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The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edge
We infer the gas kinematics, diagnostics and ionic radial profiles,distance and central star parameters, nebular photo-ionization model,spatial structure and evolutionary phase of the Planetary Nebula NGC6741 by means of long-slit ESO NTT+EMMI high-resolution spectra at nineposition angles, reduced and analysed according to the tomographic and3-D methodologies developed at the Astronomical Observatory of Padua(Italy). NGC 6741 (distance≃2.0 kpc, age≃ 1400 yr, ionizedmass Mion≃ 0.06 Mȯ) is a dense(electron density up to 12 000 cm-3), high-excitation,almost-prolate ellipsoid (0.036 pc × 0.020 pc × 0.018 pc,major, intermediate and minor semi-axes, respectively), surrounded by asharp low-excitation skin (the ionization front), and embedded in aspherical (radius≃ 0.080 pc), almost-neutral, high-density (n(HI)≃ 7 ×103 atoms cm-3) halo containinga large fraction of the nebular mass (Mhalo≥ 0.20Mȯ). The kinematics, physical conditions and ionicstructure indicate that NGC 6741 is in a deep recombination phase,started about 200 years ago, and caused by the rapid luminosity drop ofthe massive (M*=0.66{-}0.68 Mȯ), hot (logT* ≃ 5.23) and faint (logL*/Lȯ ≃ 2.75) post-AGB star, which hasexhausted the hydrogen-shell nuclear burning and is moving along thewhite dwarf cooling sequence. The general expansion law of the ionizedgas in NGC 6741, Vexp(km s-1)=13 × R arcsec,fails in the innermost, highest-excitation layers, which move slowerthan expected. The observed deceleration is ascribable to the luminositydrop of the central star (the decreasing pressure of the hot-bubble nolonger balances the pressure of the ionized gas), and appears instriking contrast to recent reports inferring that acceleration is acommon property of the Planetary Nebulae innermost layers. A detailedcomparative analysis proves that the "U"-shaped expansion velocity fieldis a spurious, incorrect result due to a combination of: (a) simplisticassumptions (spherical shell hypothesis for the nebula); (b) unfitreduction method (emission profiles integrated along the slit); and (c)inappropriate diagnostic choice (λ4686 Å of He II, i.e. athirteen fine-structure components recombination line). Some generalimplications for the shaping mechanisms of Planetary Nebulae arediscussed.
| Molecular hydrogen kinematics in the ring-like planetary nebula NGC 6781
This paper presents the radial velocity mapping of the molecularhydrogen in the ring-like planetary nebula NGC 6781. Observations of theH2 v=1{-}0 S(1) emission line (2.122 μm, Δ λ=0.02 μm)were obtained by Fabry-Pérot spectroscopy. These observationsprovide a very detailed map of the kinematic structure of the molecularhydrogen envelope in NGC 6781. The kinematic structure is explained witha model consisting of a thin hollow cylinder whose axis is tilted withrespect to the line of sight, and gas expanding radially outward with avelocity proportional to the distance to the central star. Molecularhydrogen emission was detected at LSR velocities ranging from -6.2 to+57.1 km s-1, with peak intensities between +15.7 and +25.5km s-1. There is evidence that the molecular hydrogenenvelope is excited by shocks from the ionization front. The mass ofmolecular gas is estimated to be of the order of 0.2 M_ȯ. Theemergent picture from the H2 kinematic structure supports the scenariowhere the structure of NGC 6781 was formed from the ionization anddestruction of an ellipsoidal molecular envelope that began with thedestruction and ionization of the least dense polar caps and willcontinue until the densest molecular material at nebular waist is fullyionized.
| Planetary nebula distances re-examined: an improved statistical scale
The 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 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}
| Angular dimensions of planetary nebulae
We have measured angular dimensions of 312 planetary nebulae from theirimages obtained in Hα (or Hα + [NII]). We have appliedthree methods of measurements: direct measurements at the 10% level ofthe peak surface brightness, Gaussian deconvolution and second-momentdeconvolution. The results from the three methods are compared andanalysed. We propose a simple deconvolution of the 10% levelmeasurements which significantly improves the reliability of thesemeasurements for compact and partially resolved nebulae. Gaussiandeconvolution gives consistent but somewhat underestimated diameterscompared to the 10% measurements. Second-moment deconvolution givesresults in poor agreement with those from the other two methods,especially for poorly resolved nebulae. From the results of measurementsand using the conclusions of our analysis we derive the final nebulardiameters which should be free from systematic differences between small(partially resolved) and extended (well resolved) objects in our sample.Table 1 is only available in electronic form athttp://www.edpsciences.org
| 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.
| The distance scale of planetary nebulae
By collecting distances from the literature, a set of 73 planetarynebulae with mean distances of high accuracy is derived. This sample isused for recalibration of the mass-radius relationship, used by manystatistical distance methods. An attempt to correct for a statisticalpeculiarity, where errors in the distances influences the mass-radiusrelationship by increasing its slope, has been made for the first time.Distances to PNe in the Galactic Bulge, derived by this new method aswell as other statistical methods from the last decade, are then usedfor the evaluation of these methods as distance indicators. In order ofachieving a Bulge sample that is free from outliers we derive newcriteria for Bulge membership. These criteria are much more stringentthan those used hitherto, in the sense that they also discriminateagainst background objects. By splitting our Bulge sample in two, onewith optically thick (small) PNe and one with optically thin (large)PNe, we find that our calibration is of higher accuracy than most othercalibrations. Differences between the two subsamples, we believe, aredue to the incompleteness of the Bulge sample, as well as the dominanceof optical diameters in the ``thin'' sample and radio diameters in the``thick'' sample. Our final conclusion is that statistical methods givedistances that are at least as accurate as the ones obtained from manyindividual methods. Also, the ``long'' distance scale of Galactic PNe isconfirmed.
| An analysis of the observed radio emission from planetary nebulae
We have analysed the radio fluxes for 264 planetary nebulae for whichreliable measurements of fluxes at 1.4 and 5 GHz, and of nebulardiameters are available. For many of the investigated nebulae, theoptical thickness is important, especially at 1.4 GHz. Simple modelslike the one specified only by a single optical thickness or spherical,constant density shells do not account satisfactorily for theobservations. Also an r-2 density distribution is ruled out.A reasonable representation of the observations can be obtained by atwo-component model having regions of two different values of opticalthickness. We show that the nebular diameters smaller than 10arcsec areuncertain, particularly if they come from photographic plates orGaussian fitting to the radio profile. While determining theinterstellar extinction from an optical to radio flux ratio, cautionshould be paid regarding optical thickness effects in the radio. We havedeveloped a method for estimating the value of self absorption. At 1.4GHz self absorption of the flux is usually important and can exceed afactor of 10. At 5 GHz self absorption is negligible for most of theobjects, although in some cases it can reach a factor of 2. The Galacticbulge planetary nebulae when used to calibrate the Shklovsky method givea mean nebular mass of 0.14 Msun. The statistical uncertaintyof the Shklovsky distances is smaller than a factor of 1.5. Table 1 isonly available in electronic form at http://www.edpsciences.org.
| CO content of bipolar planetary nebulae
We report high-sensitivity millimeter-wave CO observations of recentlydiscovered bipolar planetary nebulae (PNe). Three objects (BV 5-1, K3-94 and K 3-24) have been detected, and one of them (BV 5-1) isresolved by the ~ 10{''} telescope beam. The envelopes of thethree newly detected objects display values of the molecular to ionizedmass ratio of ~ 0.2, and sizes of ~ 0.1 pc. This indicates that thesePNe are rather evolved, with K 3-24 being the youngest of the three ( ~103 years) according to its CO line profile and kinematicages. In BV 5-1, the molecular gas appears to be distributed in anirregular disk or ring surrounding the central star and perpendicular tothe bipolar nebula. This is similar to the situation found in otherbetter studied bipolar PNe (e.g. NGC 2346, M 2-9, KjPn8), and thus seemsto be the rule in this class of objects. This suggests a commonmechanism for the rings and the bipolar flows that shape the opticalnebula.
| New observations of CO(1-0) line emission in planetary nebulae.
Not Available
| The dust content of planetary nebulae: a reappraisal
We have performed a statistical analysis using broad band IRAS data onabout 500 planetary nebulae with the aim of characterizing their dustcontent. Our approach is different from previous studies in that it usesan extensive grid of photoionization models to test the methods forderiving the dust temperature, the dust-to-gas mass ratio and theaverage grain size. In addition, we use only distance independentdiagrams. With our models, we show the effect of contamination by atomiclines in the broad band IRAS fluxes during planetary nebula evolution.We find that planetary nebulae with very different dust-to-gas massratios exist, so that the dust content is a primordial parameter for theinterpretation of far infrared data of planetary nebulae. In contrastwith previous studies, we find no evidence for a decrease in thedust-to-gas mass ratio as the planetary nebulae evolve. We also showthat the decrease in grain size advocated by Natta & Panagia(\cite{NattaPanagia}) and Lenzuni et al. (\cite{Lenzuni}) is an artefactof their method of analysis. Our results suggest that the timescale fordestruction of dust grains in planetary nebulae is larger than theirlifetime. Table~1 is only accessible in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Onset of Molecular Hydrogen Emission from Proto-planetary Nebulae
This paper presents results obtained from high-resolution spectroscopyof the 2.1218 μm emission line of molecular hydrogen toward fivebipolar pre-planetary nebulae. Our detections of IRAS 17441-2411 andAFGL 6815S bring to four, including AFGL 618 and AFGL 2688, the numberof bipolar preplanetaries that have been detected as sources of H_2emission at 2.1218 mum. The spatially resolved H_2 line profile of AFGL6815S suggests that the bulk of the detected emission arises in anexpanding molecular torus surrounding the central star. All of theH_2-emitting pre-planetary nebulae are found at low galactic latitudes,consistent with previous results for planetary nebulae, and have centralstars with intermediate or early spectral types. We were unable todetect H_2 emission from the bipolar, post-main-sequence sources OH231.8+4.2, IRAS 07131-0147, and IRAS 09371+1212, all of which haveM-type central stars. These results suggest that the event that triggersthe production of emission from shocked H_2 occurs at an intermediateevolutionary epoch in the postasymptotic giant branch evolution oftransition objects, after the generation of bipolar structure and beforethe nebular envelopes are ionized. From constraints imposed by thegalactic distribution of H_2-emitting planetaries, we estimate distancesto the four pre-planetary nebulae that display H_2 emission.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Planetary Nebulae in the NRAO VLA Sky Survey
The 1.4 GHz NRAO VLA Sky Survey (NVSS) images and source catalog wereused to detect radio emission from the 885 planetary nebulae north ofJ2000 declination delta = -40 deg in the Strasbourg-ESO Catalogue ofGalactic Planetary Nebulae. We identified 680 radio sources brighterthan about S = 2.5 mJy beam-1 (equivalent to T ~ 0.8 K in the 45" FWHMNVSS beam) with planetary nebulae by coincidence with accurate opticalpositions measured from Digitized Sky Survey (DSS) images. Totalextinction coefficients c at lambda = 4861 Angstroms were calculated forthe 429 planetary nebulae with available H beta fluxes and low free-freeoptical depths at 1.4 GHz. The variation of c with Galactic latitude andlongitude is consistent with the extinction being primarily interstellarand not intrinsic.
| 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.
| Orientation of planetary nebulae within the Galaxy
Narrow-band CCD images of 209 axially symmetrical planetary nebulae(PNe) have been examined in order to determine the orientation of theiraxes within the disc of the Galaxy. The nebulae have been divided intothe bipolar (B) and elliptical (E) PNe morphological types, according tothe scheme of Corradi & Schwarz. In both classes, contrary to theresults of Melnick & Harwit and Phillips we do not find any strongevidence for non-random orientations of the nebulae in the Galaxy.Compared with previous work in this field, the present study takesadvantage of the use of larger and morphologically more homogeneoussamples and offers a more rigorous statistical analysis.
| 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.
| ^12^CO J = 1-0 synthesis images of a dense torus in M2-9.
We have mapped the ^12^CO J=1-0 emission in the bipolar planetary nebulaM2-9 using the IRAM interferometer. From the maps we were able toinvestigate in detail the morphology and the kinematics of the moleculargas. The data are best explained by assuming that the molecular gas isconcentrated in an expanding, clumpy torus. The torus, which surroundsthe nucleus of M2-9, has a mean diameter of about 6". Its symmetry axisis tilted by 17° with respect to the plane of the sky. Thede-projected expansion velocity is 7km/s, and its kinematical age isabout 2100years assuming a distance of 1kpc. The lower limit for thetotal mass of the molecular gas is estimated to be9x10^-3^Msun_, i.e. at least comparable to the ionized massin the nebula.
| Planetary nebulae morphologies, central star masses and nebular properties.
We have constituted a sample of about 80 PN with defined morphologiesand well observed basic parameters (fluxes, angular radii, expansionvelocities and magnitudes of the central stars). For these PN, we havederived the central star masses by comparing the observed set ofparameters with those predicted by a simple evolutionary model of a PN,expanding at the same velocity as the observed one. We have thenexamined the relations between the PN morphological types and otherproperties, linked to the central star mass. Bipolar PN are shown tohave a wider distribution of central star masses than the rest of PN,and shifted towards higher values. They lie closer to the Galactic planeand tend to have larger N/O ratios. Point symmetric PN, which have notbeen much studied so far, are found to constitute an outstanding class.They show an almost perfect M_*_-v_exp_ correlation. They correspond toa rather short evolutionary stage of PN. They lie, on average, furtherfrom the Galactic plane than bipolar PN and tend to have lower N/O.Globally, PN with higher central star masses are found closer to theGalactic plane, and the observed relation between N/O and M_*_ isroughly consistent with the predictions from evolutionary models for AGBstars.
| The molecular envelopes of planetary nebulae.
We report the results of a survey of millimeter CO emission in 91planetary nebulae using the IRAM 30 m and SEST 15 m telescopes. Theobservations provide new detections or improved data for 23 nebulae inthe CO(2-1) and/or CO(1-0) line, and sensitive limits for those not seenin CO. Analysis of the results together with previous observationsconfirms the existence of an important class of planetary nebulae withmassive (10^-2^-a few M_sun__) envelopes of molecular gas.These nebulae typically have abundance ratios of N/O>0.3 and bipolarmorphologies indicative of a young disk population. The column densitythrough the envelopes and their mass relative to the mass of ionized gasshow dramatic decreases with increasing nebular size, documenting theexpansion of the envelopes and the growth of the optical nebulae at theexpense of the molecular gas. The molecular envelopes remain a majormass component in these objects until the nebulae reach a radius ofR=~0.1pc. The nebulae not detected in CO have little or no molecular gas(<~10^-2^-10^-3^Msun_), and their envelopes must berapidly photo-dissociated before or during the compact phase. The largedifferences in the molecular gas content of the nebulae highlight thedifferent evolutionary paths for planetary nebula formation which resultfrom the range in mass of the progenitors and the structure of theircircumstellar envelopes.
| H2 Emission from Planetary Nebulae: Signpost of Bipolar Structure
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...462..777K&db_key=AST
| Circumstellar Molecular Envelopes of AGB and Post-AGB Objects
Not Available
| IRAS-selected Galactic star-forming regions - II. Water maser detections in the extended sample
The results of the analysis of the occurrence of 22.2-GHz H_2O maseremission in a sample of 1409 IRAS sources north of declination -30 degassociated with star-forming regions are presented. Our sample containsall the IRAS sources that satisfy Emerson criteria for selectingmolecular cores associated with the earliest evolutionary stages of thestar-forming process. In a previous paper, we have reported the resultsof the observations of about one third of the sample. In the presentpaper the observations of the remaining IRAS sources are presented: 18of them are newly detected maser sources. The results show that 20 percent of all IRAS sources that satisfy the Wood & Churchwell criteriahave H_2O water masers. This is in agreement with the assumption thatthese criteria select objects that are connected with the early phasesof the evolution of high-mass star-forming regions. Moreover, about onethird of the whole sample selected according to Emerson criteriacontains IRAS sources that are not associated with massive star-formingprocesses, but probably with molecular cores in low-mass star-formingregions.
| A statistical distance scale for Galactic planetary nebulae
A statistical distance scale is proposed. It is based on the correlationbetween the ionized mass and the radius and the correlation between theradio continuum surface brightness temperature and the nebular radius.The proposed statistical distance scale is an average of the twodistances obtained while using the correlation. These correlations,calibrated based on the 1`32 planetary nebulae with well-determinedindividual distances by Zhang, can reproduce not only the averagedistance of a sample of Galactic Bulge planetary nebulae exactly at thedistance to the Galactic center, but also the expected Gaussiandistribution of their distances around the Galactic center. This newdistance scale is applied to 647 Galactic planetary nebulae. It isestimated that this distance scale can be accurate on average to35%-50%. Our statistical distance scale is in good agreement with theone recently proposed by Van de Steene and Zijlstra. The correlationsfound in this study can be attributed to the fact that the core mass ofthe central stars has a very sharp distribution, strongly peaked atapprox. 0.6 solar mass. We stress that the scatter seen in thestatistical distance scale is likely to be real. The scatter is causedby the fact that the core mass distribution, although narrow andstrongly peaked, has a finite width.
| On an alternative statistical distance scale for planetary nebulae. Catalog with statistical distances to planetary nebulae.
We have proposed a statistical method to determine distances toplanetary nebulae. The method is based on an empirical correlationbetween the radio-continuum brightness temperature and radius. Here wepresent a catalog of distance determinations calculated using thismethod.
| 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.
| Confrontation of theoretical tracks for post-AGB stars with observations of planetary nebulae
We have constructed a distance-independent diagram to test publishedtheoretical tracks for the evolution of post-AGB stars by comparing themwith the Galactic planetary nebulae data base. We have found noinconsistency between observations and the set of tracks computed bySchoenberner (1981, 1983) and Bloecker & Schoenberner (1990). On theother hand, observations do not seem support the large transition timesbetween the end of the AGB superwind and the beginning of the planetarynebula ionization phase adopted in the models of Vassiliadis & Wood(1994).
| The correlations between planetary nebula morphology and central star evolution
The morphology of 111 Galactic planetary nebulae has been studied inrelation to the evolutionary stage of their central stars. In order tolocate these stars on the log Teff - Log L/solar luminosityplane, we have calculated the Zanstra temperatures with the most up todate fluxes and magnitudes available in the literature. Distances to thenebulae were estimated with statistical methods. The different natureand evolutionary stages of central stars have been related to themorphologies of the surrounding nebulae in a statistical sense. We foundthat multiple shell nebulae contain stars that are at a differentevolutionary stage than those of single shell nebulae; we also foundthat bipolar and elliptical planetary nebulae very likely containcentral stars with a different mass distribution; furthermore, we buildan optical thickness sequence of morphological types.
| Trace of planetary nebula evolution by distance-independent parameters
Using existing infrared and radio data on a sample of 432 planetarynebulae, we derived a number of distance-independent parameters forcomparison with evolutionary models of planetary nebulae. We find thatmany of the observed properties of planetary nebulae can be explained bycurrent central star evolutionary models, even if the time scales aresubject to significant change by a factor of up to an order ofmagnitude. Specifically, we find that the evolutionary tracks are wellseparated in the radio surface brightness-central star temperatureplane, therefore allowing us to determine the core mass of individualplanetary nebulae. We also obtain the luminosity and gravity of thecentral stars of individual nebulae, from their temperature and coremass, without relying on the distance assumptions. We find that ourresults of the core mass are in good agreement with those of Mendez etal. (1992) and Tylenda et al. (1991). A systematic, large discrepancy isfound between the luminosity found in this work and that found byGathier and Pottasch (1986).
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