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 The Problem with Reddening Distances to Planetary NebulaeIt is noted that the vast majority of Galactic PNe are located outsideof the interstellar reddening layer, to the extent that local gradientsin E_{B-V} are low to undetectable. This is likely to invalidate certainprevious estimates of their reddening distances D_{RED}. It also meansthat larger scale analyses, such as those of Pottasch (1984) and Acker(1973), lead to values of D_{RED}] which are significantly too small. Asimilar critique applies to analyses based on measures of Na D-lineabsorption, and it seems likely that most of the distances of Napiwotzki& Schönberner (1995) are similarly in error. An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar MediumWe present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985. 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 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 association of IRAS sources and 12CO emission in the outer GalaxyWe have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 are only available in electronic form at the CDS viaanonymous ftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1083 A Finding List of Faint UV-bright Stars in the Galactic Plane. VI.Three additional plates from the Sandage two-color survey of theGalactic plane have been examined, yielding identifications of 113UV-bright stars. Plates were obtained using the Palomar 48 inch OschinSchmidt telescope and cover a field of view of 43 deg2. Theplates were centered at the Galactic longitude (l) and latitude (b)coordinates of 141°, +6° 164°, +0° and 164°,-6°. Sources identified range in U-B color from U-B~-0.1 to U-B~-1.2and in magnitude from mB~10 to ~20. Accurate J2000.0coordinates and finding charts are provided for all the UV candidatesincluded. Some of the more interesting sources are discussed. Previouslyidentified objects described in the literature are noted. Recessional Halos in Planetary Nebulae: An Undervalued Aspect of Nebular MorphologyMany planetary nebulae (PNs) are known to possess halos, the origins ofwhich are ill determined. We suggest that several of the brighter halos,at least, are likely to arise through contraction of ionization frontswithin the nebular shell, a situation that may arise where central startemperatures and luminosities decline at intermediate phases of PNevolution. We show that such halos may be long-lived and possessappreciable levels of emission for periods of ~103 yr or so.We also find that several well-known halo sources possess propertiesconsistent with this hypothesis and that two further sources may havedeveloped halos as a result of late helium flash. Measurements of 12C/13C in planetary nebulae: Implications on stellar and Galactic chemical evolutionWe present the results of a study aimed at determining the12C/13C ratio in two samples of planetary nebulae(PNe) by means of millimeter wave observations of 12CO and13CO. The first group includes six PNe which have beenobserved in the 3He+ hyperfine transition byBalser et al. (1997); the other group consists of 22 nebulae with richmolecular envelopes. We have determined the carbon isotopic ratio in 14objects, 9 of which are new detections. The results indicate a range ofvalues of 12C/13C between 9 and 23. We estimatethe mass of the progenitors of the PNe of our sample and combine thisinformation with the derived 12C/13C isotopicratios to test the predictions of stellar nucleosynthesis models. Wefind that the majority of PNe have isotopic ratios below the valuesexpected from current standard asymptotic giant branch models in themass range of interest. We suggest that the progenitors of the PNe musthave undergone a non-standard mixing process during their red giantphase and/or asymptotic giant phase, resulting in a significantenhancement of the 13C abundance in the surface layers. Ourstudy confirms a similar behaviour inferred from spectroscopicobservations of field population II stars and globular cluster giants,and extends it to the final stages of stellar evolution. Finally, wediscuss the implications of our results on models of Galactic chemicalevolution of 3He and 12C/13C. The dust content of planetary nebulae: a reappraisalWe 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html Spectroscopic investigation of old planetaries. IV. Model atmosphere analysisThe results of a NLTE model atmosphere analysis of 27 hydrogen-richcentral stars of old planetary nebulae (PN) are reported. These starswere selected from a previous paper in this series, where we gaveclassifications for a total of 38 central stars. Most of the analyzedcentral stars fill a previously reported gap in the hydrogen-richevolutionary sequence. Our observations imply the existence of twoseparated spectral evolutionary sequences for hydrogen-rich and -poorcentral stars/white dwarfs. This is in line with theoreticalevolutionary calculations, which predict that most post-AGB stars reachthe white dwarf domain with a thick hydrogen envelope of ~ 10(-4) M_sun.We determine stellar masses from the comparison with evolutionary tracksand derive a mass distribution for the hydrogen-rich central stars ofold PNe. The peak mass and the general shape of the distribution is inagreement with recent determinations of the white dwarf massdistribution. The properties of most analyzed stars are well explainedby standard post-AGB evolution. However, for eight stars of the sampleother scenarios have to be invoked. The properties of three of them areprobably best explained by born again post-AGB evolution. Two of theseare hybrid CSPN (hydrogen-rich PG 1159 stars), but surprisingly thethird star doesn't show any signs of chemical enrichment in itsatmosphere. The parameters of five stars are not in accordance withpost-AGB evolution. We discuss alternative scenarios such as thestripping of the hydrogen-rich envelope by a companion during the firstred giant phase or the formation of a common envelope with a possiblemerging of both components. Two stars (HDW 4 andHaWe 5) remain mysterious after all. They resembleordinary hot DA white dwarfs, but due to very large evolutionary agesthe presence of a PN cannot be explained. We speculate that the nebulaemay be shells produced by ancient nova outbursts. A wide spread ofhelium abundances is observed in the photospheres of central stars ofold PNe. It is shown that a good correlation between helium abundancesand luminosity is present. It is inferred that when the stars'luminosities fall below L~ 300 L_sun depletion starts and the heliumabundance steadily decreases with decreasing luminosity. The existenceof this correlation is in qualitative agreement with recent theoreticalcalculations of gravitational settling in the presence of a stellarwind. Infrared Planetary Nebulae in the NRAO VLA Sky SurveyIn 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. A HUBBLE SPACE TELESCOPE Survey for Resolved Companions of Planetary Nebula NucleiWe 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. Creation of ^7LI and Destruction of ^3HE, ^9BE, ^10B, and ^11B in Low-Mass Red Giants, Due to Deep CirculationIt has been demonstrated that ^7Li can be created in low-mass red giantstars via the Cameron-Fowler mechanism, due to extra deep mixing and theassociated cool bottom processing.'' Under certain conditions, this^7Li creation can take the place of the ^7Li destruction normallyexpected. Note that such extra mixing on the red giant branch (RGB) haspreviously been invoked to explain the observed ^13C enhancement. Thisnew ^7Li production can account for the recent discovery of surprisinglyhigh lithium abundances in some low-mass red giants (a few of which aresuperrich lithium stars, with abundances higher than that in theinterstellar medium). The amount of ^7Li produced can exceedlogepsilon(^7Li)~4, but depends critically on the details of the extramixing mechanism (mixing speeds, geometry, episodicity). If the deepcirculation is a relatively long-lived, continuous process, lithium-richRGB stars should be completely devoid of beryllium and boron. Coolbottom processing leads to ^3He destruction in low-mass stars; incontrast to the ^7Li creation, the extent of ^3He depletion is largelyindependent of the details of the extra mixing mechanism. The overallcontribution from solar-metallicity stars (from 1 to 40 M_solar) isexpected to be a net destruction of ^3He, with an overall ^3He survivalfraction g_3~0.9+/-0.2 (weighted average over all stellar masses); thisis in contrast to the conclusion from standard dredge-up theory, whichwould predict that stars are net producers of ^3He (withg^dr_3~2.4+/-0.5). Population II stars experience even more severe ^3Hedepletion, with 0.3<~g_3<~0.7. Destruction of ^3He in low-massstars is consistent with the requirements of Galactic chemical evolutionmodels; it would also result in some relaxation of the upper bound onthe primordial (D+^3He)/H abundance, thus relaxing the lower bound onthe cosmic baryon density Omega_b from big bang nucleosynthesiscalculations. For reference, we also present the effects of standardfirst and second dredge-up on the helium, lithium, beryllium, and boronisotopes. Electron densities in planetary nebulae, and the unusual characteristics of the [S BT II] emission zone} ] densities in planetary nebulaeWe 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 nebulaeWe 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html Planetary Nebulae in the NRAO VLA Sky SurveyThe 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 NebulaeWe 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. Measurements of the 12C/13C Ratio in Planetary Nebulae and Implications for Stellar EvolutionWe present the results of a study aimed at determining the 12C/13C ratioin two samples of planetary nebulae (PNe) by means of mm-waveobservations of 12CO and 13CO. The first group includes six PNe whichhave been observed in the 3He+ hyperfine transition; the other groupconsists of 23 nebulae with rich molecular envelopes. We have determinedthe isotopic ratio in 14 objects and the results indicate a range ofvalues between 9 and 23. In particular, three PNe have ratios well belowthe value predicted by standard evolutionary models (>~ 20),indicating that some extra-mixing process has occurred in these stars.We briefly discuss the implications of our results for standard andnonstandard stellar nucleosynthesis. Properties That Cannot Be Explained by the Progenitors of Planetary NebulaeI 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. The 3He Abundance in Planetary NebulaeWe report measurements of the abundance of 3He for a sample of sixGalactic planetary nebulae: IC 289, NGC 3242, NGC 6543, NGC 6720, NGC7009, and NGC 7662. Based on observations of the 8.665 GHz hyperfinetransition of 3He+, we derive 3He/H abundances ranging from ~0.1 to 1.0x 10-3 by number. These abundances are more than an order of magnitudelarger than those found in any H II region, the local interstellarmedium, or the proto--solar system. If planetary nebulae are surroundedby large, low-density, ionized halos, modeling suggests that theseabundances will decrease by a factor of about 2. Our source sample ishighly biased in that we selected objects on the basis of severalcriteria that maximized the likelihood of 3He detections. The abundancesare nonetheless consistent with the idea that 3He is produced insignificant quantities by stars of 1--2 Mȯ. We conclude that thereis some stellar production of 3He. The Timescale Correlation Method: Distances to Planetary Nebulae with HalosWe present deep narrow-band CCD images of six PNe obtained with thePalomar 5 m telescope in the light of [N II] lambda 6584 + H alpha and[O III] lambda 5007. Several previously undetected structures are found,including faint multiple envelope structures surrounding three of thetargets. The inclusion of these sources with data for other multipleenvelope PNe published in the literature permit a statistical evaluationof the relationship between PN "shells" and the thermal pulses of the PNnucleus. In this paper, we investigate the dynamical timescale betweensuccessive envelopes, tau dyn, and the nuclear interpulse timescale, tauip. Our results show that the deviations from the relation tau dyn = tauip can be explained by the uncertainty in the Shklovsky distance toindividual PNe, which is sigma _{d_{{sh}}}/d_{{sh}}~2 . By imposing theconstraint that tau dyn = tau ip, we find that it is possible to derivea PN distance indicator, which we name the timescale correlationdistance, dTC. The derivation for dTC is independent of ad hoc and oftenunsupported assumptions inherent to the Shklovsky method. 3He in Planetary Nebulae: A Challenge to Stellar Evolution ModelsThe discrepancy between the observed abundances of 3He in theinterstellar medium and those predicted by stellar and Galactic chemicalevolution remains largely unexplained. In this paper, we attempt to shedsome light on this unsolved problem by presenting a quantitativecomparison of the 3He abundances recently measured in six planetarynebulae (IC 289, NGC 3242, NGC 6543, NGC 6720, NGC 7009, NGC 7662) withthe corresponding predictions of stellar evolution theory. Thedetermination of the mass of the planetary nebulae progenitors allows usto dismiss, with a good degree of confidence, the hypothesis that theabundance of 3He in the envelopes of all low-mass stars (M <~ 2.5Mȯ) is strongly reduced with respect to the standard theoreticalvalues by some mixing mechanism acting in the latest phases of stellarevolution. The abundance versus mass correlation, with allowance madefor the limitation of the sample, is in fact found to be fullyconsistent with the classical prediction of stellar evolution. Weexamine the implications of this result on the Galactic evolution of 3Hewith the help of a series of models with standard and nonstandard (i.e.,3He-depleted) nucleosynthesis prescriptions in varying percentages oflow-mass stars. The results are found to be consistent with theabundances determined in the presolar material and in the localinterstellar medium only if the vast majority of low-mass stars (morethan 70%--80%) follow nonstandard prescriptions. This implies thateither the sample of planetary nebulae under examination is highlybiased and therefore not representative of the whole population oflow-mass stars, or the solution to the 3He problem lies elsewhere. Detection in H alpha of a Supershell Associated with W4From H I observations, Normandeau, Taylor, & Dewdney have identifieda possible Galactic chimney emanating from W4. We observed a 10 degdiameter field centered on this region in the H alpha line using a CCDcamera sensitive to faint extended emission. Our image shows an apparentshell of H II, which we interpret as the ionized inner wall of asuperbubble produced by stellar winds from the very young star clusterOCl 352. An analysis of the ionization balance indicates that much ofthe Lyman continuum radiation from the star cluster is absorbed and doesnot escape from the disk. The shell appears to close 6 deg (or about 230pc) above the star cluster, and at a Galactic latitude of 7 deg. Theshell is quite elongated, with its major axis approximatelyperpendicular to the Galactic plane, as predicted for a superbubbleformed in a stratified Galactic disk. The large size of the shell leadsto an estimated age between 6.4 and 9.6 Myr, which exceeds that of OCl352 (<~2.5 Myr). The reason for this discrepancy is unclear, althoughit is possible that an earlier epoch of stellar outflow has contributedto the growth of the W4 superbubble. Non-LTE model atmospheres analyses of faint PN central stars observed with Keck HIRESNot Available H2 Emission from Planetary Nebulae: Signpost of Bipolar StructureAbstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...462..777K&db_key=AST Radiation gasdynamics of planetary nebulae - VI. The evolution of aspherical planetary nebulaeThis paper reports the results of the numerical study of the formationof aspherical planetary nebulae through the generalized interactingwinds model, taking into account the effects caused by the evolvingcentral star and fast wind. The results show for the first time thataspherical nebulae do form within the required time-scale. Considerationof the development of the nebula shows that in the early stages it isthe ionization of the aspherical AGB wind that contributes considerablyto the shaping of the nebula. Furthermore, the passing through of theionization front may modify the density distribution in the slow wind,leading to the formation of a surrounding envelope, and sometimes adifferent morphology for the nebula from that to be expected from theinitial conditions. I consider how the different phases of ionizationfronts and wind swept bubbles can be observationally distinguished. Investigating the Near-Infrared Properties of Planetary Nebulae. I. Narrowband ImagesAbstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS..100..159L&db_key=AST A statistical distance scale for Galactic planetary nebulaeA 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 132 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.
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