01 dex kpc-1. These flat PNgradients are irreconcilable with Galactic metallicity gradientsflattening with time.

The distances of less-evolved planetary nebulae: a further test of statistical distance scales
It has recently been pointed out that a number of the methods used todetermine planetary nebulae (PNe) distances may be appreciably in error.Whilst the scales of Zhang (1995), Bensby & Lundstrom (2001) andothers are appropriate for higher radio brightness temperaturesTB, those of Phillips and Daub are more relevant whereTB is small.We note, in the following, that the absolute bolometric magnitudes ofless-evolved PNe are likely to be similar. The mean value of can therefore be used to constrain PNe distancesD, and confirm the distance scales for higher TB outflows. Wehave used this procedure to evaluate distances to a further 47 PNe, andwe find that the mean values of are consistent with those ofCahn, Kaler & Stanghellini (1992), Zhang (1995), Phillips et al.(2004) and van de Steene & Zijlstra (1995). They are, as expected,inconsistent with the lower TB scale of Phillips (2002a).

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.

Imaging Fabry-Perot spectrometric studies of velocity fields in gaseous nebulae
In order to study the spatio-kinematics of extended gaseous nebulae suchas the HII regions associated with giant molecular clouds and planetarynebulae, we had designed and built an Imaging Fabry-Perot Spectrometerfor the 1.2m Mt Abu Telescope. We describe here some of the significantscientific results that came out of these studies in the past one decadeat Mt Abu.

Unresolved Hα Enhancements at High Galactic Latitude in the WHAM Sky Survey Maps
We have identified 85 regions of enhanced Hα emission at|b|>10deg subtending approximately 1° or less on theWisconsin Hα Mapper (WHAM) sky survey. These high-latitude ``WHAMpoint sources'' have Hα fluxes of 10-11-10-9ergs cm-2 s-1, radial velocities within about 70km s-1 of the LSR, and line widths that range from less than20 to about 80 km s-1 (FWHM). Twenty-nine of theseenhancements are not identified with either cataloged nebulae or hotstars and appear to have kinematic properties that differ from thoseobserved for planetary nebulae. Another 14 enhancements are near hotevolved low-mass stars that had no previously reported detections ofassociated nebulosity. The remainder of the enhancements are catalogedplanetary nebulae and small, high-latitude H II regions surroundingmassive O and early B stars.

Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.

Chemical abundances of planetary nebulae towards the Galactic anticenter
In this paper we report new observations and derive chemical abundancesfor a sample of 26 planetary nebulae (PN) located in the anticenterdirection. Most of these nebulae are far away objects, located atGalactocentric distances greater than about 8 kpc, so that they areparticularly useful for the determination of the radial gradients atlarge distances from the galactic center. A comparison of the presentresults with previously determined abundances suggests that the radialabundance gradients flatten out at distances larger than about 10 kpcfrom the center.Based on observations made at the European Southern Observatory (Chile)and Laboratório Nacional de Astrofísica (Brazil).The full Table 2 is 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/423/199

Wind accretion by a binary stellar system and disc formation
I calculate the specific angular momentum of mass accreted by a binarysystem embedded in the dense wind of a mass-losing asymptotic giantbranch star. The accretion flow is of the Bondi-Hoyle-Lyttleton type.For most of the space of the relevant parameters the flow is basicallyan isothermal high Mach number accretion flow. I find that when theorbital plane of the accreting binary system and the orbital plane ofthe triple system are not parallel to each other, the accreted mass onto one or two of the binary system components has high specific angularmomentum. For a large fraction of triple-star systems, accretion discswill be formed around one or two of the stars in the binary system,provided that the mass ratio of the two stars in the accreting binarysystem is >~0.5. Such discs may blow jets which shape the descendantplanetary nebula (PN). The axis of jets will be almost parallel to theorbital plane of the triple-star system. One jet is blown outwardrelative to the wind, while the other jet passes near the mass-losingstar, and is more likely to be slowed down or deflected. I find thatduring the final asymptotic giant branch phase, when the mass-loss rateis very high, an accretion disc may form for orbital separation betweenthe accreting binary systems and the mass-losing star of up to ~400-800au. I discuss the implications for the shape of the descendant PN, andlist several PN which may have been shaped by an accreting binary-starsystem, i.e. by a triple-star system.

A Spatiokinematic Study of the Planetary Nebula NGC 1514
New spatiokinematic observations were undertaken of the planetary nebulaNGC 1514 in the [O III] line at 5007 Å using an imagingFabry-Pérot spectrometer. Our results show an inner ellipsoidalshell and polar blobs that do not conform to bipolar morphology. It isargued that the nebula is a descendant of a common envelope binarysystem the periodicity of which is estimated to be about 10 days, with aprogenitor mass of 4.5 Msolar.

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}

Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical properties
We 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.

Comparative Absorption and Emission Abundance Analyses of Nebulae: Ion Emission Densities for IC 418
Recent analyses of nebular spectra have resulted in discrepantabundances from CNO forbidden and recombination lines. We considerindependent methods of determining ion abundances for emission nebulae,comparing ion emission measures with column densities derived fromresonance absorption lines viewed against the central star continuum.Separate analyses of the nebular emission lines and the stellar UVabsorption lines yield independent abundances for ions, and their ratiocan be expressed in terms of a parameterem, the ``emission density'' for eachion. Adequate data for this technique are still scarce, but separateanalyses of spectra of the planetary nebula and central star of IC 418do show discrepant abundances for several ions, especially Fe II. Thediscrepancies are probably due to the presence of absorbing gas thatdoes not emit and/or to uncertain atomic data and excitation processes,and they demonstrate the importance of applying the technique ofcombining emission- and absorption-line data in deriving abundances fornebulae.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the STScI, which is operated by AURA, Inc., under NASAcontract NAS 5-26555, and on observations made at CTIO/NOAO, which isoperated by AURA, Inc., under cooperative agreement with the NSF.

The Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic Sample
Northern Galactic planetary nebulae (PNs) are studied to disclosepossible correlations between the morphology of the nebulae and theevolution of the central stars (CSs). To this end, we have built thebest database available to date, accounting for homogeneity andcompleteness. We use updated statistical distances and an updatedmorphological classification scheme, and we calculate Zanstratemperatures for a large sample of PNs. With our study we confirm thatround, elliptical, and bipolar PNs have different spatial distributionswithin the Galaxy, with average absolute distances to the Galactic planeof 0.73, 0.38, and 0.21 kpc, respectively. We also find evidence thatthe distributions of the CS masses are different across thesemorphological groups, although we do not find that CSs hosted by bipolarPNs are hotter, on average, than CSs within round and elliptical PNs.Our results are in broad agreement with previous analyses, indicatingthat round, elliptical, and bipolar PNs evolve from progenitors indifferent mass ranges and might belong to different stellar populations,as also indicated by the helium and nitrogen abundances of PNs ofdifferent morphology.

HST Observations of Sakurai's Object (V4334 Sgr) and Related Stars
We describe our ongoing program of HST observations of Sakurai's Object(V4334 Sgr). Direct WFPC2 imaging from August 1996 through August 2000reveals no transient features (such as light echoes), and documents thedecline of the star to below 24th visual magnitude in 2000. Thesurrounding planetary nebula has shown no changes from 1996 through2000. There are no obvious peculiar features (such as blobs or knots) inthe immediate vicinity of the star. We also have in place atarget-of-opportunity program to obtain UV spectra with HST in the eventthat the star begins to retrace its evolution back to high surfacetemperature. We also present older HST FOC imaging of V605 Aql. Thecentral object is a resolved nebula that emits in [O III] (but not inhydrogen), whose 0''.6 diameter is consistent with a dust cloud ejectedduring the 1919 outburst. The central star itself is not seen due to itsbeing embedded in the nebula. Several other central stars (including H3-75, IC 2120, and Abell 14) have late-type nuclei and no evidence forhot companions. They may be further candidates for `born-again'red-giant nuclei.

Spectroscopic investigations of planetary nebulae
The morphologies of three selected asymmetrical planetary nebulae (PNe),namely NGC 4361, NGC 1514 and NGC 246, were studied usingspatio-kinematic observations made with an Imaging Fabry-PerotSpectrometer. From the 3D spatio-kinematic models, plaussible nebularformation scenarios were proposed for NGC 4361 and NGC 1514 based on theinteracting binary progenitor model. A few parameters regarding thebinary, viz. the progenitor's mass and the binary separation, weresuggested from this study. The elemental concentration of N and He inthe nebulae were used to get some information regarding theirprogenitors. It is suggested from our study that the binary progenitorhypothesis gives more consistent / explanation of the observedstructures of these PNe than the single star hypothesis. The physicalcondition of a PNe interacting with the ambient interstellar medium hasbeen investigated in NGC 246. Its distorted morphology is attributed toits interaction with the Interstellar Medium (ISM).

The rise and rise of the deep sky image
Presidential Address to the British Astronomical Association, 2000October 25

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

A HUBBLE SPACE TELESCOPE Survey for Resolved Companions of Planetary Nebula Nuclei
We report the results of a Hubble Space Telescope ``snapshot'' surveyaimed at finding resolved binary companions of the central stars ofGalactic planetary nebulae (PNe). Using the the Wide Field and PlanetaryCamera and Wide Field Planetary Camera 2, we searched the fields of 113PNe for stars whose close proximity to the central star suggests aphysical association. In all, we find 10 binary nuclei that are verylikely to be physically associated and another six that are possiblebinary associations. By correcting for interstellar extinction andplacing the central stars' companions on the main sequence (or, in onecase, on the white dwarf cooling curve), we derive distances to theobjects, and thereby significantly increase the number of PNe withreliable distances. Comparison of our derived distances with thoseobtained from various statistical methods shows that all of the latterhave systematically overestimated the distances, by factors ranging upto a factor of 2 or more. We show that this error is most likely due tothe fact that the properties of our PNe with binary nuclei aresystematically different from those of PNe used heretofore to calibratestatistical methods. Specifically, our PNe tend to have lower surfacebrightnesses at the same physical radius than the traditionalcalibration objects. This difference may arise from a selection effect:the PNe in our survey are typically nearby, old nebulae, whereas most ofthe objects that calibrate statistical techniques are low-latitude, highsurface brightness, and more distant nebulae. As a result, thestatistical methods that seem to work well with samples of distant PNe,for example, those in the Galactic bulge or external galaxies, may notbe applicable to the more diverse population of local PNe. Our distancedeterminations could be improved with better knowledge of themetallicities of the individual nebulae and central stars, measurementsof proper motions and radial velocities for additional candidatecompanions, and deeper HST images of several of our new binary nuclei.

Possible trends in expansion velocities of planetary nebulae evolved from common envelope binary systems
Not Available

Giant Holes and Emission Structures Around Planetary Nebulae on IRAS SkyView Images
Years ago, on a POSS I print, we were attracted by a long very faintfilament about 1.2 pc away from the high-galactic-latitude PN NGC 4361;if somehow connected to NGC 4361, this would correspond to the hugedistance of ca. 25 pc. In addition, on high-contrast copies of the POSSof the region around this PN, we found that it appears to be located ina ``hole" of low surface brightness. This hole turned out to be visibleon a 100mum IRAS SkyView map too and might thus be caused by someprocess that has either destroyed the (interstellar) dust or swept itaway (Zanin and Weinberger 1997, Proc. IAU Symp. 180, 290). Recently,Clayton and de Marco (1997, AJ, 114, 2679) decribed an approximatelycircular 40' large ``evacuated" area around the PN A 58 (=V605 Aql),visible on an IRAS 100mu image; 40' would correspond to 40 pc at theassumed distance. They suppose that one sees the result of swept up ISMdust, originating from the wind from the PN progenitor star. - - Arethere more examples of this new phenomenon? We have started a systematicsearch of areas around PNe using the IRAS SkyView (brightness scaling:``Hist. Eq.", colour table: ``B-W linear" and ``Stern special").Although by now we have examined only a fraction of the known PNe, wediscovered several ``holes" that are, in a few cases, too well definedto be projection effects. Particularly intriguing are, in addition toNGC 4361, a distinct hole, best seen at 25mum and 20' across, perfectlycentered on NGC 2899, and a huge (ca. 10^o large) hole plusfilament around the close (400 pc) PN LoTr 5. We also found giantemission structures, like a 1^o large ``spot" centered on NGC1514 at 12mum, a 1.5^o arc east of Lo 4, etc. Severalexamples are shown on the poster. - Models to explain the holes and theemission structures are in preparation.

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 Size and Age of Sakurai's Planetary Nebula and the Temperature of Its Central Star
We present high-resolution, spatially resolved spectra of the nebulasurrounding Sakurai's object. We find the expansion velocity of thenebula to be 30.9 +/- 0.7 km s^-1, and the nebula extent to be 44" indiameter. We discuss the wide range of reported distances to Sakurai'sobject and the dilemma that these present, since some evidence stronglyfavors a short distance (1.1 kpc) while other data strongly favor alonger distance (5-8 kpc). We also present spectra of Sakurai's starcentered near the lambda4216 CN band that suggests the star had cooledsignificantly between 1996 October and 1997 May, but had not cooledfurther by 1997 July.

A sample of planetary nebulae observed by HIPPARCOS
By using HIPPARCOS data (magnitudes, parallaxes, proper motions), (1) wedetermine new values of the apparent luminosity of the central stars of19 planetary nebulae, (2) we discuss their distance and position on theT/L diagram. By comparison with the distances determined by individualor statistical methods, we see that most of these ``ground" distanceslook overestimated when we trust the HIPPARCOS trigonometric parallaxes.It seems that for compact nebulae, the nebula itself could influence theparallax measurement. In particular, the very small HIPPARCOS distancesto SwSt 1 and Hu 2-1 are unexpected and are not trustworthy. Peculiarmotions are analyzed, in terms of astrophysical parameters of the binarysystem A 35, and in relation with asymmetric morphology.

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

A Morphological Study of Planetary Nebulae
We have produced simulated images of 110 planetary nebulae using theellipsoidal shell model. This process has allowed us to remove theprojection effects from the morphological classification of planetarynebulae and has provided quantitative measures of the intrinsicasymmetries of the nebulae. It is shown that the morphology of mostplanetary nebulae can be reproduced with pole-to-equator density ratiosof 0.1-1. Many planetary nebulae also show a modest departure from axialsymmetry. Contrary to previous findings by Khromov & Kohoutek, thesky orientation of planetary nebulae in this sample is consistent with apurely random distribution. Extremely bipolar nebulae (e.g., those ofbutterfly shape) point to a steep density profile in the AGB envelopeand are more likely to be type I (high helium and/or nitrogen abundance)nebulae. We found evidence that these nebulae are likely to have moremassive progenitors and are at a more advanced stage of dynamicalevolution.

Observational studies of planetary nebulae with moderate-size telescopes
Not Available

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.

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Right ascension:04h09m16.98s
Apparent magnitude:10

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