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NGC 7009 (Saturn Nebula)



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Proper Motions of the Ansae in the Planetary Nebula NGC009
For the planetary nebula NGC 7009, we present a comparison of twounpublished Very Large Array archive data sets taken with a timeseparation of 8.09 years to confirm the proper motions of its ansaeobserved in the optical. We determine values of 23+/-6 and 34+/-10 masyr^{-1} for the eastern and western ansae, respectively. There ismarginal evidence suggesting that the flux densities of the jets thatconnect the ansae with the main body of the nebula diminished in about30% over the period between the two observations. We also set an upperlimit to the expansion of the main body of the planetary nebula, settinga lower limit of ˜ 700 pc for its distance.

X-ray emission from planetary nebulae calculated by 1D spherical numerical simulations
We calculate the X-ray emission from both constant and time-evolvingshocked fast winds blown by the central stars of planetary nebulae (PNe)and compare our calculations with observations. Using sphericallysymmetric numerical simulations with radiative cooling, we calculate theflow structure and the X-ray temperature and luminosity of the hotbubble formed by the shocked fast wind. We find that a constant fastwind gives results that are very close to those obtained from theself-similar solution. We show that in order for a fast shocked wind toexplain the observed X-ray properties of PNe, rapid evolution of thewind is essential. More specifically, the mass-loss rate of the fastwind should be high early on when the speed is ~300-700 kms-1, and then it needs to drop drastically by the time the PNage reaches ~1000 yr. This implies that the central star has a veryshort pre-PN (post-asymptotic giant branch) phase.

Chemical abundances in planetary nebulae and stellar evolution.
I will review basic aspects of galactic Planetary Nebulae focussing onstatus of art on their chemical abundances and relationship with thestellar evolution theory.

Low-mass binary-induced outflows from asymptotic giant branch stars
A significant fraction of planetary nebulae (PNe) and protoplanetarynebulae (PPNe) exhibit aspherical, axisymmetric structures, many ofwhich are highly collimated. The origin of these structures is notentirely understood, however, recent evidence suggests that manyobserved PNe harbour binary systems, which may play a role in theirshaping. In an effort to understand how binaries may produce suchasymmetries, we study the effect of low-mass (<0.3Msolar)companions (planets, brown dwarfs and low-mass main-sequence stars)embedded into the envelope of a 3.0-Msolar star during threeepochs of its evolution [red giant branch, asymptotic giant branch(AGB), interpulse AGB]. We find that common envelope evolution can leadto three qualitatively different consequences: (i) direct ejection ofenvelope material resulting in a predominately equatorial outflow, (ii)spin-up of the envelope resulting in the possibility of powering anexplosive dynamo-driven jet and (iii) tidal shredding of the companioninto a disc which facilitates a disc-driven jet. We study how thesefeatures depend on the secondary's mass and discuss observationalconsequences.

Evolution of elemental abundances in planetary nebulae
We study the evolution of elemental abundances in an ensemble ofGalactic planetary nebulae as a function of the masses of the centralstars (M cs) and their progenitors (M ini). We derive the dependences ofthe C, N, Ne, Cl, Ar, and S abundances on M cs and M ini for a largesample of nebulae. We calculate the theoretical elemental abundances innebulae under the assumption of complete mixing of theprogenitor’s matter ejected at different stages of its evolution.The theoretical dependences of the C and N abundances on M ini have beenfound to correspond to the observed ones. At the same time, the observedmean O abundance is approximately half its theoretical value. The Ne,Cl, Ar, and S abundances monotonically increase with increasing mass ofthe progenitor star, which reflects an increase in the mean abundancesof heavy elements during the chemical evolution of the Galaxy. We havederived the relation between the abundances of the elements underconsideration in planetary nebulae and the masses of their centralstars. This relation is used to construct the mass function for thenuclei of planetary nebulae.

Observed Planetary Nebulae as Descendants of Interacting Binary Systems
We examine recent studies on the formation rate of planetary nebulae andfind this rate to be about one-third of the formation rate of whitedwarfs. This implies that only about one-third of all planetary nebulaethat evolve to form white dwarfs are actually bright enough to beobserved. This finding corresponds with the claim that it is necessaryfor a binary companion to interact with the asymptotic giant branchstellar progenitor for the descendant planetary nebulae to be brightenough to be detected. The finding about the formation rate alsostrengthens O. De Marco's conjecture that the majority of observedplanetary nebulae harbor binary systems. In other words, single starsalmost never form observed planetary nebulae.

Electron temperature fluctuations in H II regions. The feasibility of t2 estimates from point-to-point observations
Context: . The spatial electron temperature fluctuations in ionizednebulae that were initially proposed to explain the discrepancies amongtemperatures obtained from different sensors have been pointed to as thecause of huge inconsistencies among abundances of heavy elementscalculated from recombination and forbidden emission lines. Recently,there have been some attempts of direct detection and quantification ofspatial temperature fluctuations with point-to-point temperaturemeasurements across the nebula. Aims: . In this paper, we assessthe feasibility of estimating the temperature fluctuation parameter t^2,the total variance of the spatial distribution of temperature relativeto the mean, from the distribution of temperatures measured on the planeof the sky with different sensors. Methods: . Point-to-pointmeasurements of the electron temperature at a series of contiguous andequally spaced rectangular apertures along a radius of homogeneous andspherically symmetric nebulae were numerically simulated for sixdifferent temperature sensors. Results: . Variances of projectedtemperature distributions were obtained and compared with totalt2 for different values for the density, ionization parameterand temperature of the ionizing star. The projected profiles of electrontemperature obtained from indicators associated with ions occurring atlarge fractions of the nebula, such as [O III] (λ 4959 + λ5007)/λ 4363, [Ar III] (λ 7136 + λ 7751)/λ5192, and the Balmer jump (Iλ (3646^-) -Iλ (3646^+))/Hβ, are good tracers of the internalgradient of temperature. The variances t^2s of the projectedtemperature distributions measured from these sensors correspond tosignificant fractions of the total temperature variance, for typicalnebulae of the order of t^2_s/t2 ≈ 30%, 25%, and 15% forthe Balmer jump and the [O III] and [Ar III] ratios, in that order. Onthe other hand, the temperature profiles obtained from sensorscorresponding to low ionization ions, such as [N II] (λ 6548 +λ 6584)/λ 5755, [O I] (λ 6300 + λ6364)/λ 5577, and [C I] (λ 9823 + λ 9849)/λ8728, are almost constant at the values of temperature of the outerparts of the nebula, fail to reproduce the true temperature gradient,and have a t^2s that is always less than 10% and is usuallyaround 1-3% of t^2.

Chemical abundances for Hf 2-2, a planetary nebula with the strongest-known heavy-element recombination lines
We present high-quality optical spectroscopic observations of theplanetary nebula (PN) Hf2-2. The spectrum exhibits many prominentoptical recombination lines (ORLs) from heavy-element ions. Analysis ofthe HI and HeI recombination spectrum yields an electron temperature of~900 K, a factor of 10 lower than given by the collisionally excited[OIII] forbidden lines. The ionic abundances of heavy elements relativeto hydrogen derived from ORLs are about a factor of 70 higher than thosededuced from collisionally excited lines (CELs) from the same ions, thelargest abundance discrepancy factor (adf) ever measured for a PN. Bycomparing the observed OIIλ4089/λ4649 ORL ratio totheoretical value as a function of electron temperature, we show thatthe OII ORLs arise from ionized regions with an electron temperature ofonly ~630 K. The current observations thus provide the strongestevidence that the nebula contains another previously unknown componentof cold, high-metallicity gas, which is too cool to excite anysignificant optical or ultraviolet CELs and is thus invisible via suchlines. The existence of such a plasma component in PNe provides anatural solution to the long-standing dichotomy between nebular plasmadiagnostics and abundance determinations using CELs on the one hand andORLs on the other.

X-ray emission by a shocked fast wind from the central stars of planetary nebulae
We calculate the X-ray emission from the shocked fast wind blown by thecentral stars of planetary nebulae (PNe) and compare with observations.Using spherically symmetric self-similar solutions, we calculate theflow structure and X-ray temperature for a fast wind slamming into apreviously ejected slow wind. We find that the observed X-ray emissionof six PNe can be accounted for by shocked wind segments that wereexpelled during the early-PN phase, if the fast wind speed is moderate,v2~ 400-600kms-1, and the mass-loss rate is a fewtimes 10-7Msolaryr-1. We find, asproposed previously, that the morphology of the X-ray emission is in theform of a narrow ring inner to the optical bright part of the nebula.The bipolar X-ray morphology of several observed PNe, which indicates animportant role of jets, rather than a spherical fast wind, cannot beexplained by the flow studied here.

The structure of planetary nebulae: theory vs. practice
Context.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.

Galactic Planetary Nebulae with Wolf-Rayet Nuclei III. Kinematical Analysis of a Large Sample of Nebulae
Expansion velocities (V_{exp}) of different ions and line widths at thebase of the lines are measured and analyzed for 24 PNe with [WC]-typenuclei (WRPNe), 9 PNe ionized by WELS (WLPNe) and 14 ordinary PNe. Acomparative study of the kinematical behavior of the sample clearlydemonstrates that WRPNe have on average 40-45% larger V_{exp}, andpossibly more turbulence than WLPNe and ordinary PNe. WLPNe havevelocity fields very much like the ones of ordinary PNe, rather than theones of WRPNe. All the samples (WRPNe, WLPNe and ordinary PNe) showexpansion velocities increasing with age indicators, for example is larger for low-density nebulae and also it is largerfor nebulae around high-temperature stars. This age effect is muchstronger for evolved WRPNe, suggesting that the [WC] winds have beenaccelerating the nebulae for a long time, while for non-WRPNe theacceleration seems to stop at some point when the star reaches atemperature of about 90,000 - 100,000. Non-WR nebulae reach a maximumV_{exp} ≤ 30 km s(-1) evolved WRPNe reach maximum V_{exp} about 40km s(-1) . For all kinds of objects (WRPNe and non-WRPNe) it is foundthat on average V_{exp}(N(+) ) is slightly larger than V_{exp}(O(++) ),indicating that the nebulae present acceleration of the external shells.

Accreting White Dwarfs among the Planetary Nebulae Most Luminous in [O III] λ5007 Emission
I propose that some of the most luminous planetary nebulae (PNs) areactually proto-PNs, where a companion white dwarf (WD) accretes mass ata relatively high rate from the post-asymptotic giant branch star thatblew the nebula. The WD sustains a continuous nuclear burning andionizes the nebula. The WD is luminous enough to make the dense nebulaluminous in the [O III] λ5007 line. In young stellar populationsthese WD accreting systems account for a small fraction of [OIII]-luminous PNs, but in old stellar populations these binaries mightaccount for most, or even all, of the [O III]-luminous PNs. This mightexplain the puzzling constant cutoff (maximum) [O III] λ5007luminosity of the planetary nebula luminosity function across differentgalaxy types.

Chemical abundances in seven galactic planetary nebulae
An observational study of chemical abundances in the galactic planetarynebulae NGC 1535, NGC 2438,NGC 2440, NGC 3132, NGC3242, NGC 6302, and NGC7009 based on long-slit spectra of high signal-to-noise ratioin the 3100 to 6900 Å range is presented. We determined the N, O,Ne, S, and Cl abundances from collisionally excited lines and the He andO++ abundances from recombination lines. TheO++/H+ estimates derived from recombination linesare about a factor of four and two higher than those derived fromforbidden lines for NGC 7009 and NGC3242, respectively. Spatial profiles ofO++/H+ abundance from O II permitted lines andfrom [O III] forbidden lines were obtained for the planetary nebulaNGC 7009. The differences betweenO++/H+ derived from recombination and fromforbidden lines present smooth variations along the nebular surface ofNGC 7009, with the differences decreasing from thecenter to the edges of the nebula. If these abundance differences areexplained by the presence of electron temperature fluctuations,quantified by the parameter t2, a value of aboutt2=0.09 is required for NGC 3242 andNGC 7009.

Photospheric opacity and over-expanded envelopes of asymptotic giant branch stars
I suggest that the behavior of the photospheric opacity in oxygen-rich(similar to solar abundance) upper asymptotic giant branch stars maycause these stars to substantially expand for a few thousand years. Iterm this process over-expansion. This may occur when the photospheric(effective) temperature drops to Tp ˜ 3000 K, and becausethe opacity sharply increases as temperature further decreases down toTp ˜ 2000 K. The much higher opacity implies a much lowerphotospheric density, which stabilizes the envelope structure. As massloss proceeds, the star eventually contracts to become a post-asymptoticgiant branch star. Some possible outcomes of the over-expanded phase arediscussed: (1) The over-expanded phase may be connected to the formationof semi-periodic concentric arcs (rings; shells); (2) the over-expandedphase may be related to the positive correlation between the mass lossrate and the transition to axisymmetric mass loss geometry; and (3) anover-expanded asymptotic giant branch star, which doubles its radius, issomewhat more likely to swallow a low mass companion.

Iron Depletion in the Hot Bubbles in Planetary Nebulae
We have searched for the emission from [Fe X] λ6374 and [Fe XIV]λ5303 that is expected from the gas emitting in diffuse X-rays inBD +30°3639, NGC 6543, NGC 7009, and NGC 7027. Neither line wasdetected in any object. Models that fit the X-ray spectra of theseobjects indicate that the [Fe X] λ6374 emission should be belowour detection thresholds, but the predicted [Fe XIV] λ5303emission exceeds our observed upper limits (1 σ) by factors of atleast 3.5-12. The best explanation for the absence of [Fe XIV]λ5303 is that the X-ray plasma is depleted in iron. In principle,this result provides a clear chemical signature that may be used todetermine the origin of the X-ray gas in either the nebular gas or thestellar wind. At present, although various lines of evidence appear tofavor a nebular origin, the lack of atmospheric and nebular ironabundances in the objects studied here precludes a definitiveconclusion.Based on observations obtained at the Observatorio AstronómicoNacional in San Pedro Mártir, Baja California, Mexico.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.

Slitless Grism Spectroscopy with the Hubble Space Telescope Advanced Camera for Surveys
The Advanced Camera for Surveys on board the Hubble Space Telescope isequipped with one grism and three prisms for low-resolution, slitlessspectroscopy in the range 1150-10500 Å. The G800L grism providesoptical spectroscopy between 5500 Å and >1 μm, with a meandispersion of 39 and 24 Å pixel-1 (in the firstspectral order) when coupled with the Wide Field and the High ResolutionChannels, respectively. Given the lack of any on-board calibration lampsfor wavelength and narrowband flat-fielding, the G800L grism can only becalibrated using astronomical targets. In this paper, we describe thestrategy used to calibrate the grism in orbit, with special attentiongiven to the treatment of the field dependence of the grism flat field,wavelength solution, and sensitivity in both channels.The Hubble Space Telescope is a project of international cooperationbetween NASA and the European Space Agency (ESA). The Space TelescopeScience Institute is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555.

On the nitrogen abundance of fast, low-ionization emission regions: the outer knots of the planetary nebula NGC 7009
We have constructed a 3D photoionization model of a planetary nebula(PN) similar in structure to NGC 7009 with its outer pair of knots (alsoknown as FLIERs - fast, low-ionization emission regions). The work ismotivated by the fact that the strong [NII]λ6583 line emissionfrom FLIERs in many PNe has been attributed to a significant localoverabundance of nitrogen. We explore the possibility that the apparentenhanced nitrogen abundance previously reported in the FLIERs may be dueto ionization effects. The model is constrained by the results obtainedby Gonçalves et al. from the analysis of both Hubble SpaceTelescope (HST) [OIII] and [NII] images, and long-slit spectra of NGC7009. Our model is indeed able to reproduce the main spectroscopic andimaging characteristics of the bright inner rim of NGC 7009 and itsouter pairs of knots, assuming homogeneous elemental abundancesthroughout the nebula, for nitrogen as well as all the other elementsincluded in the model.We also study the effects of a narrow slit on our non-sphericallysymmetric density distribution, via the convolution of the model resultswith the profile of the long slit used to obtain the spectroscopicobservations that constrained our model. This effect significantlyenhances the [NII]/Hβ emission, more in the FLIERs than in theinner rim.Because of the fact that the (N+/N)/(O+/O) ratiopredicted by our models is 0.60 for the rim and is 0.72 for the knots,so clearly in disagreement with the N+/N = O+/Oassumption of the ionization correction factor (icf) method, the icfswill be underestimated by the empirical scheme, in both components, rimand knots, but more so in the knots. This effect is partly responsiblefor the apparent inhomogeneous N abundance empirically derived. Thedifferences in the above ratio in these two components of the nebula maybe due to a number of effects including charge exchange - as pointed outpreviously by other authors - and the difference in the ionizationpotentials of the relevant species - which makes this ratio extremelysensitive to the shape of the local radiation field. Because of thelatter, a realistic density distribution is essential to the modellingof a non-spherical object, if useful information is to be extracted fromspatially resolved observations, as in the case of NGC 7009.

The Subdwarf Database: Released
The work on the Subdwarf Database, presented at the previous meeting,has been completed, and the tool is now publicly available. The firstrelease contains data from close to 240 different literature sources,but more still awaits entry. The database interface includes advancedsearch capabilities in coordinate, magnitude and color space. Outputtables can be generated in HTML with hyperlinks to automaticallygenerated finding charts, the Aladin viewer and a detailed data sheetthat displays all registered data for each target, including physicaldata such as temperature, gravity and helium abundance, together with afinding chart. Search results can be visualized automatically asinteractive position, magnitude or color diagrams.

X-Ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40
We present the results of Chandra X-Ray Observatory observations of theplanetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-typeWolf-Rayet central stars that are currently driving fast (~1000 kms-1), massive winds into denser, slow-moving (~10 kms-1) material ejected during recently terminated asymptoticgiant branch (AGB) evolutionary phases. Hence, these observationsprovide key tests of models of wind-wind interactions in PNs. In NGC 40,we detect faint, diffuse X-ray emission distributed within a partialannulus that lies nested within a ~40" diameter ring of nebulosityobserved in optical and near-infrared images. Hen 2-99 is not detected.The inferred X-ray temperature (TX~106 K) andluminosity (LX~2×1030 ergs s-1)of NGC 40 are the lowest measured thus far for any PN displaying diffuseX-ray emission. These results, combined with the ringlike morphology ofthe X-ray emission from NGC 40, suggest that its X-ray emission arisesfrom a ``hot bubble'' that is highly evolved and is generated by ashocked, quasi-spherical fast wind from the central star, as opposed toAGB or post-AGB jet activity. In contrast, the lack of detectable X-rayemission from Hen 2-99 suggests that this PN has yet to enter a phase ofstrong wind-wind shocks.

Blowing up warped disks in 3D. Three-dimensional AMR simulations of point-symmetric nebulae
The Generalized Interacting Stellar Winds model has been very successfulin explaining observed cylindrical and bipolar shapes of planetarynebulae. However, many nebulae have a multipolar or point-symmetricshape. Previous two-dimensional calculations showed that these seeminglyenigmatic forms can be reproduced by a two-wind model in which theconfining disk is warped, as is expected to occur in irradiated disks.In this paper we present the extension to fully three-dimensionalAdaptive Mesh Refinement simulations using the publicly availablehydrodynamics package Flash. We briefly describe the mechanism leadingto a radiation driven warped disk, and give an equation for its shape.We derive time scales related to the disk and compare them to theradiative cooling time scale of the gas, thereby determining therelevant part of parameter space. By comparing two-dimensionalcalculations including realistic radiative cooling through a coolingcurve, with ones employing a low value for the adiabatic index γ,we show that the latter, computationally less expensive approach, is avalid approximation for treating cooling in our nebulae. The results ofthe fully three-dimensional wind-disk simulations show our mechanism tobe capable of producing a plethora of multipolar (and quadrupolar)morphologies, which can explain the observed shape of a number of(proto-)planetary nebulae.

Electron temperature fluctuations in planetary nebulae
An observational study of the spatial variation of the electrontemperature and density in 10 galactic planetary nebulae is presented.The data consist of long-slit spectra of high signal-to-noise ratio inthe 3100 to 6900 Å range. Electron temperatures were determinedfrom the [O III](λ 4959 + λ 5007)/λ 4363 and [NII](λ 6548 + λ 6583)/λ 5755 ratios and from theBalmer discontinuity. Electron densities were estimated from the [SII]λ 6716/λ 6731, [Cl III]λ 5517/λ 5537, and[Ar IV]λ 4711/λ 4740 ratios. Electron temperaturevariations of low amplitude were found across the nebular surface in theplanetary nebulae studied. The temperature distribution across eachnebula presents a variance relative to the mean corresponding to 0.0003≤ t2s(Bal) ≤ 0.0078, 0.0003 ≤t2s(N II) ≤ 0.0097, and 0.0011 ≤t2s(O III) ≤ 0.0050. A systematic spatialvariation of electron density has been detected in most of objects(NGC 1535, NGC 2438, NGC2440, NGC 3132, NGC3242, NGC 6302, NGC6563, and NGC 7009). The remaining objects(NGC 6781 and NGC 6853) have notshown any significant electron density dependence on position.NGC 2438, NGC 6563, NGC6781, and NGC 6853 are in general the mostdiffuse and probably evolved objects studied here, with low meandensities in the range Ne(S II) ≈ 95-158~cm-3.An anti-correlation between temperature and density was found forNGC 2438 and NGC 3132, with theelectron temperature increasing with the decrease of electron densityand a correlation between temperature and density was found forNGC 2440, NGC 3242, NGC6302, and NGC 7009, with the electrontemperature increasing with the increase of electron density. Theserelationships seem to be associated with the structure of the nebula.The nebulae in which the correlation between temperature and density ispresent are ring shaped. The anti-correlation between temperature anddensity is found in bipolar planetary nebulae that are denser in thecentre of the nebula.

Oxygen Recombination Line Abundances in Gaseous Nebulae
The determination of the heavy element abundances from giantextragalactic H II regions has been generally based on collisionallyexcited lines. We will discuss the reasons to study the characteristicsof recombination lines, and then use these lines to determine chemicalabundances. Of these lines the oxygen (specifically the O II) lines arethe most important; and, of them, the lines of multiplet 1 of O II arethe most accessible. It has often been assumed that by measuring theintensity of a single line within a multiplet the intensities of all thelines in the multiplet can be determined; in recent studies we havefound that the intensity ratios of lines within a multiplet can dependon density; we will present empirical density-intensity relationshipsfor multiplet 1 based on recent observations of H II regions andplanetary nebulae. From observations of H II regions we find that thecritical density for collisional redistribution of the multiplet 1 O IIrecombination lines amounts to 2800+/-500 cm-3. We point out that theO/H recombination abundances of H II regions in the solar vicinity arein excellent agreement with the O/H solar value, while the abundancesderived from collisionally excited lines are not. We present acalibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8range based on O recombination lines.

Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio
Airborne and space-based low-resolution spectroscopy in the 1980sdiscovered tantalizing quantitative relationships between the gas phaseC/O abundance ratio in planetary nebulae (PNe) and the fractions oftotal far-infrared (FIR) luminosity radiated by the 7.7- and 11.3-μmbands (the C = C stretch and C-H bend, respectively), of polycyclicaromatic hydrocarbons (PAHs). Only a very small sample of nebulae wasstudied in this context, limited by airborne observations of the7.7-μm band, or the existence of adequate IRAS Low ResolutionSpectrometer data for the 11.3-μm band. To investigate these trendsfurther, we have expanded the sample of planetaries available for thisstudy using Infrared Space Observatory (ISO) low-resolution spectrasecured with the Short Wavelength Spectrometer and the Long WavelengthSpectrometer. The new sample of 43 PNe, of which 17 are detected in PAHemission, addresses the range from C/O = 0.2-13 with the objective oftrying to delineate the pathways by which carbon dust grains might haveformed in planetaries. For the 7.7-μm and 11.3-μm bands, weconfirm that the ratio of band strength to total infrared (IR)luminosity is correlated with the nebular C/O ratio. Expressed inequivalent width terms, the cut-on C/O ratio for the 7.7-μm band isfound to be 0.6+0.2-0.4, in good accord with thatfound from sensitive ground-based measurements of the 3.3-μ band.

The abundance discrepancy - recombination line versus forbidden line abundances for a northern sample of galactic planetary nebulae
We present deep optical spectra of 23 galactic planetary nebulae, whichare analysed in conjunction with archival infrared and ultravioletspectra. We derive nebular electron temperatures based on standardcollisionally excited line (CEL) diagnostics as well as the hydrogenBalmer jump and find that, as expected, the Balmer jump almost alwaysyields a lower temperature than the [OIII] nebular-to-auroral lineratio. We also make use of the weak temperature dependence of helium andOII recombination line ratios to further investigate the temperaturestructure of the sample nebulae. We find that, in almost every case, thederived temperatures follow the relation , which is the relationpredicted by two-component nebular models in which one component is coldand hydrogen-deficient. Te(OII) may be as low as a fewhundred Kelvin, in line with the low temperatures found for thehydrogen-deficient knots of Abell 30 by Wesson, Liu and Barlow.Elemental abundances are derived for the sample nebulae from both CELsand optical recombination lines (ORLs). ORL abundances are higher thanCEL abundances in every case, by factors ranging from 1.5 to 12. Fiveobjects with O2+ abundance discrepancy factors greater than 5are found. DdDm 1 and Vy 2-2 are both found to have a very largeabundance discrepancy factor of 11.8.We consider the possible explanations for the observed discrepancies.From the observed differences between Te(OIII) andTe(BJ), we find that temperature fluctuations cannot resolvethe abundance discrepancies in 22 of the 23 sample nebulae, implyingsome additional mechanism for enhancing ORL emission. In the oneambiguous case, the good agreement between abundances derived fromtemperature-insensitive infrared lines and temperature-sensitive opticallines also points away from temperature fluctuations being present. Theobserved recombination line temperatures, the large abundancediscrepancies and the generally good agreement between infrared andoptical CEL abundances all suggest instead the existence of a coldhydrogen-deficient component within the `normal' nebular gas. The originof this component is as yet unknown.

The mean properties of planetary nebulae as a function of Peimbert class
Planetary 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.

Modelling of aspherical nebulae - I. A quick pseudo-3D photoionization code
We describe a pseudo-3D photoionization code, NEBU&LOWBAR;3D, and itsassociated visualization tool, VISNEB&LOWBAR;3D, which are able to treata wide variety of nebular geometries easily and rapidly, by combiningmodels obtained with a 1D photoionization code. The only requirement forthe code to work is that the ionization source is unique and notextended. It is applicable as long as the diffuse ionizing radiationfield is not dominant and strongly inhomogeneous. As examples of thecapabilities of these new tools, we consider two very differenttheoretical cases. One is that of a high-excitation planetary nebulathat has an ellipsoidal shape with two polar density knots. The other isthat of a blister HII region, for which we have also constructed aspherical model (the spherical impostor), which has exactly the sameHβ surface brightness distribution as the blister model and thesame ionizing star.We present and comment upon line intensity maps corresponding todifferent viewing angles. We also use the computed line intensities toderive physical properties of the model in the same way as an observerwould do for a real object. For example, we derive the `apparent' valueof N/O for the entire nebula and along spectral slits of differentorientations. For this, we take the electron temperature and densityderived from the [NII]5755Å/[NII]6583Åand[OII]3726Å/[OII]3729Åratios, respectively, and we adopt thecommon recipe: N/O = N+/O+. Interestingly, we findthat, in the case of our high-excitation nebula, the derived N/O iswithin 10-20 per cent of the real value, even when the slit crosses thehigh-density knots. On the other hand, for the blister HII region andits spherical impostor, we find that the apparent N/O is much smallerthan the true one (about 0.68 and 0.5 of it, respectively).These two examples warn against preconceived ideas when interpretingspectroscopic and imaging data of HII regions and planetary nebulae. Thetools NEBU&LOWBAR;3D and VISNEB&LOWBAR;3D, which will be made publiclyavailable in the future, should facilitate the performance of numericalexperiments, to yield a better understanding of the physics ofaspherical ionized nebulae.

The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edge
We 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 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.

Recombination Line versus Forbidden Line Abundances in Planetary Nebulae
Recombination lines (RLs) of C II, N II, and O II in planetary nebulae(PNs) have been found to give abundances that are much larger in somecases than abundances from collisionally excited forbidden lines (CELs).The origins of this abundance discrepancy are highly debated. We presentnew spectroscopic observations of O II and C II recombination lines forsix planetary nebulae. With these data we compare the abundances derivedfrom the optical recombination lines with those determined fromcollisionally excited lines. Combining our new data with publishedresults on RLs in other PNs, we examine the discrepancy in abundancesderived from RLs and CELs. We find that there is a wide range in themeasured abundance discrepancyΔ(O+2)=logO+2(RL)-logO+2(CEL),ranging from approximately 0.1 dex (within the 1 σ measurementerrors) up to 1.4 dex. This tends to rule out errors in therecombination coefficients as a source of the discrepancy. Most RLsyield similar abundances, with the notable exception of O II multipletV15, known to arise primarily from dielectronic recombination, whichgives abundances averaging 0.6 dex higher than other O II RLs. Wecompare Δ(O+2) against a variety of physical propertiesof the PNs to look for clues as to the mechanism responsible for theabundance discrepancy. The strongest correlations are found with thenebula diameter and the Balmer surface brightness; high surfacebrightness, compact PNs show small values of Δ(O+2),while large low surface brightness PNs show the largest discrepancies.An inverse correlation of Δ(O+2) with nebular densityis also seen. A marginal correlation of Δ(O+2) is foundwith expansion velocity. No correlations are seen with electrontemperature, He+2/He+, central star effectivetemperature and luminosity, stellar mass-loss rate, or nebularmorphology. Similar results are found for carbon in comparing C II RLabundances with ultraviolet measurements of C III].

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Observation and Astrometry data

Right ascension:21h04m10.88s
Apparent magnitude:8

Catalogs and designations:
Proper NamesSaturn Nebula
NGC 2000.0NGC 7009

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