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# M76 (Little Dumbbell)

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 The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB StarsWe review the observed properties of extremely hot, hydrogen-deficientpost-asymptotic giant branch (AGB) stars of spectral type [WC] andPG1159. Their H deficiency is probably caused by a (very) latehelium-shell flash or an AGB final thermal pulse, laying bare interiorstellar regions that are usually kept hidden below the hydrogenenvelope. Thus, the photospheric elemental abundances of these starsallow us to draw conclusions about details of nuclear burning and mixingprocesses in the precursor AGB stars. We summarize the state of the artof stellar evolution models that simulate AGB evolution and theoccurrence of a late He-shell flash. We compare predicted elementalabundances to those determined by quantitative spectral analysesperformed with advanced non-LTE model atmospheres. Good qualitative andquantitative agreement is found. Future work can contribute to an evenmore complete picture of the nuclear processes in AGB stars. Asteroseismic inferences on GW Virginis variable stars in the frame of new PG 1159 evolutionary modelsAn adiabatic, nonradial pulsation study of GW Vir stars is presented.The pulsation calculations were based on PG 1159 evolutionary sequenceswith different stellar masses artificially derived from a fullevolutionary sequence of 0.5895 Mȯ that has beencomputed by taking the evolutionary history of the progenitor star intoaccount. The artificial sequences were constructed by appropriatelyscaling the stellar mass of the 0.5895-Mȯ sequence wellbefore the models reached the low-luminosity, high-gravity stage of theGW Vir domain. We computed g-mode pulsation periods appropriate to GWVir variable stars. The implications for the mode-trapping properties ofour PG 1159 models are discussed at length. We found that themode-trapping features characterizing our PG 1159 models are mostlyfixed by the stepped shape of the core chemical profile left by priorconvective overshooting. This is particularly true at least for therange of periods observed in GW Vir stars. In addition, we makeasteroseismic inferences about the internal structure of the GW Virstars PG 1159-035, PG 2131+066, PG 1707+427, and PG 0122+200. Precise radial velocities with BOES. Detection of low-amplitude pulsations in the K-giant α ArietisWe present the first results from a high-precision radial velocity studyof the K2 III giant star α Ari. Observationswere acquired over 6 nights in 2004 using the new high-resolutionspectrograph BOES (Bohyunsan Observatory Echelle Spectrograph) of the1.8-m telescope. A high radial-velocity precision was achieved by usingthe high-resolution (R=90 000) mode of BOES and an iodine gas absorptioncell. The radial velocity measurements made during JD = 2 452 948{-}2452 950 show coherent, low-amplitude variations with a period ofP1=0.571 days (or aliases at 0.445 or 0.821 days), and anamplitude of 18.9 m s-1. Observations of τ Cet over thissame interval are constant to within 3 m s-1. Aftersubtracting the contribution of the 0.57-day period, we find evidencefor a second period, P2 = 0.190 days. Observations made on asecond run during JD = 2 452 975{-}2 452 981 show that the radialvelocity variations are indeed present, but on shorter time-scales andwith a lower amplitude. Two probable periods fit the radial velocitymeasurements from the second run reasonably well: 0.185 days or an aliasof 0.256 days. The shorter period coincides with the secondary one foundin the earlier measurements. We conclude that, similar to other K-giantpulsating stars, α Ari shows unstable acousticpulsations or mode switching on time scales of tens of days. Thecalculated pulsation constants for the dominant 0.571-day period isconsistent with third-overtone pulsations, while the secondaryperiodicity found in both data runs corresponds to a high overtone(n≥ 12). A search for photometric variability of hydrogen-deficient planetary-nebula nucleiAims.We searched for photometric variability in a sample of hot,hydrogen-deficient planetary nebula nuclei (PNNi) with "PG 1159" or "OVI" spectral type, most of them embedded in a bipolar or ellipticalplanetary nebula envelope (PNe). These characteristics may indicate thepresence of a hidden close companion and an evolution affected byepisodes of interaction between them. Methods: .We obtainedtime-series photometry from a sample of 11 candidates using the NordicOptical Telescope (NOT) with the Andalucía Faint ObjectSpectrograph and Camera (ALFOSC), modified with our own control softwareto be able to observe in a high-speed multi-windowing mode. The datawere reduced on-line with the real time photometry (RTP) code, whichallowed us to detect periodic variable stars with small amplitudes fromCCD data in real time. We studied the properties of the observedmodulation frequencies to investigate their nature. Results: .Wereport the first detection of low-amplitude pulsations in the PNNiVV 47, NGC 6852, and Jn1. In addition, we investigated the photometric variability ofNGC 246. Time-series analysis shows that the powerspectra of VV 47, NGC 6852, andNGC 246 are variable on time scales of hours. Powerspectra from consecutive nights of VV 47 andNGC 6852 show significant peaks in differentfrequency regions. The same type of variability is present in NGC 246 in2 observing runs separated by 3 days. Changes are also found in thepower spectra of VV 47 and NGC 246during the same night. The VV 47 power spectra arepeculiar since they present modulation frequencies in a wide range from175 to 7600 μHz. This is different from the previously knownpulsating PNNi where no frequencies are found above ~3000 μHz. Thehigh-frequency modulation observed in VV 47 may bedue to g-modes triggered by the ɛ-mechanism, observed for thefirst time. Some implications of the introduction of scattered starlight in the spectrum of reddened starsThis paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit. The mean properties of planetary nebulae as a function of Peimbert classPlanetary nebulae are known to possess a broad range of abundances, andthese (with other characteristics) have been used to define five classesof outflow. Peimbert Type I sources, for instance, possess high N and Heabundances, filamentary structures, and low mean scaleheights above theGalactic plane, whilst those of Type III have much lower abundances,high peculiar velocities, and belong to the Galactic thick disc. Apartfrom some rather ill-defined indications, however, very little is knownconcerning their mean physical, spatial, structural, kinematic andthermal characteristics.We have performed a comprehensive study of all of these properties, andfind evidence for strong variations between the various Peimbertclasses. Certain of these differences are consistent with Type I sourceshaving the highest progenitor masses, although it seems that thesenebulae also possess the lowest rms densities and 5-GHz brightnesstemperatures. The latter results are in conflict with a range of recentmodelling. Antoine Darquier et la decouverte de la nebuleuse de la Lyre.Not Available The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their EnvelopesThe results of a study of the chemical compositions of Galacticplanetary nebulae taking into account two types of inhomogeneity in thenebular gas density in their envelopes are reported. New analyticalexpressions for the ionization correction factors have been derived andare used to determine the chemical compositions of the nebular gas inGalactic planetary nebulae. The abundances of He, N, O, Ne, S, and Arhave been found for 193 objects. The Y Z diagrams for various Heabundances are analyzed for type II planetary nebulae separately andjointly with HII regions. The primordial helium abundance Y p andenrichment ratio dY/dZ are determined, and the resulting values arecompared with the data of other authors. Radial abundance gradients inthe Galactic disk are studied using type II planetary nebulae. Discovery of Multiple Coaxial Rings in the Quadrupolar Planetary Nebula NGC 6881We report the discovery of multiple two-dimensional rings in thequadrupolar planetary nebula NGC 6881. As many as four pairs of ringsare seen in the bipolar lobes, and three rings are seen in the centraltorus. While the rings in the lobes have the same axis as one pair ofthe bipolar lobes, the inner rings are aligned with the other pair. Thetwo pairs of bipolar lobes are likely to be carved out by two separatehigh-velocity outflows from the circumstellar material left over fromthe asymptotic giant branch (AGB) wind. The two-dimensional rings couldbe the results of dynamical instabilities or the consequence of a fastoutflow interacting with remnants of discrete AGB circumstellar shells. The distances of Type I planetary nebulaeThe 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. Synthetic stellar populations: single stellar populations, stellar interior models and primordial protogalaxiesWe present a new set of stellar interior and synthesis models forpredicting the integrated emission from stellar populations in starclusters and galaxies of arbitrary age and metallicity. This workdiffers from existing spectral synthesis codes in a number of importantways, namely (1) the incorporation of new stellar evolutionary tracks,with sufficient resolution in mass to sample rapid stages of stellarevolution; (2) a physically consistent treatment of evolution in theHertzsprung-Russell (HR) diagram, including the approach to the mainsequence and the effects of mass loss on the giant and horizontal branchphases. Unlike several existing models, ours yield consistent ages whenused to date a coeval stellar population from a wide range of spectralfeatures and colour indexes. We use Hipparcos data to support thevalidity of our new evolutionary tracks. We rigorously discussdegeneracies in the age-metallicity plane and show that inclusion ofspectral features blueward of 4500 Å suffices to break anyremaining degeneracy and that with moderate S/N spectra (10 per20-Å resolution element) age and metallicity are not degenerate.We also study sources of systematic errors in deriving the age of asingle stellar population and conclude that they are not larger than10-15 per cent. We illustrate the use of single stellar populations bypredicting the colours of primordial protogalaxies and show that one canfirst find them and then deduce the form of the initial mass function(IMF) for the early generation of stars in the Universe. Finally, weprovide accurate analytic fitting formulae for ultrafast computation ofcolours of single stellar populations. Infrared Array Camera (IRAC) Observations of Planetary NebulaeWe present the initial results from the Infrared Array Camera (IRAC)imaging survey of planetary nebulae (PNs). The IRAC colors of PNs arered, especially in the 8.0 μm band. Emission in this band is likelydue to contributions from two strong H2 lines and a [Ar III]line in that bandpass. IRAC is sensitive to the emission in the halos aswell as in the ionized regions that are optically bright. In NGC 246, wehave observed an unexpected ring of emission in the 5.8 and 8.0 μmIRAC bands not seen previously at other wavelengths. In NGC 650 and NGC3132, the 8.0 μm emission is at larger distances from the centralstar compared to the optical and other IRAC bands, possibly related tothe H2 emission in that band and the tendency for themolecular material to exist outside of the ionized zones. In theflocculi of the outer halo of NGC 6543, however, this trend is reversed,with the 8.0 μm emission bright on the inner edges of the structures.This may be related to the emission mechanism, where the H2is possibly excited in shocks in the NGC 6543 halo, whereasH2 emission is likely fluorescently excited in the UV fieldsnear the central star. A Survey for Water Maser Emission toward Planetary Nebulae: New Detection in IRAS 17347-3139We report on a water maser survey toward a sample of 27 planetarynebulae (PNe) using the Robledo de Chavela and Medicina single-dishantennas, as well as the Very Large Array (VLA). Two detections havebeen obtained: the already known water maser emission in K3-35, and anew cluster of masers in IRAS 17347-3139. This low rate of detections iscompatible with the short lifetime of water molecules in PNe (~100 yr).The water maser cluster at IRAS 17347-3139 are distributed on a ellipseof size ~=0.2"×0.1", spatially associated with compact 1.3 cmcontinuum emission (simultaneously observed with the VLA). From archiveVLA continuum data at 4.9, 8.4, and 14.9 GHz, a spectral indexα=0.76+/-0.03 (Sν~να) is derivedfor this radio source, which is consistent with either a partiallyoptically thick ionized region or an ionized wind. However, the latterscenario can be ruled out by mass-loss considerations, thus indicatingthat this source is probably a young PN. The spatial distribution andthe radial velocities of the water masers are suggestive of a rotatingand expanding maser ring, tracing the innermost regions of a torusformed at the end of the asymptotic giant branch phase. Given that the1.3 cm continuum emission peak is located near one of the tips of themajor axis of the ellipse of masers, we speculate on a possible binarynature of IRAS 17347-3139, where the radio continuum emission couldbelong to one of the components and the water masers would be associatedwith a companion. Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IV. Synthesis and the Sulfur AnomalyWe have compiled a large sample of O, Ne, S, Cl, and Ar abundances thathave been determined for 85 Galactic planetary nebulae in a consistentand homogeneous manner using spectra extending from 3600 to 9600Å. Sulfur abundances have been computed using the near-IR lines of[S III] λλ9069, 9532 along with [S III] temperatures. Wefind average values, expressed logarithmically with a standarddeviation, of log(S/O)=-1.91+/-0.24, log(Cl/O)=-3.52+/-0.16, andlog(Ar/O)=-2.29+/-0.18, numbers consistent with previous studies of bothplanetary nebulae and H II regions. We also find a strong correlationbetween [O III] and [S III] temperatures among planetary nebulae. Inanalyzing abundances of Ne, S, Cl, and Ar with respect to O, we find atight correlation for Ne-O, and loose correlations for Cl-O and Ar-O.All three trends appear to be colinear with observed correlations for HII regions. S and O also show a correlation, but there is a definiteoffset from the behavior exhibited by H II regions and stars. We suggestthat this S anomaly is most easily explained by the existence ofS+3, whose abundance must be inferred indirectly when onlyoptical spectra are available, in amounts in excess of what is predictedby model-derived ionization correction factors in PNe. Finally for thedisk PNe, abundances of O, Ne, S, Cl, and Ar all show gradients whenplotted against Galactocentric distance. The slopes are statisticallyindistinguishable from one another, a result which is consistent withthe notion that the cosmic abundances of these elements evolve inlockstep. Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. III. Observations and Results for a Final SampleThis paper is the fourth in a series whose purpose is to study theinterstellar abundances of sulfur, chlorine, and argon in the Galaxyusing a sample of 86 planetary nebulae. Here we present new high-qualityspectrophotometric observations of 20 Galactic planetary nebulae withspectral coverage from 3700 to 9600 Å. A major feature of ourobservations throughout the entire study has been the inclusion of thenear-infrared lines of [S III] λλ9069, 9532, which allowsus to calculate accurate S+2 abundances and to either improveupon or convincingly confirm results of earlier sulfur abundancestudies. For each of the 20 objects here, we calculate ratios of S/O,Cl/O, and Ar/O and find average values ofS/O=(1.1+/-1.1)×10-2,Cl/O=(4.2+/-5.3)×10-4, andAr/O=(5.7+/-4.3)×10-3. For six objects, we are able tocompare abundances of S+3 calculated directly from available[S IV] 10.5 μm measurements with those inferred indirectly from thevalues of the ionization correction factors for sulfur. In the finalpaper of the series, we will compile results from all 86 objects, searchfor and evaluate trends, and use chemical evolution models to interpretour results. The relation between Zanstra temperature and morphology in planetary nebulaeWe have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology. The relation between elemental abundances and morphology in planetary nebulaeAn 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. Introductory Review (invited review)Not Available Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinatesWe have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029} Urban Astronomy: Observing the Messier Objects from the CityNot Available The Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic SampleNorthern Galactic planetary nebulae (PNs) are studied to disclosepossible correlations between the morphology of the nebulae and theevolution of the central stars (CSs). To this end, we have built thebest database available to date, accounting for homogeneity andcompleteness. We use updated statistical distances and an updatedmorphological classification scheme, and we calculate Zanstratemperatures for a large sample of PNs. With our study we confirm thatround, elliptical, and bipolar PNs have different spatial distributionswithin the Galaxy, with average absolute distances to the Galactic planeof 0.73, 0.38, and 0.21 kpc, respectively. We also find evidence thatthe distributions of the CS masses are different across thesemorphological groups, although we do not find that CSs hosted by bipolarPNs are hotter, on average, than CSs within round and elliptical PNs.Our results are in broad agreement with previous analyses, indicatingthat round, elliptical, and bipolar PNs evolve from progenitors indifferent mass ranges and might belong to different stellar populations,as also indicated by the helium and nitrogen abundances of PNs ofdifferent morphology. Gas temperature and excitation classes in planetary nebulaeEmpirical 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 A possible observational measure of evolution in bipolar nebulaeBipolar 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. Study of electron density in planetary nebulae. A comparison of different density indicatorsWe present a comparison of electron density estimates for planetarynebulae based on different emission-line ratios. We have considered thedensity indicators [O Ii]lambda 3729/lambda 3726, [S Ii]lambda6716/lambda 6731, [Cl Iii]lambda 5517/lambda 5537, [Ar Iv]lambda4711/lambda 4740, C Iii]lambda 1906/lambda 1909 and [N I]lambda5202/lambda 5199. The observational data were extracted from theliterature. We have found systematic deviations from the densityhomogeneous models, in the sense that: Ne(ion {N}i) <~Ne(ion {O}{ii}) < Ne(ion {S}{ii}, ion {C}{iii},ion {Cl}{iii} or ion {Ar}{iv}) and Ne(ion {S}{ii}) ~Ne(ion {C}{iii}) ~ Ne(ion {Cl}{iii}) ~Ne(ion {Ar}{iv}). We argue that the lower [O Ii] densityestimates are likely due to errors in the atomic parameters used. Bipolar Nebulae: The Missing PopulationWe have undertaken an analysis of observed aspect ratios for thecollimation disks in bipolar nebulae (BPNe). We find, as a result, thatmost such structures are likely to have a ratio of thickness to diameterb/a~0.23. Similarly, the paucity of disks having aspect ratios >=0.7implies that many of the sources are either unobserved or, what is moreprobably the case, misidentified. It seems likely, as a result, that thetrue population of BPNe is ~1.7 times greater than previously supposed.Similarly, we determine that as many as of 43% of annular or ringlikeplanetaries may represent bipolar sources oriented along the line ofsight. Les nebuleuses planetaires et leurs etoiles centrales.Not Available Departure from Axisymmetry in Planetary NebulaeMany 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 3-D ionization structure (in stereoscopic view) of planetary nebulae: the case of NGC 1501.Long-slit echellograms of the high excitation planetary nebula NGC 1501,reduced according to the methodology developed by Sabbadin et al.(2000a,b), allowed us to obtain the "true" distribution of the ionizedgas in the eight nebular slices covered by the spectroscopic slit. A 3-Drendering procedure is described and applied, which assembles thetomographic maps and rebuilds the spatial structure. The images of NGC1501, as seen in 12 directions separated by 15o, form a series ofstereoscopic pairs giving surprising 3-D views in as many directions.The main nebula consists of an almost oblate ellipsoid of moderateellipticity (a ~= 44 arcsec, a/b ~= 1.02, a/c ~= 1.11), brighter in theequatorial belt, deformed by several bumps, and embedded in a quitehomogeneous, inwards extended cocoon. Some reliability tests are appliedto the rebuilt nebula; the radial matter profile, the small scaledensity fluctuations and the 2-D (morphology) -3-D (structure)correlation are presented and analysed. The wide applications of the 3-Dreconstruction to the morphology, physical conditions, ionizationparameters and evolutionary status of expanding nebulae in general(planetary nebulae, nova and supernova remnants, shells aroundPopulation I Wolf-Rayet stars, nebulae ejected by symbiotic stars,bubbles surrounding early spectral type main sequence stars etc.) areintroduced.
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