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# NGC 3918

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 Crux - our southern heritage.Not Available The Problem with Reddening Distances to Planetary NebulaeIt is noted that the vast majority of Galactic PNe are located outsideof the interstellar reddening layer, to the extent that local gradientsin E_{B-V} are low to undetectable. This is likely to invalidate certainprevious estimates of their reddening distances D_{RED}. It also meansthat larger scale analyses, such as those of Pottasch (1984) and Acker(1973), lead to values of D_{RED}] which are significantly too small. Asimilar critique applies to analyses based on measures of Na D-lineabsorption, and it seems likely that most of the distances of Napiwotzki& Schönberner (1995) are similarly in error. Photospheric opacity and over-expanded envelopes of asymptotic giant branch starsI 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. On the nitrogen abundance of fast, low-ionization emission regions: the outer knots of the planetary nebula NGC 7009We 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. Hot dust haloes in planetary nebulaeWe point out that many planetary nebulae (PNe) have large infraredindices (H-KS), and that this is likely to result from thepresence of hot grains, and/or H2 S(1) line emission atλ= 2.122μm. We are able to identify two groups of sourcesassociated with each of these mechanisms, both of which appear topossess distinct physical characteristics. One difference between thesegroups concerns the near-infrared dimensions of the sources. It appearsthat hot dust outflows frequently have sizes θ(KS)> θ(H) > θ(J). Four of the sources are particularlyextreme in this regard, and show emission extending well outside of theprimary shells. We propose that this is likely to arise fromhigh-temperature grains located in low-density haloes. The location ofsuch grains at large distances from the central stars represents achallenge for any mechanism purporting to explain this phenomenon. Themost likely explanation appears to be in terms of photon heating of verysmall grains. Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratioAirborne 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 dusty MOCASSIN: fully self-consistent 3D photoionization and dust radiative transfer modelsWe present the first 3D Monte Carlo (MC) photoionization code to includea fully self-consistent treatment of dust radiative transfer (RT) withina photoionized region. This is the latest development (version 2.0) ofthe gas-only photoionization code MOCASSIN and employs a stochasticapproach to the transport of radiation, allowing both the primary andsecondary components of the radiation field to be treatedself-consistently, whilst accounting for the scattering of radiation bydust grains mixed with the gas, as well as the absorption and emissionof radiation by both the gas and the dust components. An escapeprobability method is implemented for the transfer of resonance linesthat may be absorbed by the grains, thus contributing to their energybalance. The energetics of the co-existing dust and gas components mustalso take into account the effects of dust-gas collisions andphotoelectric emission from the dust grains, which are dependent on thegrain charge. These are included in our code using the average grainpotential approximation scheme.A set of rigorous benchmark tests have been carried out for dust-onlyspherically symmetric geometries and 2D disc configurations. The resultsof MOCASSIN are found to be in agreement with those obtained bywell-established dust-only RT codes that employ various approaches tothe solution of the RT problem.A model of the dust and of the photoionized gas components of theplanetary nebula NGC 3918 is also presented as a means of testing thecorrect functioning of the RT procedures in a case where both gas anddust opacities are present. The two components are coupled via theheating of dust grains by the absorption of both UV continuum photonsand resonance line photons emitted by the gas. The MOCASSIN results showagreement with those of a 1D dust and gas model of this nebula publishedpreviously, showing the reliability of the new code, which can beapplied to a variety of astrophysical environments. Fluorine Abundances in Planetary NebulaeWe have determined fluorine abundances from the [F II] λ4789 and[F IV] λ4060 nebular emission lines for a sample of planetarynebulae (PNe). Our results show that fluorine is generally overabundantin PNe, thus providing new evidence for the synthesis of fluorine inasymptotic giant branch (AGB) stars. [F/O] is found to be positivelycorrelated with the C/O abundance ratio, in agreement with thepredictions of theoretical models of fluorine production in thermallypulsing AGB stars. A large enhancement of fluorine is observed in theWolf-Rayet PN NGC 40, suggesting that high mass-loss rates probablyfavor the survival of fluorine. 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 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. Helium recombination spectra as temperature diagnostics for planetary nebulaeElectron 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 NebulaeRecombination 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]. Physical conditions in Photo-Dissociation Regions around Planetary NebulaeWe present observations of the infrared fine-structure lines of [Si II](34.8 μm), [O I] (63.2 and 145.5 μm) and [C II] (157.7 μm)obtained with the ISO SWS and LWS spectrographs of nine PlanetaryNebulae (PNe). These lines originate in the Photo-Dissociation Regions(PDRs) associated with the nebulae and provide useful information on theevolution and excitation conditions of the ejected material in theseregions. In order to interpret the observations, the measured lineintensities have been compared with those predicted byphoto-dissociation models. This comparison has been done taking intoaccount the C/O content in the nebulae. The densities derived with thiscomparison show a large scatter for some nebulae, probably because thedensity is higher than the critical density. Therefore, they are nolonger sensitive to this parameter implying that transitions from otherspecies with higher critical density should be used. The possiblecontribution of shocks to the observed emission characteristics of thesePNe is briefly discussed and it is shown that the radiation field is themain driving force responsible for the atomic lines in the PNe that havebeen studied. In addition, data on the pure rotational lines ofH2 in three nebulae (NGC 7027, NGC 6302 and Hb 5) are alsopresented. Assuming local thermal equilibrium the rotational temperatureand densities have been derived. We have derived the mass of atomic gasin the PDR associated with these PNe and compared those to ionic massesderived from Hβ and molecular masses derived from low J COobservations. This comparison shows that for these nebulae, the PDR isthe main reservoir of gas surrounding these objects. A comparison of theresults of these evolved PNe with very young PNe from the literaturesuggests that as the nebula ages the relative amount of ionic gasincreases at the expense of the atomic and molecular mass.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http: / /cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/523 Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418We present high signal-to-noise ratio echelle spectra of the compacthigh surface brightness, low-ionization planetary nebula (PN) IC 418.These reveal 807 emission lines down to intensities less than10-5 that of Hβ for which we determine widths andrelative intensities. We show that line profiles are a valuableparameter for making line identifications and in constraining theexcitation mechanism of the lines. We present evidence that indicatesthat many supposed high-level recombination lines may in fact be excitedby a process other than recombination. We contend from the detection ofdielectronic recombination lines that their relatively low intensitiesargue against their making a significant contribution to levelpopulations of the heavy ions in this object. Following similar analysesof other PNe we find that IC 418 shows a small discrepancy in ionabundances derived from forbidden versus recombination lines of theheavy elements. Chemical abundances of planetary nebulae from optical recombination lines - II. Abundances derived from collisionally excited lines and optical recombination linesIn Paper I, we presented spectrophotometric measurements of emissionlines from the ultraviolet (UV) to the far-infrared for 12 Galacticplanetary nebulae (PNe) and derived nebular thermal and densitystructures using a variety of plasma diagnostics. The measurements andplasma diagnostic results are used in the current paper to determineelemental abundances in these nebulae. Abundance analyses are carriedout using both strong collisionally excited lines (CELs) and weakoptical recombination lines (ORLs) from heavy element ions.Assuming electron temperatures and densities derived from HIrecombination spectra (line and continuum), we are able to determine theORL C abundance relative to hydrogen for all the PNe in our sample, Nand O abundances for 11 of them and Ne abundances for nine of them. Inall cases, ORL abundances are found to be systematically higher than thecorresponding values deduced from CELs. In NGC 40, the discrepancybetween the abundances derived from the two types of emission linereaches a factor of 17 for oxygen. For the other 10 PNe, thediscrepancies for oxygen vary from 1.6 to 3.1. In general, collisionallyexcited infrared fine-structure lines, which have excitation energiesless than 103 K and consequently emissivities that areinsensitive to electron temperature and temperature fluctuations, yieldionic abundances comparable to those derived from optical/UV CELs. For agiven nebula, the discrepancies between the ORL and CEL abundances areof similar magnitude for different elements. In other words, relativeabundance ratios such as C/O, N/O and Ne/O deduced from the traditionalmethod based on strong CELs are comparable to those yielded by ORLs, fora wide range of ORL to CEL oxygen abundance ratios, varying from nearunity to over a factor of 20.We have also determined ORL abundances relative to hydrogen for thethird-row element magnesium for 11 nebulae in our sample. In strongcontrast to the cases for second-row elements, Mg abundances derivedfrom the MgII 3d-4f λ4481 ORL are nearly constant for all the PNeanalysed so far and agree within the uncertainties with the solarphotospheric value.In accordance with results from previous studies, the ORL to CELabundance ratio is correlated with the difference between the electrontemperatures derived from the [OIII] forbidden-line ratio, on the onehand, and from the hydrogen recombination Balmer discontinuity, on theother. We find that the discrepancy between the ORL and CEL abundancesis correlated with nebular absolute diameter, surface brightness, theelectron density derived from [SII] CELs, and excitation class. Theresults confirm that the dichotomy of temperatures and heavy elementalabundances determined from the two types of emission line, which hasbeen widely observed in PNe, is a strong function of nebular evolution,as first pointed out by Garnett and Dinerstein.Our analyses show that temperature fluctuations and/or densityinhomogeneities are incapable of explaining the large discrepanciesbetween the heavy elemental abundances and electron temperaturesdetermined from the two types of emission line. Our analyses support thebi-abundance model of Liu et al., who have proposed that PNe containanother previously unseen component of ionized gas which, highlyenriched in heavy elements, has an electron temperature of<~103 K and emits strongly in recombination lines but notin CELs. Our determinations of low average emission temperatures fromthe observed line intensity ratios of HeI and OII ORLs lend furthersupport to this scenario. A deep survey of heavy element lines in planetary nebulae - II. Recombination-line abundances and evidence for cold plasmaIn our Paper I, we presented deep optical observations of the spectra of12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe,carrying out an abundance analysis using the collisionally excitedforbidden lines. Here, we analyse the relative intensities of faintoptical recombination lines (ORLs) from ions of carbon, nitrogen andoxygen in order to derive the abundances of these ions relative tohydrogen. The relative intensities of four high-l CII recombinationlines with respect to the well-known 3d-4f λ4267 line are foundto be in excellent agreement with the predictions of recombinationtheory, removing uncertainties about whether the high C2+abundances derived from the λ4267 line could be due tonon-recombination enhancements of its intensity.We define an abundance discrepancy factor (ADF) as the ratio of theabundance derived for a heavy element ion from its recombination linesto that derived for the same ion from its ultraviolet, optical orinfrared collisionally excited lines (CELs). All of the PNe in oursample are found to have ADFs that exceed unity. Two of the PNe, NGC2022 and LMC N66, have O2+ ADFs of 16 and 11, respectively,while the remaining 13 PNe have a mean O2+ ADF of 2.6, withthe smallest value being 1.8.Garnett and Dinerstein found that for a sample of about 12 PNe themagnitude of the O2+ ADF was inversely correlated with thenebular Balmer line surface brightness. We have investigated this for alarger sample of 20 PNe, finding weak correlations with decreasingsurface brightness for the ADFs of O2+ and C2+.The C2+ ADFs are well correlated with the absolute radii ofthe nebulae, although no correlation is present for the O2+ADFs. We also find both the C2+ and O2+ ADFs to bestrongly correlated with the magnitude of the difference between thenebular [OIII] and Balmer jump electron temperatures (ΔT),corroborating a result of Liu et al. for the O2+ ADF.ΔT is found to be weakly correlated with decreasing nebularsurface brightness and increasing absolute nebular radius.There is no dependence of the magnitude of the ADF upon the excitationenergy of the ultraviolet, optical or infrared CEL transition used,indicating that classical nebular temperature fluctuations - i.e. in achemically homogeneous medium - are not the cause of the observedabundance discrepancies. Instead, we conclude that the main cause of thediscrepancy is enhanced ORL emission from cold ionized gas located inhydrogen-deficient clumps inside the main body of the nebulae, as firstpostulated by Liu et al. for the high-ADF PN, NGC 6153. We havedeveloped a new electron temperature diagnostic, based upon the relativeintensities of the OII 4f-3d λ4089 and 3p-3s λ4649recombination transitions. For six out of eight PNe for which bothtransitions are detected, we derive O2+ ORL electrontemperatures of <=300 K, very much less than the O2+forbidden-line and H+ Balmer jump temperatures derived forthe same nebulae. These results provide direct observational evidencefor the presence of cold plasma regions within the nebulae, consistentwith gas cooled largely by infrared fine-structure transitions; at suchlow temperatures, recombination transition intensities will besignificantly enhanced due to their inverse power-law temperaturedependence, while ultraviolet and optical CELs will be significantlysuppressed. Planetary nebula distances re-examined: an improved statistical scaleThe distances of planetary nebulae (PNe) are still quite uncertain.Although observational estimates are available for a small proportion ofPNe, based on statistical parallax and the like, such distances are verypoorly determined for the majority of galactic PNe. In particular,estimates of so-called statistical' distance appear to differ byfactors of ~2.7.We point out that there is a well-defined correlation between the 5-GHzluminosity of the sources, L5, and their brightnesstemperatures, TB. This represents a different trend to thoseinvestigated in previous statistical analyses, and permits us todetermine independent distances to a further 449 outflows. Thesedistances are shown to be closely comparable to those determined using aTB-R correlation, providing that the latter trend is taken tobe non-linear.This non-linearity in the TB-R plane has not been noted inprevious analyses, and is likely responsible for the broad (andconflicting) ranges of distance that have previously been published.Finally, we point out that there is a close accord between observedtrends within the L5-TB and TB-Rplanes, and the variation predicted through nebular evolutionarymodelling. This is used to suggest that observational biases areprobably modest, and that our revised distance scale is reasonablytrustworthy. Flux Ratio [Nev] 14.3/24.3 as a Test of Collision StrengthsFrom ISO [Nev] 14.3/24.3 μm line flux ratios, we find that 10 out of20 planetary nebulae (PNs) have measured ratios below the low-electrondensity (Ne) theoretical predicted limit. Such astronomicaldata serve to provide important tests of atomic data, collisionstrengths in this case. In principle, well-calibrated measurements ofthe [Nev] 14.3/24.3 flux ratio could improve upon the existing atomicdata. 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. Classification of Spectra from the Infrared Space Observatory PHT-S DatabaseWe have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA). 12C/13C Ratio in Planetary Nebulae from the IUE ArchivesWe 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. 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. Rings in the haloes of planetary nebulaeWe present a search for rings or arcs in the haloes of planetary nebulae(PNe). We discovered such structures in eight PNe, tripling the sampleof PNe with known rings. This shows that, contrary to what was believedto date, the occurrence of mass loss fluctuations with timescales of102-103 yrs at the end of the asymptotic giantbranch phase (AGB) is common. We estimate a lower limit of theoccurrence rate of rings in PN haloes to be ˜35%. Using these newdetections and the cases previously known, we discuss the statisticalproperties of ring systems in PNe haloes. We estimate that the massmodulation producing the rings takes place during the last 10 000 or 20000 yrs of AGB evolution. In PNe, the spacing between rings ranges from<0.01 pc to 0.06 pc, significantly larger than those seen inproto-PNe. This, together with the finding of a possible positivecorrelation of spacing with the post-AGB age of the nebulae, suggeststhat the spacing of the rings increases with time. These properties, aswell as the modest surface brightness amplitudes of rings, areconsistent with the predictions of the dust-driven wind instabilitymodel explored by Meijerink et al. (\cite{Me03}), but do not immediatelyexclude other proposed models.Based on observations obtained at: the 2.5 INT telescope of the IsaacNewton Group and the 2.6 m NOT telescope operated by NOTSA in theSpanish Observatorio del Roque de Los Muchachos of the Instituto deAstrofísica de Canarias; the 3.5 m NTT and the 2.2 MPG/ESO at theEuropean Southern Observatory in Chile; and the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by AURA for NASA under contract NAS5-26555. A deep survey of heavy element lines in planetary nebulae - I. Observations and forbidden-line densities, temperatures and abundancesWe present deep optical spectrophotometry of 12 Galactic planetarynebulae (PNe) and three Magellanic Cloud PNe. Nine of the Galactic PNewere observed by scanning the slit of the spectrograph across thenebula, yielding relative line intensities for the entire nebula thatare suitable for comparison with integrated nebular fluxes measured inother wavelength regions. In this paper we use the fluxes ofcollisionally excited lines (CELs) from the nebulae to derive electrondensities and temperatures, and ionic abundances. We find that thenebular electron densities derived from optical CEL ratios aresystematically higher than those derived from the ratios of the infrared(IR) fine-structure (FS) lines of [OIII]. The latter have lower criticaldensities than the typical nebular electron densities derived fromoptical CELs, indicating the presence of significant density variationswithin the nebulae, with the IR CELs being biased towards lower densityregions.We find that for several nebulae the electron temperatures obtained from[OII] and [NII] optical CELs are significantly affected by recombinationexcitation of one or more of the CELs. When allowance is made forrecombination excitation, much better agreement is obtained with theelectron temperatures obtained from optical [OIII] lines. We alsocompare electron temperatures obtained from the ratio of optical nebularto auroral [OIII] lines with temperatures obtained from the ratio of[OIII] optical lines to [OIII] IR FS lines. We find that when the latterare derived using electron densities based on the [OIII]52 μm/88μm line ratio, they yield values that are significantly higher thanthe optical [OIII] electron temperatures. In contrast to this, [OIII]optical/IR temperatures derived using the higher electron densitiesobtained from optical [ClIII]λ5517/λ5537 ratios show muchcloser agreement with optical [OIII] electron temperatures, implyingthat the observed [OIII] optical/IR ratios are significantly weighted bydensities in excess of the critical densities of both [OIII] FS lines.Consistent with this, ionic abundances derived from [OIII] and [NIII] FSlines using electron densities from optical CELs show much betteragreement with abundances derived for the same ions from optical andultraviolet CELs than do abundances derived from the FS lines using thelower electron densities obtained from the observed [OIII]52 μm/88μm ratios. The behaviour of these electron temperatures, obtainedmaking use of the temperature-insensitive [OIII] IR FS lines, providesno support for significant temperature fluctuations within the nebulaebeing responsible for derived Balmer jump electron temperatures that arelower than temperatures obtained from the much more temperaturesensitive [OIII] optical lines. 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} The post-AGB evolution of AGB mass loss variationsWe present new numerical hydrodynamical modelling of the evolution ofAsymptotic Giant Branch (AGB) mass loss fluctuations during thepost-AGB/Planetary Nebula phase. These models show that afterionization, the observable effects of the mass loss fluctuationsdisappear in a few thousand years, consistent with the fact that onlyfew PNe have been found to be surrounded by `rings''. We derive theobservational characteristics of these rings, and compare them toreported observations of the rings around NGC 6543, finding a good matchof emission properties and line shapes. We predict small variations inthe observable electron temperatures. 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 Abundance Anomalies in the X-Ray Spectra of Planetary Nebulae NGC 7027 and BD +30°3639We revisit Chandra observations of planetary nebulae NGC 7027 and BD+30°3639 in order to address the question of abundance anomalies inthe X-ray-emitting gas. Enhanced abundances relative to solar ofmagnesium (Mg) for NGC 7027 and neon (Ne) for BD +30°3639 arerequired to fit their X-ray spectra, whereas observations at optical andinfrared wavelengths show depleted Mg and Ne in these systems. Weattribute the enhancement of Mg in NGC 7027 in the X-ray, relative tothe optical, to the depletion of Mg onto dust grains within the opticalnebula. For BD +30°3639, we speculate that the highly enhanced Necomes from a white dwarf companion, which accreted a fraction of thewind blown by the asymptotic giant branch progenitor and went through anovalike outburst that enriched the X-ray-emitting gas with Ne. Three-dimensional photoionization modelling of the planetary nebula NGC 3918The three-dimensional Monte Carlo photoionization code MOCASSIN has beenapplied to construct a realistic model of the planetary nebula NGC 3918.Three different geometric models were tried, the first being thebiconical density distribution already used some years ago by Clegg etal. In this model the nebula is approximated by a biconical structure ofenhanced density, embedded in a lower-density spherical region.Spindle-like density distributions were used for the other two models(models A and B). Model A used a mass distribution slightly modifiedfrom one of Mellema's hydrodynamical models that had already beenadopted by Corradi et al. for their observational analysis of NGC 3918.Our spindle-like model B instead used an analytical expression todescribe the shape of the inner shell of this object as consisting of anellipsoid embedded in a sphere.The effects of the interaction of the diffuse fields coming from twoadjacent regions of different densities were investigated. These arefound to be non-negligible, even for the relatively uncomplicated caseof a biconical geometry. We found that the ionization structure oflow-ionization species near the boundaries is particularly affected.It is found that all three models provided acceptable matches to theintegrated nebular optical and ultraviolet spectrum. Large discrepancieswere found between all of the model predictions of infraredfine-structure line fluxes and ISO SWS measurements. This was found tobe largely due to an offset of ~14 arcsec from the centre of the nebulathat affected all of the ISO observations of NGC 3918.For each model, we also produced projected emission-line maps andposition-velocity diagrams from synthetic long-slit spectra, which couldbe compared to recent HST images and ground-based long-slit echellespectra. This comparison showed that spindle-like model B provided thebest match to the observations. Although the integrated emission-linespectrum of NGC 3918 can be reproduced by all three of thethree-dimensional models investigated in this work, the capability ofcreating projected emission-line maps and position-velocity diagramsfrom synthetic long-slit spectra was found to be crucial in allowing usto constrain the structure of this object.
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 Сазвежђа: Кентаур Ректацензија: 11h50m17.73s Deклинација: -57Â°10'56.9" Apparent магнитуда: 8

Каталог и designations:
 Proper имена (Edit) NGC 2000.0 NGC 3918 → Захтевај још каталога од VizieR