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The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their Envelopes
The 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.
| Helium recombination spectra as temperature diagnostics for planetary nebulae
Electron temperatures derived from the HeI recombination line ratios,designated Te(HeI), are presented for 48 planetary nebulae(PNe). We study the effect that temperature fluctuations inside nebulaehave on the Te(HeI) value. We show that a comparison betweenTe(HeI) and the electron temperature derived from the Balmerjump of the HI recombination spectrum, designated Te(HI),provides an opportunity to discriminate between the paradigms of achemically homogeneous plasma with temperature and density variations,and a two-abundance nebular model with hydrogen-deficient materialembedded in diffuse gas of a `normal' chemical composition (i.e.~solar), as the possible causes of the dichotomy between the abundancesthat are deduced from collisionally excited lines and those deduced fromrecombination lines. We find that Te(HeI) values aresignificantly lower than Te(HI) values, with an averagedifference of = 4000 K. Theresult is consistent with the expectation of the two-abundance nebularmodel but is opposite to the prediction of the scenarios of temperaturefluctuations and/or density inhomogeneities. From the observeddifference between Te(HeI) and Te(HI), we estimatethat the filling factor of hydrogen-deficient components has a typicalvalue of 10-4. In spite of its small mass, the existence ofhydrogen-deficient inclusions may potentially have a profound effect inenhancing the intensities of HeI recombination lines and thereby lead toapparently overestimated helium abundances for PNe.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This 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.
| A reexamination of electron density diagnostics for ionized gaseous nebulae
We 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]).
| On the O II Ground Configuration Energy Levels
The most accurate way to measure the energy levels for the O II2p3 ground configuration has been from the forbidden lines inplanetary nebulae. We present an analysis of modern planetary nebuladata that nicely constrain the splitting within the 2D termand the separation of this term from the ground4S3/2 level. We extend this method to H II regionsusing high-resolution spectroscopy of the Orion Nebula, covering all sixvisible transitions within the ground configuration. These data confirmthe splitting of the 2D term while additionally constrainingthe splitting of the 2P term. The energies of the2P and 2D terms relative to the ground(4S) term are constrained by requiring that all six linesgive the same radial velocity, consistent with independent limits placedon the motion of the O+ gas and the planetary nebula data.
| Electron temperatures and densities of planetary nebulae determined from the nebular hydrogen recombination spectrum and temperature and density variations
A method is presented to derive electron temperatures and densities ofplanetary nebulae (PNe) simultaneously, using the observed hydrogenrecombination spectrum, which includes continuum and line emission. Bymatching theoretical spectra to observed spectra around the Balmer jumpat about 3646 Å, we determine electron temperatures and densitiesfor 48 Galactic PNe. The electron temperatures based on this method -hereafter Te(Bal) - are found to be systematically lower thanthose derived from [OIII] λ4959/λ4363 and [OIII] (88 μm+ 52 μm)/λ4959 ratios - hereafterTe([OIII]na) andTe([OIII]fn). The electron densities based on thismethod are found to be systematically higher than those derived from[OII] λ3729/λ3726, [SII] λ6731/λ6716,[ClIII] λ5537/λ5517, [ArIV] λ4740/λ4711 and[OIII] 88 μm/52 μm ratios. These results suggest that temperatureand density fluctuations are generally present within nebulae. Thecomparison of Te([OIII]na) and Te(Bal)suggests that the fractional mean-square temperature variation(t2) has a representative value of 0.031. A majority oftemperatures derived from the Te([OIII]fn) ratioare found to be higher than those of Te([OIII]na),which is attributed to the existence of dense clumps in nebulae - those[OIII] infrared fine-structure lines are suppressed by collisionalde-excitation in the clumps. By comparingTe([OIII]fn), Te([OIII]na)and Te(Bal) and assuming a simple two-density-componentmodel, we find that the filling factor of dense clumps has arepresentative value of 7 × 10-5. The discrepanciesbetween Te([OIII]na) and Te(Bal) arefound to be anticorrelated with electron densities derived from variousdensity indicators; high-density nebulae have the smallest temperaturediscrepancies. This suggests that temperature discrepancy is related tonebular evolution. In addition, He/H abundances of PNe are found to bepositively correlated with the difference betweenTe([OIII]na) and Te(Bal), suggestingthat He/H abundances might have been overestimated generally because ofthe possible existence of H-deficient knots. Electron temperatures anddensities deduced from spectra around the Paschen jump regions at 8250Åare also obtained for four PNe: NGC 7027, NGC 6153, M 1-42 andNGC 7009. Electron densities derived from spectra around the Paschenjump regions are in good agreement with the corresponding values derivedfrom spectra around the Balmer jump, whereas temperatures deduced fromthe spectra around the Paschen jump are found to be lower than thecorresponding values derived from spectra around the Balmer jump for allthe four cases. The reason remains unclear.
| 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.
| 12C/13C Ratio in Planetary Nebulae from the IUE Archives
We investigated the abundance ratio of 12C/13C inplanetary nebulae by examining emission lines arising from C III2s2p3Po2,1,0-->2s21S0.Spectra were retrieved from the International Ultraviolet Explorerarchives, and multiple spectra of the same object were co-added toachieve improved signal-to-noise ratio. The 13C hyperfinestructure line at 1909.6 Å was detected in NGC 2440. The12C/13C ratio was found to be ~4.4+/-1.2. In allother objects, we provide an upper limit for the flux of the 1910Å line. For 23 of these sources, a lower limit for the12C/13C ratio was established. The impact on ourcurrent understanding of stellar evolution is discussed. The resultinghigh-signal-to-noise ratio C III spectrum helps constrain the atomicphysics of the line formation process. Some objects have the measured1907/1909 Å flux ratio outside the low-electron densitytheoretical limit for 12C. A mixture of 13C with12C helps to close the gap somewhat. Nevertheless, someobserved 1907/1909 Å flux ratios still appear too high to conformto the currently predicted limits. It is shown that this limit, as wellas the 1910/1909 Å flux ratio, are predominantly influenced byusing the standard partitioning among the collision strengths for themultiplet1S0-3PoJaccording to the statistical weights. A detailed calculation for thefine-structure collision strengths between these individual levels wouldbe valuable.
| J320 (PN G190.3-17.7) as a poly-polar planetary nebula surrounded by point-symmetric knots
Spatially resolved, long-slit spectra obtained with the Manchesterechelle spectrometer of the planetary nebula J320 have been comparedwith Hubble Space Telescope and ground-based images. The morphology andkinematics of the nebular core have been convincingly modelled using theXSHAPE code.Two and possibly three bipolar lobes, expanding at ~ 46 kms-1 but with different axial orientations, have been found toconstitute the bright central nebula. These are found to be surroundedby two sets of point-symmetric, high-speed knots reminiscent of thosethat could be generated by a bipolar, rotating, episodic jet. Anotherisolated knot exists but without a counterpart.The sequence and nature of the periodic ejections that have given riseto this complex structure are considered in view of the most recenttheories of the shaping and evolution of planetary nebulae.
| The 3-D ionization structure and evolution of NGC 7009 (Saturn Nebula)
Tomographic and 3-D analyses for extended, emission-line objects areapplied to long-slit ESO NTT + EMMI high-resolution spectra of theintriguing planetary nebula NGC 7009, covered at twelve position angles.We derive the gas expansion law, the diagnostics and ionic radialprofiles, the distance and the central star parameters, the nebularphoto-ionization model and the spatial recovery of the plasma structureand evolution. The Saturn Nebula (distance≃1.4 kpc, age≃6000yr, ionized mass≃0.18 Mȯ) consists of severalinterconnected components, characterized by different morphology,physical conditions, excitation and kinematics. We identify four``large-scale'', mean-to-high excitation sub-systems (the internalshell, the main shell, the outer shell and the halo), and as many``small-scale'' ones: the caps (strings of low-excitation knots withinthe outer shell), the ansae (polar, low-excitation, likely shockedlayers), the streams (high-excitation polar regions connecting the mainshell with the ansae), and an equatorial, medium-to-low excitationpseudo-ring within the outer shell. The internal shell, the main shell,the streams and the ansae expand at Vexp≃4.0 × Rarcsec km s-1, the outer shell, the caps and the equatorialpseudo-ring at Vexp≃3.15 × R arcsec kms-1, and the halo at Vexp≃10 kms-1. We compare the radial distribution of the physicalconditions and the line fluxes observed in the eight sub-systems withthe theoretical profiles coming from the photo-ionization code CLOUDY,inferring that all the spectral characteristics of NGC 7009 areexplainable in terms of photo-ionization by the central star, a hot (log T* ≃4.95) and luminous ( logL*/Lȯ≃3.70) 0.60-0.61Mȯ post-AGB star in the hydrogen-shell nuclear burningphase. The 3-D shaping of the Saturn Nebula is discussed within anevolutionary scenario dominated by photo-ionization and supported by thefast stellar wind: it begins with the superwind ejection (firstisotropic, then polar deficient), passes through the neutral, transitionphase ({lasting} ≃3000 yr), the ionization start (occurred≃2000 yr ago), and the full ionization of the main shell(≃1000 yr ago), at last reaching the present days: the wholenebula is optically thin to the UV stellar flux, except the caps (meanlatitude condensations in the outer shell, shadowed by the main shell)and the ansae (supersonic ionization fronts along the major axis).Based on observations made with: ESO Telescopes at the La SillaObservatories (program ID 65.I-0524), and the NASA/ESA Hubble SpaceTelescope, obtained from the data archive at the Space TelescopeInstitute. Observing programs: GO 6117 (P.I. Bruce Balick), GO 6119(P.I. Howard Bond) and GO 8390 (P.I. Arsen Hajian). STScI is operated bythe association of Universities for Research in Astronomy, Inc. underthe NASA contract NAS 5-26555. We extensively apply the photo-ionizationcode CLOUDY, developed at the Institute of Astronomy of the CambridgeUniversity (Ferland et al. 1998).
| The relation between Zanstra temperature and morphology in planetary nebulae
We have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology.
| Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinates
We have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}
| 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
| Shaping Proto-Planetary and Young Planetary Nebulae with Collimated Fast Winds
Using two-dimensional hydrodynamical simulations, we investigate theinteraction of a collimated fast wind (CFW) interacting with a sphericalasymptotic giant branch (AGB) wind as the mechanism for shapingproto-planetary nebulae (PPNs) and young planetary nebulae. Inparticular, we compare our simulations with the observations of anevolved PPN with multiple, highly collimated lobes, CRL 618. Wecharacterize our model CFW by three parameters-opening angle, velocity,and mass-loss rate-and explore the dependence of the properties of theshell on the first two. For given opening angle and velocity, themass-loss rate is chosen to give a shell velocity of about 150 kms-1 at the tip, similar to that seen in CRL 618. In oursimulations, the shell dynamics is found to depend on the velocity ofthe fast wind: we obtain a momentum-driven shell for a 300 kms-1 fast wind and a ballistic bow shock-driven shell for a1000 km s-1 fast wind. The shell driven by the collimatedfast wind is highly collimated, even though the AGB wind is spherical.Time variations in the velocity of the fast wind produce a series ofinternal shock pairs interacting with the inner surface of the shell. Asa result of radial expansion, the density of the internal shocksdecreases with distance.Various emission diagnostics have been derived from our simulations. Fora 300 km s-1 fast wind, the optical emission arises from boththe shocked AGB wind and shocked fast wind, showing one or two brightbowlike structures at the tip of the lobe. However, for a 1000 kms-1 fast wind, since the shocked fast wind is much hotter, itemits mainly in X-ray emission; the optical emission forms only onebowlike structure at the tip associated with the shocked AGB wind. Theposition-velocity (PV) diagrams derived from our simulations all show abroad range of velocities at the tip. The detailed PV structure andvelocity range at the tip depend on the shell dynamics and the relativecontributions of the shocked fast wind and shocked AGB wind.We make a detailed comparison of our simulations to the observations ofthe relatively isolated northwestern (W1) lobe of CRL 618. We find thata 300 km s-1 collimated fast wind with an opening angle of10° can readily produce a highly collimated lobe similar to the W1lobe, including the bowlike emission structure at its tip. However, ourmodels have difficulty producing the bright emission structures seenalong the body of the lobe. The [S II] λ6716/λ6730 ratiosat the tip of the lobe in all of our simulations are similar to thatobserved at the tip of the W1 lobe. The optical line ratios indicate atemperature stratification in the tip; for both the simulations andobservations, however, the temperatures at the tip of the lobe in oursimulations are higher than observed. The position-velocity (PV)diagrams derived from our simulations are all qualitatively consistentwith the current observations. The collimated fast wind in CRL 618 isunlikely to be steady and is not radiatively driven.
| Winds, Bubbles, and Outflows in Planetary Nebulae
The aim of this work is to highlight the contributions that John Dysonhas made to the study of the interstellar medium in general and, inparticular, to the field of planetary nebulae. I review a fewoutstanding problems regarding the formation and evolution of outflowsin planetary nebulae.
| A Detailed Kinematical Study of the Point-Symmetric Planetary Nebula IC4634
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| The correlation between expansion velocity and morphology in planetary nebulae
It is usually accepted that the differing morphological classes ofplanetary nebulae (PNe) arise from progenitors of differing mass. Theprimary evidence for this derives from the differing galacticdistributions of the sources. This, if true, would be expected to resultin other differences as well, including variations in the kinematics ofthe nebular envelopes. We point out here that there is now sufficientevidence to determine that this is the case. We find that BRET-typesources (i.e. nebulae possessing ``bipolar rotating episodic jets") havethe lowest velocities of expansion VEXP, followed (in orderof increasing velocity) by bipolar (BPNe), elliptical and circularnebulae. In addition to this, we find that the distributionsN(VEXP) of circular, elliptical and bipolar sources are quitedistinct, with BPNe being biased towards lower velocities, and circularsources distributed more uniformly. It appears therefore that bipolaroutflows contain, within the same shells, evidence for both the highestand lowest velocities of expansion. Whilst the outer wings of thesenebulae are expanding at ~ 175 km s-1, the brighter parts ofthe shells (probably corresponding to equatorial toroids) havevelocities of only ~ 18 km s-1.
| Emission Lines from Jets in Planetary Nebulae
Emission line profiles from collimated high-velocity outflows or jets inplanetary nebulae (PNe) have diverse characteristics that indicate theinfluence of binary cores and magnetic confinement mechanisms in theirorigin. In this contribution, a number of cases are presented thatexemplify the complexity of these outflows in PNe.
| Enigmatic Low-Velocity Jet-Like Features in Planetary Nebulae
We are developing a project aimed at studying the physical properties,origin and evolution of low-ionization structures in planetary nebulae.Within this project we have identified a number of pairs of highlycollimated low-ionization jet-like features (Gonçalves et al.2001). In spite of being very similar to real jets, they have theintriguing property of possessing expansion velocities which are verylow, or at least not significantly different from, that of the shells inwhich they are embedded. In this contribution we discuss our data onthese fake jets (Corradi et al. 1997, 1999) and compare them withexisting theoretical models for the formation of collimated structuresin PNe. These enigmatic jet-like systems are not easily accounted forwithin the theoretical scenarios that deal with collimated features inPNe.
| Gas temperature and excitation classes in planetary nebulae
Empirical methods to estimate the elemental abundances in planetarynebulae usually use the temperatures derived from the [O III] and [N II]emission-line ratios, respectively, for the high- and low-ionizationzones. However, for a large number of objects these values may not beavailable. In order to overcome this difficulty and allow a betterdetermination of abundances, we discuss the relationship between thesetwo temperatures. Although a correlation is not easily seen when asample of different PNe types is used, the situation is improved whenthey are gathered into excitation classes. From [OII]/[OIII] andHeII/HeI line ratios, we define four excitation classes. Then, usingstandard photoionization models which fit most of the data, a linearrelation between the two temperatures is obtained for each of the fourexcitation classes. The method is applied to several objects for whichonly one temperature can be obtained from the observed emission linesand is tested by recalculation of the radial abundance gradient of theGalaxy using a larger number of PNe. We verified that our previousgradient results, obtained with a smaller sample of planetary nebulae,are not changed, indicating that the temperature relation obtained fromthe photoionization models are a good approximation, and thecorresponding statistical error decreases as expected. Tables 3-5, 7 and9 are only available in electronic form at http://www.edpsciences.org
| Effective collision strengths for fine-structure forbidden transitions among the 3s23p4 levels of ClII
The ab initio R-matrix method is used to calculate effective collisionstrengths for electron-impact excitation of the sulphur-like ion ClII inthe close-coupling approximation. All 10 astrophysically importantfine-structure forbidden transitions within the3s23p4 3Pe0,1,2,1De2,1Se0 ground configuration levels areconsidered. The 12 lowest LS target states are included in thecalculation. Effective collision strengths are obtained by averaging theelectron collision strengths over a Maxwellian distribution of electronvelocities. Results are presented for electron temperatures in the rangelogT(K)=3.3 to logT(K)=5.5, appropriate for astrophysical applications.These are the only effective collision strength data that are currentlyavailable.
| The 3-D ionization structure of the planetary nebula NGC 6565
A 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), the``transition'' 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.
| A Catalogue of IJK Photometry of PNe with DENIS
Near-infrared photometry of planetary nebulae (PNe) allows theclassification of those objects (Whitelock 1985; Peña &Torres-Peimbert 1987). We present the largest homogeneous sample.
| The distance scale of planetary nebulae
By collecting distances from the literature, a set of 73 planetarynebulae with mean distances of high accuracy is derived. This sample isused for recalibration of the mass-radius relationship, used by manystatistical distance methods. An attempt to correct for a statisticalpeculiarity, where errors in the distances influences the mass-radiusrelationship by increasing its slope, has been made for the first time.Distances to PNe in the Galactic Bulge, derived by this new method aswell as other statistical methods from the last decade, are then usedfor the evaluation of these methods as distance indicators. In order ofachieving a Bulge sample that is free from outliers we derive newcriteria for Bulge membership. These criteria are much more stringentthan those used hitherto, in the sense that they also discriminateagainst background objects. By splitting our Bulge sample in two, onewith optically thick (small) PNe and one with optically thin (large)PNe, we find that our calibration is of higher accuracy than most othercalibrations. Differences between the two subsamples, we believe, aredue to the incompleteness of the Bulge sample, as well as the dominanceof optical diameters in the ``thin'' sample and radio diameters in the``thick'' sample. Our final conclusion is that statistical methods givedistances that are at least as accurate as the ones obtained from manyindividual methods. Also, the ``long'' distance scale of Galactic PNe isconfirmed.
| An analysis of the observed radio emission from planetary nebulae
We have analysed the radio fluxes for 264 planetary nebulae for whichreliable measurements of fluxes at 1.4 and 5 GHz, and of nebulardiameters are available. For many of the investigated nebulae, theoptical thickness is important, especially at 1.4 GHz. Simple modelslike the one specified only by a single optical thickness or spherical,constant density shells do not account satisfactorily for theobservations. Also an r-2 density distribution is ruled out.A reasonable representation of the observations can be obtained by atwo-component model having regions of two different values of opticalthickness. We show that the nebular diameters smaller than 10arcsec areuncertain, particularly if they come from photographic plates orGaussian fitting to the radio profile. While determining theinterstellar extinction from an optical to radio flux ratio, cautionshould be paid regarding optical thickness effects in the radio. We havedeveloped a method for estimating the value of self absorption. At 1.4GHz self absorption of the flux is usually important and can exceed afactor of 10. At 5 GHz self absorption is negligible for most of theobjects, although in some cases it can reach a factor of 2. The Galacticbulge planetary nebulae when used to calibrate the Shklovsky method givea mean nebular mass of 0.14 Msun. The statistical uncertaintyof the Shklovsky distances is smaller than a factor of 1.5. Table 1 isonly available in electronic form at http://www.edpsciences.org.
| ISO LWS observations of planetary nebula fine-structure lines
We have obtained 43-198μm far-infrared (IR) spectra for a sample of51 Galactic planetary nebulae (PN) and protoplanetary nebulae (PPN),using the Long Wavelength Spectrometer (LWS) on board the Infrared SpaceObservatory (ISO). Spectra were also obtained of the former PN candidateLo 14. The spectra yield fluxes for the fine-structure lines [Nii]122μm, [Niii] 57μm and [Oiii] 52 and 88μm emitted in theionized regions and the [Oi] 63- and 146-μm and [Cii] 158-μm linesfrom the photodissociation regions (PDRs), which have been used todetermine electron densities and ionic abundances for the ionizedregions and densities, temperatures and gas masses for the PDRs. Thestrong [Niii] and [Oiii] emission lines detected in the LWS spectrumtaken centred on Lo 14 could be associated with the nearby strong radioand infrared source G 331.5-0.1. We find that the electron densitiesyielded by the [Oiii] 88μm/52μm doublet ratio are systematicallylower than those derived from the optical [Ariv]λ4740/λ4711 and [Cliii] λ5537/λ5517 doubletratios, which have much higher critical densities than the 52- and88-μm lines, suggesting the presence of density inhomogeneities inthe nebulae. Ionic abundances, N+/H+,N2+/H+ and O2+/H+, as wellas the N2+/O2+ abundance ratio, which provides agood approximation to the N/O elemental abundance ratio, are derived.Although ionic abundances relative to H+ deduced from thefar-IR fine-structure lines are sensitive to the adopted electrondensity and the presence of density inhomogeneities, the strongdependence on the nebular physical conditions is largely cancelled outwhen N2+/O2+ is calculated from the57μm/(52μm+88μm) flux ratio, owing to the similarity of thecritical densities of the lines involved. The temperatures and densitiesof the PDRs around 24 PN have been determined from the observed [Oi] and[Cii] line intensity ratios. Except for a few objects, the deducedtemperatures fall between 200 and 500K, peaking around 250K. Thedensities of the PDRs vary from104-105cm-3, reaching3×105cm-3 in some young compact PN. With aderived temperature of 1600K and a density of105cm-3, the PDR of NGC 7027 is one of the warmestand at the same time one of the densest amongst the nebulae studied. Formost of the PN studied, the [Cii]-emitting regions contain only modestamounts of material, with gas masses <~0.1Msolar.Exceptional large PDR masses are found for a few nebulae, including NGC7027, the bipolar nebulae M2-9 and NGC 6302, the young dense planetarynebulae BD+30°3639, IC 418 and NGC 5315, and the old, probablyrecombining, nebulae IC 4406 and NGC 6072.
| 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.
| Low-Ionization Structures in Planetary Nebulae: Confronting Models with Observations
Around 50 planetary nebulae (PNs) are presently known to possess``small-scale'' low-ionization structures (LISs) located inside oroutside their main nebular bodies. We consider here the different kindsof LISs (jets, jetlike systems, symmetrical and nonsymmetrical knots)and present a detailed comparison of the existing model predictions withthe observational morphological and kinematical properties. We find thatnebulae with LISs appear indistinctly spread among all morphologicalclasses of PNs, indicating that the processes leading to the formationof LISs are not necessarily related to those responsible for theasphericity of the large-scale morphological components of PNs. We showthat both the observed velocities and locations of most nonsymmetricalsystems of LISs can be reasonably well reproduced assuming either fossilcondensations originated in the asymptotic giant branch (AGB) wind or insitu instabilities. The jet models proposed to date (hydrodynamical andmagnetohydrodynamical interacting winds or accretion disk collimatedwinds) appear unable to account simultaneously for several keycharacteristics of the observed high-velocity jets, such as theirkinematical ages and the angle between the jet and the symmetry axes ofthe nebulae. The linear increase in velocity observed in several jetsfavors magnetohydrodynamical confinement compared to pure hydrodynamicalinteracting wind models. On the other hand, we find that the formationof jetlike systems characterized by relatively low expansion velocities(similar to those of the main shells of PNs) cannot be explained by anyof the existing models. Finally, the knots that appear in symmetricaland opposite pairs of low velocity could be understood as the survivalof fossil (symmetrical) condensations formed during the AGB phase or asstructures that have experienced substantial slowing down by the ambientmedium.
| Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.
Not Available
| The optical spectrum of the planetary nebula NGC 6543
With the Hamilton echelle spectrograph at the Lick Observatory,emission-rich spectral lines of the planetary nebula NGC 6543 weresecured in the wavelength range from 3550 to 10100Å. We chose twobright regions, ~8arcsec east and ~13arcsec north of the central star,the physical conditions and chemical abundances of which may differ as aresult of the different physical characteristics involving the massejection of different epochs. By combining Hamilton echelle observationswith archive UV data secured with the International Ultraviolet Explorer(IUE), we obtain improved diagnostics and chemical compositions for thetwo observed regions. The diagnostic diagram gives the average value ofTe=8000~8300K, and the electron number density nearNe~5000cm-3 for most ions, while somelow-excitation lines indicate much higher temperatures, i.e.Te~10000K. With the construction of a photoionization model,we try to fit the observed spectra in a self-consistent way: thus, formost elements, we employ the same chemical abundances in the nebularshell; and we adopt an improved Sobolev approximation model atmospherefor the hydrogen-deficient Wolf-Rayet type central star. Within theobservational errors, the chemical abundances do not seem to show anypositional variation except for helium. The chemical abundances of NGC6543 appear to be the same as in average planetary nebulae. Theprogenitor star may have been an object of one solar mass, most of theheavier elements of which were less plentiful than in the Sun.
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