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# NGC 6565

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 An Atlas of [N II] and [O III] Images and Spectra of Planetary NebulaeWe 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 structure of planetary nebulae: theory vs. practiceContext.This paper is the first in a short series dedicated to thelong-standing astronomical problem of de-projecting the bi-dimensional,apparent morphology of a three-dimensional mass of gas. Aims.Wefocus on the density distribution in real planetary nebulae (and alltypes of expanding nebulae). Methods. We introduce some basictheoretical notions, discuss the observational methodology, and developan accurate procedure for determining the matter radial profile withinthe sharp portion of nebula in the plane of the sky identified by thezero-velocity-pixel-column (zvpc) of high-resolution spectral images.Results. The general and specific applications of the method (andsome caveats) are discussed. Moreover, we present a series of evolutivesnapshots, combining illustrative examples of both model and trueplanetary nebulae. Conclusions. The zvpc radial-densityreconstruction - added to tomography and 3D recovery developed at theAstronomical Observatory of Padua (Italy) - constitutes a very usefultool for looking more closely at the spatio-kinematics, physicalconditions, ionic structure, and evolution of expanding nebulae. Morpho-Kinematic Modeling of Gaseous Nebulae with SHAPEWe present a powerful new tool to analyse and disentangle the 3-Dgeometry and kinematic structure of gaseous nebulae. The method consistsin combining commercially available digital animation software tosimulate the 3-D structure and expansion pattern of the nebula with adedicated, purpose-built rendering software that produces the finalimages and long slit spectra which are compared to the real data. Weshow results for the complex planetary nebulae NGC 6369 and Abell 30based on long slit spectra obtained at the San Pedro MártirObservatory. Morpho-kinematic modelling of gaseous nebulae with ShapeWe present a powerful new tool to disentangle the 3-D geometry andkinematic structure of gaseous nebulae. The method consists of combiningcommercially available digital animation software to simulate the 3-Dstructure and expansion pattern of the nebula with a dedicated, purposebuilt rendering software that produces the final images and long slitspectra which are compared to the real data. In this contribution weshow results for the complex planetary nebula NGC369 based on long slitspectra obtained at the San Pedro Mártir observatory. 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. The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edgeWe infer the gas kinematics, diagnostics and ionic radial profiles,distance and central star parameters, nebular photo-ionization model,spatial structure and evolutionary phase of the Planetary Nebula NGC6741 by means of long-slit ESO NTT+EMMI high-resolution spectra at nineposition angles, reduced and analysed according to the tomographic and3-D methodologies developed at the Astronomical Observatory of Padua(Italy). NGC 6741 (distance≃2.0 kpc, age≃ 1400 yr, ionizedmass Mion≃ 0.06 Mȯ) is a dense(electron density up to 12 000 cm-3), high-excitation,almost-prolate ellipsoid (0.036 pc × 0.020 pc × 0.018 pc,major, intermediate and minor semi-axes, respectively), surrounded by asharp low-excitation skin (the ionization front), and embedded in aspherical (radius≃ 0.080 pc), almost-neutral, high-density (n(HI)≃ 7 ×103 atoms cm-3) halo containinga large fraction of the nebular mass (Mhalo≥ 0.20Mȯ). The kinematics, physical conditions and ionicstructure indicate that NGC 6741 is in a deep recombination phase,started about 200 years ago, and caused by the rapid luminosity drop ofthe massive (M*=0.66{-}0.68 Mȯ), hot (logT* ≃ 5.23) and faint (logL*/Lȯ ≃ 2.75) post-AGB star, which hasexhausted the hydrogen-shell nuclear burning and is moving along thewhite dwarf cooling sequence. The general expansion law of the ionizedgas in NGC 6741, Vexp(km s-1)=13 × R arcsec,fails in the innermost, highest-excitation layers, which move slowerthan expected. The observed deceleration is ascribable to the luminositydrop of the central star (the decreasing pressure of the hot-bubble nolonger balances the pressure of the ionized gas), and appears instriking contrast to recent reports inferring that acceleration is acommon property of the Planetary Nebulae innermost layers. A detailedcomparative analysis proves that the "U"-shaped expansion velocity fieldis a spurious, incorrect result due to a combination of: (a) simplisticassumptions (spherical shell hypothesis for the nebula); (b) unfitreduction method (emission profiles integrated along the slit); and (c)inappropriate diagnostic choice (λ4686 Å of He II, i.e. athirteen fine-structure components recombination line). Some generalimplications for the shaping mechanisms of Planetary Nebulae arediscussed. 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. Evolution from AGB to planetary nebula in the MSX surveyWe investigate the evolution of oxygen- and carbon-rich AGB stars,post-AGB objects, and planetary nebulae using data collected mainly fromthe MSX catalogue. Magnitudes and colour indices are compared with thosecalculated from a grid of synthetic spectra that describe the post-AGBevolution beginning at the onset of the superwind. We find that carbonstars and OH/IR objects form two distinct sequences in the (K-[8.3])×([8.3]-[14.7]) MSX colour diagram. OH/IR objects are distributedin two groups: the bluest ones are crowded near [14.7]-[21.3]≃ 1and [8.3]-[14.7]≃ 2, and a second, redder group is spread over alarge area in the diagram, where post-AGB objects and planetary nebulaeare also found. High mass-loss rate OH/IR objects, post-AGB stars, andplanetary nebulae share the same region in the (K-[8.3])×([8.3]-[14.7]) and [14.7]-[21.3]×([8.3]-[14.7]) colour-colourdiagrams. This region in the diagram is clearly separated from a bluerone where most OH/IR stars are found. We use a grid of models ofpost-AGB evolution, which are compared with the data. The gap in thecolour-colour diagrams is interpreted as the result of the rapidtrajectory in the diagram of the stars that have just left the AGB.Based on results obtained by the MSX survey.Tables 1 to 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/565 A reexamination of electron density diagnostics for ionized gaseous nebulaeWe present a comparison of electron densities derived from opticalforbidden line diagnostic ratios for a sample of over a hundred nebulae.We consider four density indicators, the [O II]λ3729/λ3726, [S II] λ6716/λ6731, [Cl III]λ5517/λ5537 and [Ar IV] λ4711/λ4740 doubletratios. Except for a few H II regions for which data from the literaturewere used, diagnostic line ratios were derived from our own high qualityspectra. For the [O II] λ3729/λ3726 doublet ratio, we findthat our default atomic data set, consisting of transition probabilitiesfrom Zeippen (\cite{zeippen1982}) and collision strengths from Pradhan(\cite{pradhan}), fit the observations well, although at high electrondensities, the [O II] doublet ratio yields densities systematicallylower than those given by the [S II] λ6716/λ6731 doubletratio, suggesting that the ratio of transition probabilities of the [OII] doublet, A(λ3729)/A(λ3726), given by Zeippen(\cite{zeippen1982}) may need to be revised upwards by approximately 6per cent. Our analysis also shows that the more recent calculations of[O II] transition probabilities by Zeippen (\cite{zeippen1987a}) andcollision strengths by McLaughlin & Bell (\cite{mclaughlin}) areinconsistent with the observations at the high and low density limits,respectively, and can therefore be ruled out. We confirm the earlierresult of Copetti & Writzl (\cite{copetti2002}) that the [O II]transition probabilities calculated by Wiese et al. (\cite{wiese}) yieldelectron densities systematically lower than those deduced from the [SII] λ6716/λ6731 doublet ratio and that the discrepancy ismost likely caused by errors in the transition probabilities calculatedby Wiese et al. (\cite{wiese}). Using our default atomic data set for [OII], we find that Ne([O II])  Ne([S II]) ≈Ne([Cl III])< Ne([Ar IV]). The abundance distributions of Galactic bulge and disc planetary nebulaeWe present an abundance analysis of a sample of Galactic bulge planetarynebulae (GBPNe). The observational data set consists of spectra of 88nebulae obtained with the FLAIR II multiobject spectrograph on the UKSchmidt Telescope, together with spectra of 42 nebulae obtained with theRGO Spectrograph on the Anglo-Australian Telescope. After selectingthose for which reliable nebular electron temperatures could be derived,collisionally excited line abundances were derived for 45 GBPNe.These were then compared with similarly derived abundances for 54Galactic disc PNe (GDPNe), taken from the work of Kingsburgh &Barlow. We find that within the errors the abundances have the sameaverage values, essentially the same distributions - including that forthe mass-sensitive N/O ratio - and show the same relationships. Thewidth of the nitrogen distribution exceeds that arising from errors,which could be a consequence of the range of precursor masses. The ratioof Type-I to non-Type-I PNe in the bulge and disc samples is similar, 18and 25 per cent respectively. For the GDPNe, we find larger N/H and N/Oratios for the small number of those with He/H > 0.14, compared withthose with He/H < 0.14.For neither disc nor bulge sample is there any strong evidence for adepletion of oxygen for the higher-mass precursor stars (Type I PNe). Wefind no correlation between O/H and N/O or He/H. On the N/O to He/Hplane, the bulge and disc PNe show a distribution whereby the low N/Ovalues only occur for low He/H values, but at N/O > 0.25 the wholerange of He/H values were sampled. The theoretical tracks to which wecompare our data do not explain the PNe with low He/H abundance and highN/O ratio.Realistic uncertainties in collisionally excited lines (CEL) abundancesfor individual PNe are quite large, of the order of 40 per cent foroxygen. Large samples are therefore required to get good statisticalaccuracy. This is usually achieved by combining many studies, and so wehave compared the results of a number of published studies with our own,to search for any systematic differences. The average abundances arefound to be the same within the errors except for cases where theabundance derivation methods are dissimilar, where systematicdifferences can occur. The N/O ratio is especially sensitive to thedetails of the abundance derivations.Our bulge PN sample shows no evidence for either very-low-metallicityobjects or for super-metal-rich objects - the implied mass and agedistributions of the bulge PN precursor stars are indistinguishablewithin the observational errors from those in the local Galactic disc. 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. 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. 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 fourlarge-scale'', mean-to-high excitation sub-systems (the internalshell, the main shell, the outer shell and the halo), and as manysmall-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 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. 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} Angular dimensions of planetary nebulaeWe 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 3-D ionization structure of NGC 6818: A Planetary Nebula threatened by recombinationLong-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. 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 New Planetary Nebulae towards the galactic bulge. II. Objects surrounding the central areaIn the second part of the objective-prism survey of PNe towards thegalactic centre we present the discovery of further 44 new planetarynebulae found in six ESO fields surrounding the central area and give arough description of these objects. Besides, we give coordinates of manyknown objects in the area of the survey. The discussion of the angulardiameters leads to the belief that the majority of our PNe is veryprobably located in or close to the galactic bulge. The 3-D ionization structure of the planetary nebula NGC 6565A detailed study of the planetary nebula NGC 6565 has been carried outon long-slit echellograms (lambda /Delta lambda =60 000, spectral range= lambda lambda 3900-7750 Å) at six, equally spaced positionangles. The expansion velocity field, the c(Hβ ) distribution andthe radial profile of the physical conditions (electron temperature anddensity) are obtained. The distance, radius, mass and filling factor ofthe nebula and the temperature and luminosity of the central star arederived. The radial ionization structure is analyzed using both theclassical method and the photo-ionization code CLOUDY. Moreover, wepresent the spatial structure in a series of images from differentdirections, allowing the reader to see'' the nebula in 3-D. NGC 6565results to be a young (2000-2500 years), patchy, optically thicktriaxial ellipsoid (a=10.1 arcsec, a/b=1.4, a/c=1.7) projected almostpole-on. The matter close to major axis was swept-up by someaccelerating agent (fast wind? ionization? magnetic fields?), formingtwo faint and asymmetric polar cups. A large cocoon of almost neutralgas completely embeds the ionized nebula. NGC 6565 is in a recombinationphase, because of the luminosity drop of the massive powering star,which is reaching the white dwarf domain (log T* =~ 5.08 K;log L*/Lsun =~ 2.0). The stellar decline startedabout 1000 years ago, but the main nebula remained optically thin forother 600 years before the recombination phase occurred. In the nearfuture the ionization front will re-grow, since the dilution factor dueto the expansion will prevail on the slower and slower stellar decline.NGC 6565 is at a distance of 2.0 (+/-0.5) kpc and can be divided intothree radial zones: the fully ionized'' one, extending up to0.029-0.035 pc at the equator (0.050 pc at the poles), thetransition'' one, up to 0.048-0.054 pc (0.080 pc), the halo'',detectable up to 0.110 pc. The ionized mass ( =~ 0.03 Msun)is only a fraction of the total mass (>= 0.15 Msun), whichhas been ejected by an equatorial enhanced superwind of 4 (+/-2) x10-5 Msun yr-1 lasted for 4 (+/-2) x103 years. Based on observations made with ESO Telescopes atthe La Silla Observatories, under programme ID 65.I-0524, and onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive at the Space Telescope Institute (observingprogram GO 7501; P.I. Arsen Hajian). STScI is operated by theassociation of Universities for Research in Astronomy, Inc. under theNASA contract NAS 5-26555. We have applied the photoionization codeCLOUDY, developed at the Institute of Astronomy of the CambridgeUniversity. Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IIB. Abundances in a Southern SampleWe have undertaken a large spectroscopic survey of over 80 planetarynebulae with the goal of providing a homogeneous spectroscopic databasebetween 3600 and 9600 Å, as well as a set of consistentlydetermined abundances, especially for oxygen, sulfur, chlorine, andargon. In the current paper we calculate and report the S/O, Cl/O, andAr/O abundance ratios for 45 southern planetary nebulae (predominantlytype II), using our own recently observed line strengths published in acompanion paper. One of the salient features of our work is the use ofthe near-IR lines of [S III] λλ9069, 9532 coupled with the[S III] temperature, to determine the S+2 ionic abundance. Wefind the following average abundances for these objects:S/O=0.011+/-0.0064, Cl/O=0.00031+/-0.00012, and Ar/O=0.0051+/-0.0020. Sulfur, Chlorine, and Argon in Planetary Nebulae. IIA. Observations of a Southern SampleIn this paper we present fully reduced and dereddened emission linestrengths for a sample of 45 southern type II planetary nebulae(PNs).The spectrophotometry for these PNs covers an extendedoptical/near-IR range from 3600 to 9600 Å. This PN study andsubsequent analysis (presented in a companion paper), together with asimilar treatment for a northern PN sample, is aimed at addressing thelack of homogeneous, consistently observed, reduced, and analyzed datasets that include the near-IR [S III] lines at 9069 and 9532 Å.The use of type II objects only is intended to select disk nebulae thatare uncontaminated by nucleosynthetic products of the progenitor star.Extending spectra redward to include the strong [S III] lines enables usto look for consistency between S+2 abundances inferred fromthese lines and from the more accessible, albeit weaker, [S III] line atλ6312. Helium contamination from the progenitor stars of planetary nebulae: The He/H radial gradient and the ΔY / ΔZ enrichment ratioIn 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. The distance scale of planetary nebulaeBy collecting distances from the literature, a set of 73 planetarynebulae with mean distances of high accuracy is derived. This sample isused for recalibration of the mass-radius relationship, used by manystatistical distance methods. An attempt to correct for a statisticalpeculiarity, where errors in the distances influences the mass-radiusrelationship by increasing its slope, has been made for the first time.Distances to PNe in the Galactic Bulge, derived by this new method aswell as other statistical methods from the last decade, are then usedfor the evaluation of these methods as distance indicators. In order ofachieving a Bulge sample that is free from outliers we derive newcriteria for Bulge membership. These criteria are much more stringentthan those used hitherto, in the sense that they also discriminateagainst background objects. By splitting our Bulge sample in two, onewith optically thick (small) PNe and one with optically thin (large)PNe, we find that our calibration is of higher accuracy than most othercalibrations. Differences between the two subsamples, we believe, aredue to the incompleteness of the Bulge sample, as well as the dominanceof optical diameters in the thin'' sample and radio diameters in thethick'' sample. Our final conclusion is that statistical methods givedistances that are at least as accurate as the ones obtained from manyindividual methods. Also, the `long'' distance scale of Galactic PNe isconfirmed. An analysis of the observed radio emission from planetary nebulaeWe have analysed the radio fluxes for 264 planetary nebulae for whichreliable measurements of fluxes at 1.4 and 5 GHz, and of nebulardiameters are available. For many of the investigated nebulae, theoptical thickness is important, especially at 1.4 GHz. Simple modelslike the one specified only by a single optical thickness or spherical,constant density shells do not account satisfactorily for theobservations. Also an r-2 density distribution is ruled out.A reasonable representation of the observations can be obtained by atwo-component model having regions of two different values of opticalthickness. We show that the nebular diameters smaller than 10arcsec areuncertain, particularly if they come from photographic plates orGaussian fitting to the radio profile. While determining theinterstellar extinction from an optical to radio flux ratio, cautionshould be paid regarding optical thickness effects in the radio. We havedeveloped a method for estimating the value of self absorption. At 1.4GHz self absorption of the flux is usually important and can exceed afactor of 10. At 5 GHz self absorption is negligible for most of theobjects, although in some cases it can reach a factor of 2. The Galacticbulge planetary nebulae when used to calibrate the Shklovsky method givea mean nebular mass of 0.14 Msun. The statistical uncertaintyof the Shklovsky distances is smaller than a factor of 1.5. Table 1 isonly available in electronic form at http://www.edpsciences.org. Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.Not Available On the abundance gradient of the galactic diskEstimates of the gas temperature in planetary nebulae obtained from the[O III] emission line ratio and from the Balmer discontinuity indicatedifferences reaching up to 6000 K (Liu & Danziger 1993). The [O III]temperature is commonly used to obtain the ionic fractions of highlyionized ions, particularly the O++ and Ne++ ions when using theempirical method to calculate the elemental abundances of photoionizedgas from the observed emission line intensities. However, if the gastemperature is overestimated the elemental abundances may beunderestimated. In particular this may lead to an incorrect elementalabundance gradient for the Galaxy, usually used as a constraint for thechemical evolution models. Using Monte Carlo simulations, we calculatethe systematic error introduced in the abundance gradient obtained fromplanetary nebulae by an overestimation of the gas temperature. Theresults indicate that the abundance gradient in the Galaxy should besteeper than previously assumed. ROSAT Observations of X-Ray Emission from Planetary NebulaeWe have searched the entire ROSAT archive for useful observations tostudy X-ray emission from Galactic planetary nebulae (PNs). The searchyields a sample of 63 PNs, which we call the ROSAT PN sample. About20%-25% of this sample show X-ray emission; these include 13 definitedetections and three possible detections (at a 2 σ level). AllX-ray sources in these PNs are concentrated near the central stars. OnlyA30, BD +30°3639, and NGC 6543 are marginally resolved by the ROSATinstruments. Three types of X-ray spectra are seen in PNs. Type 1consists of only soft X-ray emission (<0.5 keV), peaks at 0.1-0.2keV, and can be fitted by blackbody models at temperatures1-2×105 K. Type 2 consists of harder X-ray emission,peaks above 0.5 keV, and can be fitted by thin plasma emission models attemperatures of a few times 106 K. Type 3 is a composite of abright type 1 component and a fainter type 2 component. Unresolved softsources with type 1 spectra or the soft component of type 3 spectra aremost likely photospheric emission from the hot central stars. Absorptioncross sections are large for these soft-energy photons; therefore, onlylarge, tenuous, evolved PNs with hot central stars and small absorptioncolumn densities have been detected. The origin of hard X-ray emissionfrom PNs is uncertain. PNs with type 2 spectra are small, dense, youngnebulae with relatively cool (<<105 K) central stars,while PNs with type 3 X-ray spectra are large, tenuous, evolved nebulaewith hot central stars. The hard X-ray luminosities are also differentbetween these two types of PNs, indicating perhaps different origins oftheir hard X-ray emission. Future Chandra and XMM observations with highspatial and spectral resolution will help to understand the origin ofhard X-ray emission from PNs. Distances of Galactic Planetary Nebulae Based on a Relationship Between the Central Star Mass and the N/O AbundanceIn 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. The dust content of planetary nebulae: a reappraisalWe have performed a statistical analysis using broad band IRAS data onabout 500 planetary nebulae with the aim of characterizing their dustcontent. Our approach is different from previous studies in that it usesan extensive grid of photoionization models to test the methods forderiving the dust temperature, the dust-to-gas mass ratio and theaverage grain size. In addition, we use only distance independentdiagrams. With our models, we show the effect of contamination by atomiclines in the broad band IRAS fluxes during planetary nebula evolution.We find that planetary nebulae with very different dust-to-gas massratios exist, so that the dust content is a primordial parameter for theinterpretation of far infrared data of planetary nebulae. In contrastwith previous studies, we find no evidence for a decrease in thedust-to-gas mass ratio as the planetary nebulae evolve. We also showthat the decrease in grain size advocated by Natta & Panagia(\cite{NattaPanagia}) and Lenzuni et al. (\cite{Lenzuni}) is an artefactof their method of analysis. Our results suggest that the timescale fordestruction of dust grains in planetary nebulae is larger than theirlifetime. Table~1 is only accessible in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
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