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Time variation of radial gradients in the Galactic disk: electron temperatures and abundances
Aims.We investigate the electron temperature gradient in the galacticdisk as measured by young HII regions on the basis of radiorecombination lines and the corresponding gradient in planetary nebulae(PN) based on [OIII] electron temperatures. The main goal is toinvestigate the time evolution of the electron temperature gradient andof the radial abundance gradient, which is essentially a mirror image ofthe temperature gradient. Methods: The recently derived electrontemperature gradient from radio recombination lines in HII regions iscompared with a new determination of the corresponding gradient fromplanetary nebulae for which the progenitor star ages have beendetermined. Results: The newly derived electron temperature gradientfor PN with progenitor stars with ages in the 4-5 Gyr range is muchsteeper than the corresponding gradient for HII regions. These electrontemperature gradients are converted into O/H gradients in order to makecomparisons with previous estimates of the flattening rate of theabundance gradient. Conclusions: .It is concluded that the O/H gradienthas flattened out in the past 5 Gyr at a rate of about 0.0094 dexkpc-1 Gyr-1, in good agreement with our previousestimates.
| The Macquarie/AAO/Strasbourg Hα Planetary Nebula Catalogue: MASH
We present the Macquarie/AAO/Strasbourg Hα Planetary NebulaCatalogue (MASH) of over 900 true, likely and possible new Galacticplanetary nebulae (PNe) discovered from the AAO/UKST Hα survey ofthe southern Galactic plane. The combination of depth, resolution,uniformity and areal coverage of the Hα survey has opened up ahitherto unexplored region of parameter space permitting the detectionof this significant new PN sample. Away from the Galactic bulge the newPNe are typically more evolved, of larger angular extent, of lowersurface brightness and more obscured (i.e. extinguished) than those inmost previous surveys. We have also doubled the number of PNe in theGalactic bulge itself and although most are compact, we have also foundmore evolved examples. The MASH catalogue represents the culmination ofa seven-year programme of identification and confirmatory spectroscopy.A key strength is that the entire sample has been derived from the same,uniform observational data. The 60 per cent increase in known GalacticPNe represents the largest ever incremental sample of such discoveriesand will have a significant impact on many aspects of PN research. Thisis especially important for studies at the faint end of the PNluminosity function which was previously poorly represented.
| On the progenitor of V838 Monocerotis
We summarize and analyze the available observational data on theprogenitor and the environment of V838 Mon. From the availablephotometric data for the progenitor of V838 Mon we exclude thepossibility that the object before eruption was an evolved red giantstar (AGB or RGB star). We find that most likely it was a main sequenceor pre-main sequence star of 5-10~ Mȯ. From thelight echo structure and evolution we conclude that the reflecting dustis of interstellar nature rather than blown by V838 Mon in the past. Wediscuss the IRAS and CO data for interstellar medium observed near theposition of V838 Mon. Several interstellar molecular regions have radialvelocities similar to that of V838 Mon, so dust seen in the light echomight be related to one of them.
| Star formation in RCW 108: Triggered or spontaneous?
We present visible, near infrared and mm-wave observations of RCW 108, amolecular cloud complex in the Ara OB1 association that is being erodedby the energetic radiation of two O-type stars in the nearby cluster NGC6193. The western part of the RCW108 molecular cloud, for which wederive a mass of ~8000 Mȯ, contains an embedded compactHII region, IRAS 16362-4845, ionized by an aggregate of early-type starsfor which we estimate a mass of 210 Mȯ. Thespectral type of the earliest star is O9, as confirmed by the visiblespectrum of the compact HII region. We notice a lack of stars later thanA0 in the aggregate, at least having the moderate reddenings that arecommon among its B-type stars, and we speculate that this might be aconsequence of the extreme youth of the aggregate. We also note theexistence of a dense ionized clump (n > 104cm-3) appearing near the main ionizing star of the compactHII region. We examine the distribution of stars displaying infraredexcesses projected across the molecular cloud. While many of them arelocated in the densest (n 104-5 cm-3) areaof the molecular cloud near the position of IRAS 16362-4845, we alsofind a group concentrating towards the edge of the cloud that faces NGC6193, as well as some other stars beyond the edge of the molecularcloud. The intense ionizing radiation field by the O stars in NGC 6193is a clear candidate trigger of star formation in the molecular cloud,and we suggest that the existence and arrangement of stars in thisregion of the molecular cloud supports a scenario in which theirformation may be a consequence of this. However, infrared excess starsare also present in some areas of the opposite side of the cloud, whereno obvious candidate external trigger is identified. The existence ofsuch tracers of recent star formation scattered across the more massivemolecular cloud associated with IRAS 16362-4845, and the low starformation efficiency that we derive, indicate that it is in a state tostill form stars. This is in contrast to the less massive cloud (660 Mȯ) close to NGC 6193, which seems to be moreevolved and mostly already recycled into stars, and whose internalkinematics show hints of having been perturbed by the presence of themassive stars formed out of it.
| 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 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 relation between elemental abundances and morphology in planetary nebulae
An investigation of the variation of elemental abundances with planetarynebula morphology is of considerable interest, since it has a bearingupon how such sources are formed, and from which progenitors they areejected. Recent advances in morphological classification now enable usto assess such trends for a statistically significant number of sources.We find, as a result, that the distribution N[log(X/H)] of sources withrespect to elemental abundance (X/H) varies between the differingmorphologies. Circular sources tend to peak towards low abundancevalues, whilst bipolar nebulae (BPNe) peak towards somewhat highervalues. This applies for most elemental species, although it is perhapsleast apparent for oxygen. In contrast, elliptical sources appear todisplay much broader functions N[log(X/H)], which trespass upon thedomains of both circular and elliptical planetary nebulae (PNe).We take these trends to imply that circular sources derive fromlower-mass progenitors, bipolar sources from higher-mass stars, and thatelliptical nebulae derive from all masses of progenitor, high and low.Whilst such trends are also evident in values of mean abundance, they are much less clear. Only in the cases of He/H, N/H,Ne/H and perhaps Ar/H is there evidence for significant abundancedifferences.Certain BPNe appear to possess low abundance ratios He/H and Ar/H, andthis confirms that a few such outflows may arise from lower-massprogenitors. Similarly, we note that ratios are quite modestin elliptical planetary nebulae, and not much different from those forcircular and bipolar PNe; a result that conflicts with the expectationsof at least one model of shell formation.
| 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
| Classification of planetary nebulae by their departure from axisymmetry
We propose a scheme to classify planetary nebulae (PNe) according totheir departure from axisymmetric structure. We consider only departurealong and near the equatorial plane, i.e. between the two sidesperpendicular to the symmetry axis of the nebula. We consider six typesof departure from axisymmetry: (1) PNe where the central star is not atthe centre of the nebula; (2) PNe having one side brighter than theother; (3) PNe having unequal size or shape of the two sides; (4) PNewhere the symmetry axis is bent, e.g. the two lobes in a bipolar PN arebent toward the same side; (5) PNe where the main departure fromaxisymmetry is in the outer regions, e.g. an outer arc; and (6) PNe thatshow no departure from axisymmetry, i.e. any departure, if it exists, ison scales smaller than the scale of blobs, filaments and otherirregularities in the nebula. PNe that possess more than one type ofdeparture are classified by the most prominent type. We discuss theconnection between departure types and the physical mechanisms that maycause them, mainly resulting from the influence of a stellar binarycompanion. We find that ~50 per cent of all PNe in the analysed samplepossess large-scale departure from axisymmetry. This number is largerthan that expected from the influence of binary companions, namely~25-30 per cent. We argue that this discrepancy comes from many PNewhere the departure from axisymmetry, mainly unequal size, shape orintensity, results from the presence of long-lived and large (hot orcool) spots on the surface of their asymptotic giant branch progenitors.Such spots locally enhance the mass-loss rate, leading to a departurefrom axisymmetry, mainly near the equator, in the descendent PN.
| A possible observational measure of evolution in bipolar nebulae
Bipolar planetary nebulae (BPNe) possess a broad range of shapes,ranging from narrow-waisted butterfly-like structures, through to thosewhich are more nearly cylindrical. We point out that these morphologiesappear to be correlated with radio surface brightness Tb, inthe sense that higher values of Tb are associated withenvelopes having narrower waists. If one interprets the variation inTb as arising from shell evolution, as is usually assumed forother planetaries, this would then imply that shell morphology varieswith time in a manner which appears not to be consistent withevolutionary models. It also remains possible, however, that differentBPNe morphologies arise as a result of differing mechanisms offormation, and that the pre-collimation of high velocity central windsgives rise to narrower waists, and higher surface brightness nuclei. Ourresults, if this is true, may then imply that central winds possess abroad range of collimations.
| Departure from Axisymmetry in Planetary Nebulae
Many planetary nebulae (PNs) exhibit symmetries that range fromunremarkable spherical and elliptical shapes to quite exotic bipolar andpoint-symmetric shapes. However, there are many that exhibit distinctlynonaxisymmetric structure in either (1) the shape of the nebula, or (2)the off-centered position of the illuminating star. By examining a largenumber of well-resolved images of PNs, we estimate that ~30%-50% of allPNs exhibit distinctly nonaxisymmetric structure. In this paper, wediscuss how such departures from axisymmetry can arise from the binarynature of the progenitors of the PNs. The scenarios include (1)relatively close binaries with eccentric orbits, and (2) longer orbitalperiod systems with either circular or eccentric orbits. In the firstmechanism, the departure from axisymmetry is caused by the variation ofmass loss and/or mass transfer with the changing distance between thecompanions in their eccentric orbit. In the second mechanism, thedeparture from axisymmetry is the result of the time-varying vectordirection of the mass-losing star, or that of a possible pair of jetsfrom the companion, as the stars move around their orbit. In order toassess the fraction of PNs whose nonaxisymmetric morphologies areexpected to arise in binary systems, we have carried out a detailedpopulation synthesis study. In this study, a large number of primordialbinaries are evolved through the lifetimes of both stars, including windmass loss. We then assess whether the primary or the secondary (or both)produces a PN. The expected deviations from axisymmetry are thenclassified for each binary and the results tabulated. We find that ~25%of elliptical and ~30%-50% of bipolar PNs are expected to acquirenonaxisymmetric structure from binary interactions.
| Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.
Not Available
| The Formation of Very Narrow Waist Bipolar Planetary Nebulae
We discuss the interaction of the slow wind blown by an asymptotic giantbranch (AGB) star with a collimated fast wind (CFW) blown by itsmain-sequence or white dwarf companion, at orbital separations in therange of several AU<~a<~200 AU. The CFW results from accretion ofthe AGB wind into an accretion disk around the companion. The fast windis collimated by the accretion disk. We argue that such systems are theprogenitors of bipolar planetary nebulae and bipolar symbiotic nebulaewith a very narrow equatorial waist between the two polar lobes. The CFWwind will form two lobes along the symmetry axis and will furthercompress the slow wind near the equatorial plane, leading to theformation of a dense slowly expanding ring. Therefore, contrary to thecommon claim that a dense equatorial ring collimates the bipolar flow,we argue that in the progenitors of very narrow waist bipolar planetarynebulae, the CFW, through its interaction with the slow wind, forms thedense equatorial ring. Only later in the evolution, and after the CFWand slow wind cease, does the mass-losing star leave the AGB and blow asecond, more spherical, fast wind. At this stage the flow structurebecomes the one that is commonly assumed for bipolar planetary nebulae,i.e., collimation of the fast wind by the dense equatorial material.However, this results in the broadening of the waist in the equatorialplane and cannot by itself account for the presence of very narrowwaists or jets. We conduct a population synthesis study of the formationof planetary nebulae in wide binary systems which quantitativelysupports the proposed model. The population synthesis code follows theevolution of both stars and their arbitrarily eccentric orbit, includingmass loss via stellar winds, for 5×104 primordialbinaries. We show the number of expected systems that blow a CFW is inaccord with the number found from observations, to within the manyuncertainties involved. Overall, we find that ~5% of all planetarynebulae are bipolars with very narrow waists. Our population synthesisnot only supports the CFW model but more generally supports the binarymodel for the formation of bipolar planetary nebulae.
| Photometric Constraints upon Binaries in Bipolar Nebulae
We summarize current information concerning the photometry, distances,and extinctions for the central stars of bipolar nebulae (BPNs). Theseare used to place tight constraints upon the properties of binarycompanions and to derive upper limit mass and spectral functions forthese stars. A comparison of these results with model binary systemssuggests that many BPN central stars may have binary companions,although these are either unrecognized or undetected. However, thephotometric limits also imply that such binaries are unlikely to undergostable mass transfer during AGB Roche lobe overflow. Constraints uponthe masses of the secondary stars also enable limits to be placed uponorbital velocities, whence it is apparent that the motion of very few ofthese binaries is likely to be detected at current levels ofobservational precision.
| 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
| HaTr 10, a Planetary Nebula with Extremely Strong Nitrogen Lines
Optical spectra and direct images reveal that a very faint, scarcelystudied object (HaTr 10) which appears as an incomplete elliptical ringof 0.4′x0.2′ is a heavily obscured planetary nebula withexceptionally strong nitrogen lines ([N II] lambda6583/Hα~6). In theHα/[N II] versus Hα/[S II] diagnostic diagram, the object doesnot resemble H II regions or supernova remnants. It is quite far fromthe locus of the bulk of (bipolar) planetary nebulae. Photoionizationmodels can well reproduce the observed emission lines, by using a gas ofdensity 500 cm^-3 and with abundances of N=8.6, O=8.2, and all otherelements with solar abundances. On our deepest CCD frames the nebula isconsiderably larger (1.5′x1.0′), showing butterfly-like faintextensions. These symmetrical lobes, the ring around its waist, and thevery high nitrogen abundance support the classification of HaTr 10 as anextreme bipolar planetary nebula. We suggest that very deep frames ofnebulae that appear morphologically akin to HaTr 10 should be obtainedto enable or support a reliable classification.
| Classification and spectral analysis of faint central stars of highly excited planetary nebulae
We present narrow-band images of nine planetary nebulae (PN) and aspectral analysis of four of their central stars (CSPN) by means ofmodel atmosphere techniques based on medium-resolution optical spectra.Four of our CSPN are unambiguously identified. They are very hot (T_eff,>100 kK) and have about 1 - 2 times solar He/H abundance ratios. (theexciting stars of PN G214.9+07.8, PNG231.8+04.1, PN G283.6+25.3, PNG293.6+10.9). Three other of our PN are candidates forAbell 35-like objects: The supposed central stars ofPN G257.5+00.6 and PN G277.1-03.8exhibit F-type spectra while probably the hotter exciting star escapesdetection in the optical. Absorption features is the spectrum of thecentral star of PN G283.6+25.3 suggest the presenceof a cool companion. The images of PN G277.1-03.8display two close stars at its center which both show F-type spectra. Itappears likely that this is a binary system but better spectra andadditional UV spectra are required in order to successfully identify theexciting central star. Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile (proposals 56.D-0717, 57.D-0148)
| An atlas of images of Planetary Nebulae
A catalogue of narrowband CCD images of 100 southern and 1 northernPlanetary Nebulae is presented. The data are images taken in the lightof the Hα , Hβ , [OIII]500.7 nm, and [NII]658.4 nm lines. Foreach image we give the exposure time, the maximum extent of the nebula,and the seeing measured from the frames themselves. For ease ofreference, we have added an alphabetical list of the objects. Our listwill be useful for morphological studies of Planetary Nebulae, and iscomplementary to previously published lists, especially the one of\cite[Schwarz et al. (1992)]{sch92}. Based on observations obtained atthe European Southern Observatory, La Silla, Chile.
| Giant Holes and Emission Structures Around Planetary Nebulae on IRAS SkyView Images
Years ago, on a POSS I print, we were attracted by a long very faintfilament about 1.2 pc away from the high-galactic-latitude PN NGC 4361;if somehow connected to NGC 4361, this would correspond to the hugedistance of ca. 25 pc. In addition, on high-contrast copies of the POSSof the region around this PN, we found that it appears to be located ina ``hole" of low surface brightness. This hole turned out to be visibleon a 100mum IRAS SkyView map too and might thus be caused by someprocess that has either destroyed the (interstellar) dust or swept itaway (Zanin and Weinberger 1997, Proc. IAU Symp. 180, 290). Recently,Clayton and de Marco (1997, AJ, 114, 2679) decribed an approximatelycircular 40' large ``evacuated" area around the PN A 58 (=V605 Aql),visible on an IRAS 100mu image; 40' would correspond to 40 pc at theassumed distance. They suppose that one sees the result of swept up ISMdust, originating from the wind from the PN progenitor star. - - Arethere more examples of this new phenomenon? We have started a systematicsearch of areas around PNe using the IRAS SkyView (brightness scaling:``Hist. Eq.", colour table: ``B-W linear" and ``Stern special").Although by now we have examined only a fraction of the known PNe, wediscovered several ``holes" that are, in a few cases, too well definedto be projection effects. Particularly intriguing are, in addition toNGC 4361, a distinct hole, best seen at 25mum and 20' across, perfectlycentered on NGC 2899, and a huge (ca. 10^o large) hole plusfilament around the close (400 pc) PN LoTr 5. We also found giantemission structures, like a 1^o large ``spot" centered on NGC1514 at 12mum, a 1.5^o arc east of Lo 4, etc. Severalexamples are shown on the poster. - Models to explain the holes and theemission structures are in preparation.
| Electron densities in planetary nebulae, and the unusual characteristics of the [S BT II] emission zone} ] densities in planetary nebulae
We investigate the radial variation of electron densities in planetarynebulae, using values of ne deriving from the [S ii]<~mbda6717/<~mbda6730 line ratio. As a result, we are able to showthat there is a sharp discontinuity in densities of order 1.4 dex closeto nebular radii R=0.1 pc. It is proposed, as a consequence, that mostnebulae contain two primary [S ii] emission zones, with densitiesdiffering by a factor ~ 10(2) . The intensity of emission from thedenser component increases by an order of magnitude where nebulae passfrom radiation to density-bound expansion regimes, resulting in acorresponding discontinuous jump in [S ii]/Hβ line ratios. Theorigins of these changes are not entirely clear, although one mechanismis investigated whereby the superwind outflows shock interact withexterior AGB envelopes. Finally, the derived trends in ne(R)are used to determine distances for a further 262 nebulae. The resultingdistance scale appears to be comparable to that of Daub (1982) and Cahnet al. (1992).
| The kinematics of 867 galactic planetary nebulae
We present a compilation of radial velocities of 867 galactic planetarynebulae. Almost 900 new measurements are included. Previously publishedkinematical data are compared with the new high-resolution data toassess their accuracies. One of the largest samples in the literatureshows evidence for a systematic velocity offset. We calculate weightedaverages between all available data. Of the final values in thecatalogue, 90% have accuracies better than 20 km s(-1) . We use thiscompilation to derive kinematical parameters of the galacticdifferential rotation obtained from least-square fitting and toestablish the Disk rotation curve; we find no significal trend for thepresence of an increasing external rotation curve. We examine also therotation of the bulge; the derived curve is consistent with a linearlyincreasing rotation velocity with l: we find V_b,r=(9.9+/-1.3)l -(6.7+/-8.5) km s(-1) . A possible steeper gradient in the innermostregion is indicated. Table 2 is available in electronic form only, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Spectral trends in planetary nebulae: The roles of radiative and shock excitation
We have investigated de-reddened spectral line ratios for some 538planetary nebulae. As a result, it has proved possible to definecomparative variations between differing transitions, evaluate theviability of radiative modeling for the generality of nebulae, andassess the importance of shocks in modifying low- excitation linestrengths. Whilst most transitions are well represented in terms ofradiative excitation, the [OI] lambda 6300 Angstroms line appears to beappreciably too strong in most of the present sample; a deviation whichmay arise through shock interaction between the primary outflow shelland enveloping superwind material. Comparison between shock modeling andline excesses also suggests that an appreciable proportion of [SII]lambda 6716/31 Angstroms emission may arise through shock excitation; aconclusion which, if confirmed, may have serious consequences fornebular density estimations. Some 14 nebulae are identified as likelyshock candidates, whilst it is proposed that the majority of bipolarnebulae may also show spectral deviations associated with shockexcitation. Line excesses for these latter sources are most consistentwith shock velocities V_s ~ 80 => 100 km s(-1) ; values which arealso comparable to observed wind velocities. Finally, sources containingFLIERs (\cite[Balick et al. 1993]{ba93}) are shown to be confined tohighly specific spectral regimes; a result which permits us to identifythree further possible FLIER sources, and propose characteristic lineratio diagnostics for the further discovery of such features.
| A self-consistent determination of distances, physical parameters, and chemical composition for a large sample of galactic planetary nebulae: chemical composition
The relative abundances of He, C, N, O, Ne, Mg, Si, S, and Ar arepresented for, respectively, 185, 65, 212, 221, 180, 13, 41, 197, and205 Galactic planetary nebulae. The observed stages of ionization weretaken into account using the relations between the relative abundancesof different ions derived from a grid of photoionization models for thenebular emission. The chemical compositions of all the planetary nebulaewere determined using the same method and the same atomic data, so thatthe results have a high degree of uniformity; this is the first timethis has been done for such a large sample of Galactic planetary nebulae(221 objects).
| The chemical structure of bipolar planetary nebulae. II. 13 objects
Long slit spectrophotometry of 13 bipolar planetary nebulae has beenused to study their physical and chemical properties. In each nebula,one to seven different spatial regions have been considered in order tosearch for possible abundance variations through the nebulae. Weobtained the following main results: - within the errors, the He, O, andN abundances are constant through all the nebulae. - the Ne, Ar, and Sabundances are also constant, within the errors, but their face valueshave systematic increases toward the outer regions of the nebulae. Thesetrends may be attributed to inaccuracies in the ionization correctionfactors, as predicted by Alexander & Balick (1997) for long-slitobservations of extended PNe. The corresponding increase of the Nabundance predicted by those authors is, however, generally not observed(with one exception). - The present sample contains some of the GalacticPNe with the highest He and N/O abundances known to date (M 3-2, He2-111, NGC 6537). The highest He overabundances cannot by reproduced byany current model of AGB evolution. - Oxygen depletion is suggested forthe nebulae with the highest N/O abundances, indicating that efficientON cycle process has occurred in their progenitors. Tables 4 to 29 areonly available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via/ttp://cdsweb.u-strasbg.fr/Abstract.html}. Based on observations made atthe European Southern Observatory
| Eccentric Binary Model for Off-Center Planetary Nebula Nuclei
We examine the influence of eccentric binary progenitors on themorphologies of their descendant planetary nebulae. In particular, weconsider how mass loss via a stellar wind by an asymptotic giant branch(AGB) star in an eccentric binary can lead to the displacement of thecentral star in the equatorial plane. We postulate that the mass-lossrate from the AGB star varies systematically with orbital phase. Suchvariations may be due to several effects, including a tidal enhancementof the stellar wind near periastron and a cessation of the stellar windwhen the Roche lobe of the AGB star encroaches on its extendedatmosphere. Our results may pertain to binary systems with semimajoraxes in the range of a ~= 7-80 AU, which corresponds to orbital periodsin the range P ~= 15-500 yr. We apply the results to planetary nebulaein general, and MyCn 18 (the Hourglass Nebula) in particular, where thecentral star was recently found by the Hubble Space Telescope to bedisplaced from the center of the nebula. The results of this paper maybe applied to circumstellar matter around more massive stars, such asprogenitors of supernovae, by rescaling the physical properties of thebinary stars and the wind velocities.
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