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An Atlas of [N II] and [O III] Images and Spectra of Planetary Nebulae
We present an atlas of Hubble Space Telescope images and ground-based,long-slit, narrowband spectra centered on the 6584 Å line of [NII] and the 5007 Å line of [O III]. The spectra were obtained fora variety of slit positions across each target (as shown on the images)in an effort to account for nonspherical nebular geometries in a robustmanner. We have extended the prolate ellipsoidal shell model originallydevised by Aaquist, Zhang, and Kwok to generate synthetic images, aswell as long-slit spectra. Using this model, we have derived basicparameters for the subsample of PNe that present ellipsoidal appearancesand regular kinematic patterns. We find differences between ourparameters for the target PNe as compared to those of previous studies,which we attribute to increased spatial resolution for our image dataand the inclusion of kinematic data in the model fits. The data andanalysis presented in this paper can be combined with detections ofnebular angular expansion rates to determine precise distances to the PNtargets.
| The Cool ISM in Elliptical Galaxies. I. A Survey of Molecular Gas
We present preliminary results from a survey of CO emission from membersof a volume-limited sample of non-cluster elliptical galaxies. Ourintent is to compare the gas properties of these ellipticals to a sampleof lenticulars selected using similar criteria. The data, although stillsparse, suggest that the cool gas in ellipticals shows the same puzzlingupper mass cutoff found in the lenticular galaxies. We find, however,significantly lower detection rates and possibly much lowerH2/H I mass ratios in the ellipticals. The detection rate ishigher among the lower mass galaxies, as has been found previously. Thisseems puzzling given that the deeper potential wells of the largergalaxies ought to make gas retention easier, but perhaps that effect isoverwhelmed by feedback from the central supermassive black hole. As wehave observed ~40% of our full sample, the conclusions are necessarilytentative at this time.
| Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79northern Galactic planetary nebulae (PNe) whose morphological classeshave been uniformly determined. Ionic abundances and plasma diagnosticswere derived from selected optical line strengths in the literature, andelemental abundances were estimated with the ionization correctionfactor developed by Kingsbourgh & Barlow in 1994. We compare theelemental abundances to the final yields obtained from stellar evolutionmodels of low- and intermediate-mass stars, and we confirm that mostbipolar PNe have high nitrogen and helium abundance and are the likelyprogeny of stars with main-sequence mass greater than 3Msolar. We derive =0.27 and discuss the implication of such ahigh ratio in connection with the solar neon abundance. We determine theGalactic gradients of oxygen and neon and foundΔlog(O/H)/ΔR=-0.01 dex kpc-1 andΔlog(Ne/H)/ΔR=-0.01 dex kpc-1. These flat PNgradients are irreconcilable with Galactic metallicity gradientsflattening with time.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We 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 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 distances of Type I planetary nebulae
The distances D of planetary nebulae (PNe) are still extremelyuncertain. Although a variety of methods have been used to evaluate thisparameter, these are often in conflict, and subject to large random andsystematic errors. It is therefore important to evaluate D using as manyindependent procedures as possible. We outline here one further way inwhich this parameter may be assessed. It is noted that where the nebularmass range is narrow, then one might expect observed PNe radii to beroughly similar. This, where it occurs, would also result in acorrelation between their angular diameters Θ, and distances D.We find that just such a trend occurs for Type I nebulae, and we employthis to determine distances to a further 44 such outflows. Our meanvalues of D appear similar to those of Zhang [ApJS 98 (1995) 659],implying a relatively long PNe distance scale.
| A reanalysis of chemical abundances in galactic PNe and comparison with theoretical predictions
New determinations of chemical abundances for He, N, O, Ne, Ar and Sare derived for all galactic planetary nebulae (PNe) so far observedwith a relatively high accuracy, in an effort to overcome differences inthese quantities obtained over the years by different authors usingdifferent procedures. These include: ways to correct for interstellarextinction, the atomic data used to interpret the observed line fluxes,the model nebula adopted to represent real objects and the ionizationcorrections for unseen ions. A unique `good quality' classical-typeprocedure, i.e. making use of collisionally excited forbidden lines toderive ionic abundances of heavy ions, has been applied to allindividual sets of observed line fluxes in each specific position withineach PN. Only observational data obtained with linear detectors, andsatisfying some `quality' criteria, have been considered. Suchobservations go from the mid-1970s up to the end of 2001. Theobservational errors associated with individual line fluxes have beenpropagated through the whole procedure to obtain an estimate of theaccuracy of final abundances independent of an author's `prejudices'.Comparison of the final abundances with those obtained in relevantmulti-object studies on the one hand allowed us to assess the accuracyof the new abundances, and on the other hand proved the usefulness ofthe present work, the basic purpose of which was to take full advantageof the vast amount of observations done so far of galactic PNe, handlingthem in a proper homogeneous way. The number of resulting PNe that havedata of an adequate quality to pass the present selection amounts to131. We believe that the new derived abundances constitute a highlyhomogeneous chemical data set on galactic PNe, with realisticuncertainties, and form a good observational basis for comparison withthe growing number of predictions from stellar evolution theory. Owingto the known discrepancies between the ionic abundances of heavyelements derived from the strong collisonally excited forbidden linesand those derived from the weak, temperature-insensitive recombinationlines, it is recognized that only abundance ratios between heavyelements can be considered as satisfactorily accurate. A comparison withtheoretical predictions allowed us to assess the state of the art inthis topic in any case, providing some findings and suggestions forfurther theoretical and observational work to advance our understandingof the evolution of low- and intermediate-mass stars.
| The 3-D ionization structure and evolution of NGC 7009 (Saturn Nebula)
Tomographic and 3-D analyses for extended, emission-line objects areapplied to long-slit ESO NTT + EMMI high-resolution spectra of theintriguing planetary nebula NGC 7009, covered at twelve position angles.We derive the gas expansion law, the diagnostics and ionic radialprofiles, the distance and the central star parameters, the nebularphoto-ionization model and the spatial recovery of the plasma structureand evolution. The Saturn Nebula (distance≃1.4 kpc, age≃6000yr, ionized mass≃0.18 Mȯ) consists of severalinterconnected components, characterized by different morphology,physical conditions, excitation and kinematics. We identify four``large-scale'', mean-to-high excitation sub-systems (the internalshell, the main shell, the outer shell and the halo), and as many``small-scale'' ones: the caps (strings of low-excitation knots withinthe outer shell), the ansae (polar, low-excitation, likely shockedlayers), the streams (high-excitation polar regions connecting the mainshell with the ansae), and an equatorial, medium-to-low excitationpseudo-ring within the outer shell. The internal shell, the main shell,the streams and the ansae expand at Vexp≃4.0 × Rarcsec km s-1, the outer shell, the caps and the equatorialpseudo-ring at Vexp≃3.15 × R arcsec kms-1, and the halo at Vexp≃10 kms-1. We compare the radial distribution of the physicalconditions and the line fluxes observed in the eight sub-systems withthe theoretical profiles coming from the photo-ionization code CLOUDY,inferring that all the spectral characteristics of NGC 7009 areexplainable in terms of photo-ionization by the central star, a hot (log T* ≃4.95) and luminous ( logL*/Lȯ≃3.70) 0.60-0.61Mȯ post-AGB star in the hydrogen-shell nuclear burningphase. The 3-D shaping of the Saturn Nebula is discussed within anevolutionary scenario dominated by photo-ionization and supported by thefast stellar wind: it begins with the superwind ejection (firstisotropic, then polar deficient), passes through the neutral, transitionphase ({lasting} ≃3000 yr), the ionization start (occurred≃2000 yr ago), and the full ionization of the main shell(≃1000 yr ago), at last reaching the present days: the wholenebula is optically thin to the UV stellar flux, except the caps (meanlatitude condensations in the outer shell, shadowed by the main shell)and the ansae (supersonic ionization fronts along the major axis).Based on observations made with: ESO Telescopes at the La SillaObservatories (program ID 65.I-0524), and the NASA/ESA Hubble SpaceTelescope, obtained from the data archive at the Space TelescopeInstitute. Observing programs: GO 6117 (P.I. Bruce Balick), GO 6119(P.I. Howard Bond) and GO 8390 (P.I. Arsen Hajian). STScI is operated bythe association of Universities for Research in Astronomy, Inc. underthe NASA contract NAS 5-26555. We extensively apply the photo-ionizationcode CLOUDY, developed at the Institute of Astronomy of the CambridgeUniversity (Ferland et al. 1998).
| 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}
| Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical properties
We present a comprehensive observational study of haloes aroundplanetary nebulae (PNe). Deep Hα+[NII] and/or [OIII] narrow-bandimages have been obtained for 35 PNe, and faint extended haloes havebeen newly discovered in the following 10 objects: Cn 1-5, IC 2165, IC2553, NGC 2792, NGC 2867, NGC 3918, NGC 5979, NGC 6578, PB 4, andpossibly IC 1747. New deep images have also been obtained of other knownor suspected haloes, including the huge extended emission around NGC3242 and Sh 2-200. In addition, the literature was searched, andtogether with the new observations an improved data base containing some50 PN haloes has been compiled.The halo sample is illustrated in an image atlas contained in thispaper, and the original images are made available for use by thescientific community at http://www.ing.iac.es/~rcorradi/HALOES/.The haloes have been classified following the predictions of modernradiation-hydrodynamical simulations that describe the formation andevolution of ionized multiple shells and haloes around PNe. According tothe models, the observed haloes have been divided into the followinggroups: (i) circular or slightly elliptical asymptotic giant branch(AGB) haloes, which contain the signature of the last thermal pulse onthe AGB; (ii) highly asymmetrical AGB haloes; (iii) candidaterecombination haloes, i.e. limb-brightened extended shells that areexpected to be produced by recombination during the late post-AGBevolution, when the luminosity of the central star drops rapidly by asignificant factor; (iv) uncertain cases which deserve further study fora reliable classification; (v) non-detections, i.e. PNe in which no halois found to a level of <~10-3 the peak surface brightnessof the inner nebulae.We discuss the properties of the haloes: detection rate, morphology,location of the central stars in the Hertzsprung-Russell diagram, sizes,surface brightness profiles, and kinematical ages. Among the mostnotable results, we find that, as predicted by models, ionized AGBhaloes are a quite common phenomenon in PNe, having been found in 60 percent of elliptical PNe for which adequately deep images exist. Another10 per cent show possible recombination haloes. In addition, using thekinematical ages of the haloes and inner nebulae, we conclude that mostof the PNe with observed AGB haloes have left the AGB far from a thermalpulse, at a phase when hydrogen burning is the dominant energy source.We find no significant differences between the AGB haloes ofhydrogen-poor and hydrogen-rich central stars.
| The 3-D ionization structure of NGC 6818: A Planetary Nebula threatened by recombination
Long-slit NTT+EMMI echellograms of NGC 6818 (the Little Gem) at nineequally spaced position angles, reduced according to the 3-D methodologyintroduced by Sabbadin et al. (\cite{Sabbadin00}a,b), allowed us toderive: the expansion law, the diagnostics and ionic radial profiles,the distance and the central star parameters, the nebularphoto-ionization model, the 3-D reconstruction in He II, [O III] and [NII], the multicolor projection and a series of movies. The Little Gemresults to be a young (3500 years), optically thin (quasi-thin in somedirections) double shell (Mion =~ 0.13 Msun) ata distance of 1.7 kpc, seen almost equatorial on: a tenuous and patchyspherical envelope (r =~ 0.090 pc) encircles a dense and inhomogeneoustri-axial ellipsoid (a/2 =~ 0.077 pc, a/b =~ 1.25, b/c =~ 1.15)characterized by a hole along the major axis and a pair of equatorial,thick moustaches. NGC 6818 is at the start of the recombination phasefollowing the luminosity decline of the 0.625 Msun centralstar, which has recently exhausted the hydrogen shell nuclear burningand is rapidly moving toward the white dwarf domain (log T*=~ 5.22 K; log L*/Lsun =~ 3.1). The nebula isdestined to become thicker and thicker, with an increasing fraction ofneutral, dusty gas in the outermost layers. Only over some hundreds ofyears the plasma rarefaction due to the expansion will prevail againstthe slower and slower stellar decline, leading to a gradual re-growingof the ionization front. The exciting star of NGC 6818 (mV =~17.06) is a visual binary: a faint, red companion (mV =~17.73) appears at 0.09 arcsec in PA =190degr , corresponding to aseparation ge 150 AU and to an orbital period ge 1500 years.Based on observations made with ESO Telescopes at the La SillaObservatories, under programme ID 65.I-0524, and on observations madewith the NASA/ESA Hubble Space Telescope, obtained from the data archiveat the Space Telescope Institute (observing programs GO 7501 and GO8773; P.I. Arsen Hajian). STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under the NASA contract NAS5-26555. We have applied the photo-ionization code CLOUDY, developed atthe Institute of Astronomy of the Cambridge University.
| The association of IRAS sources and 12CO emission in the outer Galaxy
We 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
| 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.
| Enigmatic Low-Velocity Jet-Like Features in Planetary Nebulae
We are developing a project aimed at studying the physical properties,origin and evolution of low-ionization structures in planetary nebulae.Within this project we have identified a number of pairs of highlycollimated low-ionization jet-like features (Gonçalves et al.2001). In spite of being very similar to real jets, they have theintriguing property of possessing expansion velocities which are verylow, or at least not significantly different from, that of the shells inwhich they are embedded. In this contribution we discuss our data onthese fake jets (Corradi et al. 1997, 1999) and compare them withexisting theoretical models for the formation of collimated structuresin PNe. These enigmatic jet-like systems are not easily accounted forwithin the theoretical scenarios that deal with collimated features inPNe.
| Gas temperature and excitation classes in planetary nebulae
Empirical methods to estimate the elemental abundances in planetarynebulae usually use the temperatures derived from the [O III] and [N II]emission-line ratios, respectively, for the high- and low-ionizationzones. However, for a large number of objects these values may not beavailable. In order to overcome this difficulty and allow a betterdetermination of abundances, we discuss the relationship between thesetwo temperatures. Although a correlation is not easily seen when asample of different PNe types is used, the situation is improved whenthey are gathered into excitation classes. From [OII]/[OIII] andHeII/HeI line ratios, we define four excitation classes. Then, usingstandard photoionization models which fit most of the data, a linearrelation between the two temperatures is obtained for each of the fourexcitation classes. The method is applied to several objects for whichonly one temperature can be obtained from the observed emission linesand is tested by recalculation of the radial abundance gradient of theGalaxy using a larger number of PNe. We verified that our previousgradient results, obtained with a smaller sample of planetary nebulae,are not changed, indicating that the temperature relation obtained fromthe photoionization models are a good approximation, and thecorresponding statistical error decreases as expected. Tables 3-5, 7 and9 are only available in electronic form at http://www.edpsciences.org
| Helium contamination from the progenitor stars of planetary nebulae: The He/H radial gradient and the ΔY / ΔZ enrichment ratio
In this work, two aspects of the chemical evolution of 4He inthe Galaxy are considered on the basis of a sample of disk planetarynebulae (PN). First, an application of corrections owing to thecontamination of 4He from the evolution of the progenitorstars shows that the He/H abundance by number of atoms is reduced by0.012 to 0.015 in average, leading to an essentially flat He/H radialdistribution. Second, a determination of the helium to heavy elementenrichment ratio using the same corrections leads to values in the range2.8 < ΔY / ΔZ < 3.6 for Y p = 0.23 and 2.0< ΔY / ΔZ < 2.8 for Y p = 0.24, in goodagreement with recent independent determinations and theoretical models.
| Low-Ionization Structures in Planetary Nebulae: Confronting Models with Observations
Around 50 planetary nebulae (PNs) are presently known to possess``small-scale'' low-ionization structures (LISs) located inside oroutside their main nebular bodies. We consider here the different kindsof LISs (jets, jetlike systems, symmetrical and nonsymmetrical knots)and present a detailed comparison of the existing model predictions withthe observational morphological and kinematical properties. We find thatnebulae with LISs appear indistinctly spread among all morphologicalclasses of PNs, indicating that the processes leading to the formationof LISs are not necessarily related to those responsible for theasphericity of the large-scale morphological components of PNs. We showthat both the observed velocities and locations of most nonsymmetricalsystems of LISs can be reasonably well reproduced assuming either fossilcondensations originated in the asymptotic giant branch (AGB) wind or insitu instabilities. The jet models proposed to date (hydrodynamical andmagnetohydrodynamical interacting winds or accretion disk collimatedwinds) appear unable to account simultaneously for several keycharacteristics of the observed high-velocity jets, such as theirkinematical ages and the angle between the jet and the symmetry axes ofthe nebulae. The linear increase in velocity observed in several jetsfavors magnetohydrodynamical confinement compared to pure hydrodynamicalinteracting wind models. On the other hand, we find that the formationof jetlike systems characterized by relatively low expansion velocities(similar to those of the main shells of PNs) cannot be explained by anyof the existing models. Finally, the knots that appear in symmetricaland opposite pairs of low velocity could be understood as the survivalof fossil (symmetrical) condensations formed during the AGB phase or asstructures that have experienced substantial slowing down by the ambientmedium.
| Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.
Not Available
| IUE Spectra and Emission-Line Fluxes for Eight Planetary Nebulae
We present IUE spectra and observed emission-line fluxes for theplanetary nebulae NGC 2438, NGC 2610, NGC 5307, NGC 6620, NGC 7354,K3-27, PB 8, and He 2-182. All data have been processed by the finalarchive NEWSIPS routines. PB 8 and He 2-182 display strong C IVλ1549 P Cygni profiles from which terminal wind velocities of-2835+/-500 km s-1 and -2350+/-500 km s-1,respectively, were determined.
| Recessional Halos in Planetary Nebulae: An Undervalued Aspect of Nebular Morphology
Many 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.
| Distances of Galactic Planetary Nebulae Based on a Relationship Between the Central Star Mass and the N/O Abundance
In this paper, we propose a method to determine distances of Galacticplanetary nebulae on the basis of a relationship between the centralstar mass and the nebular N/O abundance ratio. This relationship is usedin combination with some basic parameters of the central stars, such asthe lambda 5480 flux, surface gravity and visual magnitude in order toobtain distances to a sample of a hundred Galactic planetary nebulae.
| Tomography of the low excitation planetary nebula NGC 40
Spatially resolved, long-slit echellograms at different position anglesof the bright, low excitation planetary nebula NGC 40 indicate that thehigher the gas excitation, the faster the radial motion, thus confirmingthe overturn of the Wilson law already suggested by Sabbadin &Hamzaoglu (1982). New reduction procedures, giving the radial trends ofthe electron density and of the ionic and chemical abundances, wereapplied to NGC 40; they show that: - the radial matter distribution hasa sharp ``bell'' profile with peaks up to 4000 cm-3; - theionization structure is peculiar, indicating the presence of chemicalcomposition gradients within the nebula: the innermost regions, hydrogendepleted, are essentially constituted of photospheric material ejectedat high velocity by the WC8 nucleus. Moreover, detailed H+,O++ and N+ tomographic maps, giving the spatialionic distributions at four position angles, are presented and discussedwithin the interacting winds evolutionary model.
| Gas Dynamics in Planetary Nebulae: From Macro-structures to FLIERs
Purpose of this paper is to clarify how Planetary Nebulae (PNe) are veryinteresting laboratories to study cosmic gas dynamics. I first recallthe history of PNe which are generated from low and intermediate massstars through successive mass loss processes starting in the Reg Giantphase of evolution and continuing also after the termination of thepulsed AGB phase, where most of the nebular mass is believed to beejected. The correponding stellar winds are the ingredients of thenebula. Their initial properties and subsequent mutual interactions,under the action of the evolving stellar radiation field, areresponsible for the properties of the nebula. The observed structures ofPNe are considered in detail. Larger scale macroscopic structures (MACS)are examined separately from quite smaller scale microscopic structures(MICS). The formation of MACS, at least in cases of round to moderatelyelliptical PNe, is shown to be reasonably well understood in terms ofexisting hydrodynamical models. Considering the kinematical behaviour,MICS can be separated into FLIERs (Fast Low Ionization Emitting Regions)and SLOWERs (slowly moving). Attention is focussed on FLIERs and on theproposed mechanisms to interpret them. Recent observations with theHubble Space Telescope have provided us with a wealth of detailed(subarcsec) information on the nebular structures. The inner structureof FLIERs is here illustrated to consist of substructures of variousshapes with an high degree of individually from object to object, alsowithin the same PN. These new data call for deeper thoretical efforts tosolve the problems of cosmic gas dynamics, posed by their observedproperties. An ample account is given of the most relevant originalworks, in an effort to allow the non specialist reader to quickly becomeacquainted with the status of art in the various aspects of the subject.
| The dust content of planetary nebulae: a reappraisal
We have performed a statistical analysis using broad band IRAS data onabout 500 planetary nebulae with the aim of characterizing their dustcontent. Our approach is different from previous studies in that it usesan extensive grid of photoionization models to test the methods forderiving the dust temperature, the dust-to-gas mass ratio and theaverage grain size. In addition, we use only distance independentdiagrams. With our models, we show the effect of contamination by atomiclines in the broad band IRAS fluxes during planetary nebula evolution.We find that planetary nebulae with very different dust-to-gas massratios exist, so that the dust content is a primordial parameter for theinterpretation of far infrared data of planetary nebulae. In contrastwith previous studies, we find no evidence for a decrease in thedust-to-gas mass ratio as the planetary nebulae evolve. We also showthat the decrease in grain size advocated by Natta & Panagia(\cite{NattaPanagia}) and Lenzuni et al. (\cite{Lenzuni}) is an artefactof their method of analysis. Our results suggest that the timescale fordestruction of dust grains in planetary nebulae is larger than theirlifetime. Table~1 is only accessible in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Infrared Planetary Nebulae in the NRAO VLA Sky Survey
In order to construct a sample of planetary nebulae (PNe) unbiased bydust extinction, we first selected the 1358 sources in the IRAS PointSource Catalog north of J2000 declination delta=-40^deg having measuredS(25 μm)>=1 Jy and colors characteristic of PNe: detections orupper limits consistent with both S(12 μm)<=0.35S(25 μm) andS(25 μm)>=0.35S(60 μm). The majority are radio-quietcontaminating sources such as asymptotic giant branch stars. Free-freeemission from genuine PNe should make them radio sources. The 1.4 GHzNRAO VLA Sky Survey (NVSS) images and source catalog were used to rejectradio-quiet mid-infrared sources. We identified 454 IRAS sources withradio sources brighter than S~2.5 mJy beam^-1 (equivalent to T~0.8 K inthe 45" FHWM NVSS beam) by positional coincidence. They comprise 332known PNe in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulaeand 122 candidate PNe, most of which lie at very low Galactic latitudes.Exploratory optical spectroscopic observations suggest that most ofthese candidates are indeed PNe optically dimmed by dust extinction,although some contamination remains from H II regions, Seyfert galaxies,etc. Furthermore, the NVSS failed to detect only 4% of the known PNe inour infrared sample. Thus it appears that radio selection can greatlyimprove the reliability of PN candidate samples withoutsacrificingcompleteness.
| Abundance gradients in the outer galactic disk from planetary nebulae
Radial abundance gradients of the element ratios O/H, Ne/H, S/H, andAr/H are determined on the basis of a sample of disk planetary nebulae.The behaviour of the gradients at large distances from the galacticcentre, R > R_0 = 7.6 kpc, is emphasized. It is concluded that thederived gradients are consistent with an approximately constant slope inthe inner parts of the Galaxy, and some flattening for distances largerthan R_0. A comparison is made with previous determinations using bothphotoionized nebulae and young stars, and some consequences ontheoretical models for the chemical evolution of the galactic disk arediscussed. Table~1 is available only electronically at the CDS(anonymousftp 130.79.128.5 or http://cdsweb.u-strasbg.fr/Abstract.html)
| Electron densities in planetary nebulae, and the unusual characteristics of the [S BT II] emission zone} ] densities in planetary nebulae
We investigate the radial variation of electron densities in planetarynebulae, using values of ne deriving from the [S ii]<~mbda6717/<~mbda6730 line ratio. As a result, we are able to showthat there is a sharp discontinuity in densities of order 1.4 dex closeto nebular radii R=0.1 pc. It is proposed, as a consequence, that mostnebulae contain two primary [S ii] emission zones, with densitiesdiffering by a factor ~ 10(2) . The intensity of emission from thedenser component increases by an order of magnitude where nebulae passfrom radiation to density-bound expansion regimes, resulting in acorresponding discontinuous jump in [S ii]/Hβ line ratios. Theorigins of these changes are not entirely clear, although one mechanismis investigated whereby the superwind outflows shock interact withexterior AGB envelopes. Finally, the derived trends in ne(R)are used to determine distances for a further 262 nebulae. The resultingdistance scale appears to be comparable to that of Daub (1982) and Cahnet al. (1992).
| On The Kinematics of Multiple-Shell Planetary Nebulae. I. Data and Expansion Velocities
We present spatially resolved echelle spectroscopy, obtained at highspectral resolution, for 15 multiple-shell planetary nebulae. Mostexhibit faint detached halos (IC 1295, MA 3, M 2-2, M 2-40, NGC 6804,NGC 6826, NGC 6884, NGC 6891, NGC 7662, PM 1-295, and Vy 2-3).Furthermore, we have included some with attached shells (IC 1454, K1-20, K 3-73, and PM 1-276) to allow comparison of the kinematicproperties of the two subclasses of multiple-shell planetary nebulae. Inaddition, some of the nebulae in our sample show a triple-shellstructure, composed of the bright main nebula and a combination of twoattached shells (PB 9), one attached shell and one detached halo (NGC6826, NGC 6891, NGC 7662, and Vy 2-3), or two detached shells (NGC6804). A new method for computing the expansion velocities of thoseshells that do not show line splitting has been developed. This methodassumes a thick-shell model and uses the observed Hα emissionbrightness profile to compute the volume emissivity dependence,ɛ(r), with the distance from the center of the nebula. Theexpansion velocity is then worked out by modeling how much the width ofa the Hα line decreases with the radius of the shell. The radialvelocity, expansion velocities of each shell, and turbulencecontribution to the line width are presented. The expansion velocity ofthe detached halos spans from 12 to 30 km s^-1. It is worth noting thatthe expansion velocities obtained by this method are greater than ifthey were computed with a thin-shell model, as has previously been done.In relation to the attached shells, their expansion velocities span from10 to 30 km s^-1. When the expansion velocities of the outer attachedshells are related to the ellipticity of the inner shells, a trendtoward faster expansion of the outer than the inner shells at higherellipticities is found. The turbulent contribution to the line width hasalso been established. It is smaller for halos (0 km s^-1 <=sigma_tur <= 6 km s^-1) than for attached shells (0 km s^-1 <=sigma_tur <= 15 km s^-1). This suggests that large-scale hydrodynamicprocesses are more important in attached shells than in detached halos.We have also studied the kinematics of the detached halos whosemorphology is perturbed from a round shape to a dipole asymmetry,indicating its interaction with the surrounding interstellar medium. Wefound systematic differences between the kinematical behavior of theenhanced edge of the halo and the opposite side in these cases, thusrevealing the kinematic effect of the interaction of the halos with theinterstellar medium.
| Disturbed FLIERS in Planetary Nebulae
We examine the recent claim by Balick et al. that their observations offast low-ionization emission regions in four planetary nebulae poseserious problems to the so-called fast-bullet model for the FLIERs.Using timescale estimates and the results of existing numerical studies,and from the phenomenology of 11 objects, we conclude that, contrary tothe claim made in the above paper, the fast-bullet model remainsconsistent with observations.
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