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 A search for photometric variability of hydrogen-deficient planetary-nebula nucleiAims.We searched for photometric variability in a sample of hot,hydrogen-deficient planetary nebula nuclei (PNNi) with "PG 1159" or "OVI" spectral type, most of them embedded in a bipolar or ellipticalplanetary nebula envelope (PNe). These characteristics may indicate thepresence of a hidden close companion and an evolution affected byepisodes of interaction between them. Methods: .We obtainedtime-series photometry from a sample of 11 candidates using the NordicOptical Telescope (NOT) with the Andalucía Faint ObjectSpectrograph and Camera (ALFOSC), modified with our own control softwareto be able to observe in a high-speed multi-windowing mode. The datawere reduced on-line with the real time photometry (RTP) code, whichallowed us to detect periodic variable stars with small amplitudes fromCCD data in real time. We studied the properties of the observedmodulation frequencies to investigate their nature. Results: .Wereport the first detection of low-amplitude pulsations in the PNNiVV 47, NGC 6852, and Jn1. In addition, we investigated the photometric variability ofNGC 246. Time-series analysis shows that the powerspectra of VV 47, NGC 6852, andNGC 246 are variable on time scales of hours. Powerspectra from consecutive nights of VV 47 andNGC 6852 show significant peaks in differentfrequency regions. The same type of variability is present in NGC 246 in2 observing runs separated by 3 days. Changes are also found in thepower spectra of VV 47 and NGC 246during the same night. The VV 47 power spectra arepeculiar since they present modulation frequencies in a wide range from175 to 7600 μHz. This is different from the previously knownpulsating PNNi where no frequencies are found above ~3000 μHz. Thehigh-frequency modulation observed in VV 47 may bedue to g-modes triggered by the ɛ-mechanism, observed for thefirst time. The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB StarsWe review the observed properties of extremely hot, hydrogen-deficientpost-asymptotic giant branch (AGB) stars of spectral type [WC] andPG1159. Their H deficiency is probably caused by a (very) latehelium-shell flash or an AGB final thermal pulse, laying bare interiorstellar regions that are usually kept hidden below the hydrogenenvelope. Thus, the photospheric elemental abundances of these starsallow us to draw conclusions about details of nuclear burning and mixingprocesses in the precursor AGB stars. We summarize the state of the artof stellar evolution models that simulate AGB evolution and theoccurrence of a late He-shell flash. We compare predicted elementalabundances to those determined by quantitative spectral analysesperformed with advanced non-LTE model atmospheres. Good qualitative andquantitative agreement is found. Future work can contribute to an evenmore complete picture of the nuclear processes in AGB stars. 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. 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 Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic SampleNorthern Galactic planetary nebulae (PNs) are studied to disclosepossible correlations between the morphology of the nebulae and theevolution of the central stars (CSs). To this end, we have built thebest database available to date, accounting for homogeneity andcompleteness. We use updated statistical distances and an updatedmorphological classification scheme, and we calculate Zanstratemperatures for a large sample of PNs. With our study we confirm thatround, elliptical, and bipolar PNs have different spatial distributionswithin the Galaxy, with average absolute distances to the Galactic planeof 0.73, 0.38, and 0.21 kpc, respectively. We also find evidence thatthe distributions of the CS masses are different across thesemorphological groups, although we do not find that CSs hosted by bipolarPNs are hotter, on average, than CSs within round and elliptical PNs.Our results are in broad agreement with previous analyses, indicatingthat round, elliptical, and bipolar PNs evolve from progenitors indifferent mass ranges and might belong to different stellar populations,as also indicated by the helium and nitrogen abundances of PNs ofdifferent morphology. Evolutionary status of hydrogen-deficient central stars of planetary nebulaeThe observational data for the planetary nebulae with hydrogen-deficientcentral stars are analysed. We show that the general evolutionarysequence is: late-[WC], early-[WC], PG 1159. An analysis of the observeddistributions of nebular parameters leads to a conclusion that theplanetary nebulae with hydrogen-deficient nuclei are not different fromthe population of other planetary nebulae in the Galaxy. In particularthe proportion of the H-deficient stars among young nebulae is the sameas in the whole population. We have made a detailed comparison of theobserved parameters with theoretical modelling of the late He-shellflash (born again AGB) scenario. Our finding is that the [WC] nuclei arenot formed in a late He-shell flash. This scenario can, however, giveorigin to some PG 1159 objects. There are five objects known which havepresumably suffered from a late He-shell flash. The observed parametersof their nebulae imply that these stars will not become typical [WC]objects. Thus most of hydrogen-deficient central stars (at least [WC])evolve directly from the AGB as do the other planetary nebula nuclei. Wediscuss implications of this result. 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 A Catalog of Spectroscopically Identified White DwarfsA catalog of 2249 white dwarfs which have been identifiedspectroscopically is presented complete through 1996 April. Thiscompilation is the fourth edition of the Villanova Catalog ofSpectroscopically Identified White Dwarfs. For each degenerate star, thefollowing data entries with references are provided: (1) a catalogcoordinate designation or WD number, in order of right ascension; (2)the right ascension and declination for epoch 1950.0; (3) the spectraltype based upon the new system; (4) a catalog symbol denoting binarymembership; (5) a list of most names known to exist for a given star;(6) proper motion and position angle; (7) broadband UBV photometry, V,B-V, U-B (8) multichannel spectrophotometry, v(MC), g-r (9)Strömgren narrowband photometry, y, b-y, u-b (10) an absolutevisual magnitude based upon the best available color-magnitudecalibration or trigonometric parallax; (11) the observed radial velocityuncorrected for gravitational redshift or solar motion; and (12) thetrigonometric parallax with mean error when available. Notes for unusualor peculiar stars and a coded Reference Key alphabetized by the firstauthor's last name are presented, as well as an expanded tablecross-referencing all names to the catalog WD number. An introductionand full descriptions of the entries are provided in the text. 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. 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. Spectroscopic investigation of old planetaries. III. Spectral types, magnitudes, and distances.The results of a spectroscopic investigation of central stars of oldplanetary nebulae (PN) are reported. Classifications are provided for atotal of 38 central stars, most of them previously unclassified. Thesample is dominated by hydrogen-rich (pre-)white dwarfs, and the ratioof hydrogen-rich to -poor stars (=~4:1) is consistent with results ofprevious investigations of central stars and hot white dwarfs. The classof hydrogen-poor central stars of planetary nebulae (CPN) is representedby helium- and carbon-rich PG1159 stars: seven new PG1159 central starswere detected, increasing the number of known objects of this rare classby more than 100%. A previously unknown class has been established by"hybrid" central stars: their spectra resemble closely those of PG1159stars, but with strong Balmer lines indicating that they arehydrogen-rich in contrast to the PG1159 stars. We derived V magnitudesfrom our spectrophotometric observations and used the interstellar NaDlines for distance estimates of those PNe, which are close to thegalactic plane. Since this method is independent of "ad hoc" assumptionsabout nebular properties it is superior to the widely used "statistical"methods. Kinematical ages (t_kin_) are calculated from publishedexpansion velocities and angular diameters and are used to construct aM_V_-t_kin_ diagram. A comparison with theoretical evolutionary tracksyielded a mean mass of 0.62+/-0.09Msun_ in good agreementwith generally accepted values for white dwarfs. It is shown that thewidely adopted "statistical" distances of old PNe are obviously toosmall by more than a factor of two. 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. Spectra of GW VIR Type PulsatorsWe describe our effort to confine empirically the position of the GW Virinstability strip in the HR diagram. Spectra ranging from the soft X-rayto the optical region of the electromagnetic spectrum are analyzed bymeans of non-LTE model atmospheres. We compare our results for stellarparameters with those from pulsational analyses. Generally, goodagreement is found. 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. The extinction constants for galactic planetary nebulaeThe extinction constants are determined from Balmer decrementmeasurements for over 900 planetary nebulae. Comparison with publishedextinction constants shows that the results from ESO are fairlyreliable. An analysis of the extinction constants derived from theBalmer decrement and from the radio/Hβ flux ratio indicates thatthe latter tends to be systematically smaller than the former forincreasing extinction. We suggest that the radio measurements ofPottasch's group probably underestimate the radio fluxes, at least forsome (faintest) objects. A catalogue of absolute fluxes and distances of planetary nebulaeThe paper presents a complete list of averaged recalibrated absoluteH-beta fluxes, global (where possible) relative He II lambda 4686fluxes, 5 GHz radio flux densities, and H-alpha/H-beta interstellarextinction constants for 778 Galactic planetary nebulae. The catalogprovides much of the fundamental data required to generate Zanstratemperatures. When data with the lowest errors are selected, the opticaland radio/optical extinctions show a peculiar correlation, with theradio values slightly high at low extinction and notably low at highextinction. The data are used, along with the best estimates of angulardiameters, to calculate Shklovsky distances according to the Daub schemeon the scale used earlier by Cahn and Kaler (1971). Use of this distancescale shows approximate equality of the death rates of optically thickand optically thin planetary nebulae. The method gives the correctdistances to the Magellanic Clouds. Strasbourg - ESO catalogue of galactic planetary nebulae. Part 1; Part 2Not Available The absolute H-beta fluxes for galactic planetary nebulaeThe absolute H-beta flux measurements from spectrophotometricobservations of about 880 galactic planetary nebulae are presented. Thegalactic coordinates of PNe and the adopted diameters for the nebulae inarcsec are given. The final values of the H-beta flux, corrected for thenebular extension, are presented and compared to the results of othermeasurements. Properties of central stars in 13 faint, extended planetary nebulaeTemperatures and luminosities for 13 faint central stars in extendedplanetary nebulae have been derived and placed on the theoretical logL-log T diagram to infer their masses. Despite the initial expectationof finding predominantly higher-mass central stars, it was found ratherthat the masses cover almost the entire central star range, from lessthan 0.55 to about 1.1 solar mass and have a mean value of about 0.64 +or - 0.16 solar mass. This value is not significantly different from theaverage of 0.58 solar mass typically found for the mass distributionboth of planetary nebula central stars and of white dwarfs. Properties and evolution of dust grains in planetary nebulaeThe IRAS data of 233 planetary nebulae (PNs) determining the propertiesof the dust contained in their envelopes have been analyzed. The mainresult is that the dust grains undergo substantial evolution during thelifetime of a PN. In particular, the grain size and the total dust massdecrease systematically with the nebular radius whereas the number ofemitting grains increases. Two scenarios which can explain these resultsare discussed. A first possibility is that grains undergo processeswhich both 'erode' and 'shatter' them. It is argued that sputtering byfast atoms of a stellar wind and grain-grain collisions may plausiblyproduce such effects. Alernatively, the dust present in PNs may have asize distribution since the very beginning and large grains may beselectively destroyed during the PN evolution. A catalogue of expansion velocities of Galactic planetary nebulaePublished observational data on 288 Galactic PN are compiled in tables,graphs, and sketches based on spatiokinematical models and brieflycharacterized. The criteria used in selecting the data are discussed,and particular attention is given to the accuracies of the distanceestimates and their implications for theoretical models of PN or stellarevolution. Planetary nebulae in the GalaxyIt is shown that the planetary nebulae can be divided into three typesaccording to the values of the mass of shell and a central star. Thecriteria are given using which one can determine the mass type of thenebula. The distance scale of each mass-type planetary nebulae is given.The distribution of planetary nebulae in the Galaxy, their formationrate, scale-height and other physical and kinematic characteristics areinvestigated. A catalogue of planetary nebulae emitting in the ratiorange is given. New identifications of faint central stars in extended planetary nebulae.Not Available A study of some southern planetary nebulaeRadial velocities and expansions velocities (forbidden line O III,forbidden line O II, and He II) are measured for a sample of 64 southernGalactic planetary nebulae. Also, nebular electronic densities arederived from the forbidden line O II 3727, 3729 A double for 23 of theseobjects. The distance scale of Dopita et al. (1987) is used to derivedistances to 33 Galactic nebulae. Identification of faint central stars in extended, low-surface-brightness planetary nebulaeAs part of a larger program to study the properties of planetary nebulacentral stars, a search for faint central stars in extended,low-surface-brightness planetary nebulae using CCD imaging is performed.Of 25 target nebulae, central star candidates have been identified in17, with certainties ranging from 'extremely probable' to 'possible'.Observed V values in the central star candidates extend to fainter than23 mag. The identifications are presented along with the resultingphotometric measurements. A catalogue of distances of planetary nebulaeThis catalog contains distances of 468 galactic planetary nebulae pluslower limits to 61 objects and upper limits for 134 nebulae. Thedistances were calculated assuming a relationship between the nebularionized mass and radius. Index and cross-identification of planetary nebulaeThe index of 86 discovery lists of planetary nebulae contains all theknown planetary nebulae in the Galaxy, including the probable, possibleand misclassified objects. A cross-identification gives all the names ofthe 1518 PN ordered by increasing galactic longitude. Radio observations at 14.7 GHz of southern planetary nebulaePeak and integrated radio flux densities at 14.7 GHz are presented for397 celestial objects located south of declination +27 deg and listed asplanetary nebulae in the Perek and Kohoutek (1967) catalog. The objectsinclude 260 confirmed planetary nebulae (CPN's), 25 suspect faintplanetary nebulae (SFPN's), 21 nebulae of the MV (misclassified, verylow excitation) class, five diffuse nebulae, 20 stars, five peculiarobjects, 21 objects that were not detected optically, and 40unclassified nebulae. The integrated flux densities of 264 of theobjects (including 236 CPN's) are compared with 5-GHz data for the sameobjects, and all 264 objects are shown to be optically thin in theirradio emission between 5 and 14.7 GHz. It is found that the followingpercentages of the 397 objects were detected above the 2-sigma level (10mJy) of the survey: (1) 87% of the 211 CPN's classified on the basis oftheir optical spectra; (2) 84% of the 49 CPN's classified on the basisof their optical appearance; (3) 95% of the nebulae of the MV class; (4)76% of the SFPN's; (5) 100% of the diffuse nebulae; (6) 26% of the 46objects in the remaining classes; and (7) 50% of the unclassifiednebulae.
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