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The Abundances of Light Neutron-Capture Elements in Planetary Nebulae. I. Photoionization Modeling and Ionization Corrections
We have conducted a large-scale survey of 120 planetary nebulae (PNe) tosearch for the near-infrared emission lines [Kr III] 2.199 μm and [SeIV] 2.287 μm. The neutron (n)-capture elements Se and Kr may beenriched in a PN if its progenitor star experienced s-processnucleosynthesis and third dredge-up. In order to determine Se and Krabundances, we have added these elements to the atomic databases of thephotoionization codes Cloudy and XSTAR, which we use to deriveionization correction factors (ICFs) to account for the abundances ofunobserved Se and Kr ions. However, much of the atomic data governingthe ionization balance of these two elements are unknown, and have beenapproximated from general principles. We find that uncertainties in theatomic data can lead to errors approaching 0.3 dex in the derived Seabundances and up to 0.2-0.25 dex for Kr. To reduce the uncertainties inthe Kr ionization balance stemming from the approximate atomic data, wehave modeled 10 bright PNe in our sample, selected because they exhibitemission lines from multiple Kr ions in their optical and near-infraredspectra. We have empirically adjusted the uncertain Kr atomic data untilthe observed line intensities of the various Kr ions are adequatelyreproduced by our models. Using the adjusted Kr atomic data, we havecomputed a grid of models over a wide range of physical parameters(central star temperature, nebular density, and ionization parameter)and derived formulae that can be used to compute Se and Kr ICFs. In thesecond paper of this series, we will apply these ICFs to our full sampleof 120 PNe, which comprises the first large-scale survey of n-captureelements in PNe.This paper includes data taken at the McDonald Observatory of theUniversity of Texas at Austin.

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.

Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79northern Galactic planetary nebulae (PNe) whose morphological classeshave been uniformly determined. Ionic abundances and plasma diagnosticswere derived from selected optical line strengths in the literature, andelemental abundances were estimated with the ionization correctionfactor developed by Kingsbourgh & Barlow in 1994. We compare theelemental abundances to the final yields obtained from stellar evolutionmodels of low- and intermediate-mass stars, and we confirm that mostbipolar PNe have high nitrogen and helium abundance and are the likelyprogeny of stars with main-sequence mass greater than 3Msolar. We derive =0.27 and discuss the implication of such ahigh ratio in connection with the solar neon abundance. We determine theGalactic gradients of oxygen and neon and foundΔlog(O/H)/ΔR=-0.01 dex kpc-1 andΔlog(Ne/H)/ΔR=-0.01 dex kpc-1. These flat PNgradients are irreconcilable with Galactic metallicity gradientsflattening with time.

Planetary nebulae abundances and stellar evolution
A summary is given of planetary nebulae abundances from ISOmeasurements. It is shown that these nebulae show abundance gradients(with galactocentric distance), which in the case of neon, argon, sulfurand oxygen (with four exceptions) are the same as HII regions and earlytype star abundance gradients. The abundance of these elements predictedfrom these gradients at the distance of the Sun from the center areexactly the solar abundance. Sulfur is the exception to this; the reasonfor this is discussed. The higher solar neon abundance is confirmed;this is discussed in terms of the results of helioseismology. Evidenceis presented for oxygen destruction via ON cycling having occurred inthe progenitors of four planetary nebulae with bilobal structure. Theseprogenitor stars had a high mass, probably greater than 5 Mȯ. Thisis deduced from the high values of He/H and N/H found in these nebulae.Formation of nitrogen, helium and carbon are discussed. The high massprogenitors which showed oxygen destruction are shown to have probablydestroyed carbon as well. This is probably the result of hot bottomburning.

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.

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.

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.

C [III] imagery of planetary nebulae and H II regions.
Not Available

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 evolution of planetary nebulae. III. Internal kinematics and expansion parallaxes
A detailed theoretical study of the basic internal kinematics ofplanetary nebulae is presented, based on 1D radiation-hydrodynamicssimulations of circumstellar envelopes around central stars of 0.595 and0.696 Mȯ. By means of observable quantities like radialsurface-brightness distributions and emission-line profiles computedfrom the models, a comparison with real objects was performed andrevealed a reasonable agreement. This allowed to draw importantconclusions by investigating the kinematics of these models in detail.Firstly, it is shown that the determination of kinematical ages,normally considered to be simple if size and expansion rate of an objectare given, can seriously be flawed. Secondly, the expansion law of aplanetary nebula is different from what is assumed for derivingspatio-kinematical models. Thirdly and most importantly, ourhydrodynamical models help to correctly use existing angular expansionmeasurements for distance determinations. The mere combination of theangular expansion rates with the spectroscopic expansion velocitiesleads always to a serious underestimate of the distance, the degree ofwhich depends on the evolutionary state of the object. The necessarycorrection factor varies between 3 and 1.3. Individual correctionfactors can be estimated with an accuracy of about 10% by matching ourhydrodynamical models to real objects. As a result, revised distancesfor a few objects with reliable angular expansion rates are presented.But even these corrected distances are not always satisfying: they stillappear to be inconsistent with other distance determinations and, evenmore disturbing, with the accepted theory of post-asymptotic giantbranch evolution. As a byproduct of the angular expansion measurements,the transition times from the vicinity of the asymptotic giant branch tothe planetary-nebula regime could be estimated. They appear to beshorter than assumed in the present evolutionary calculations.

Fluorine Abundances in Planetary Nebulae
We 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.

Integral Field Spectroscopy of Faint Halos of Planetary Nebulae
We present the first integral field spectroscopy observations of the twoplanetary nebulae NGC 3242 and NGC 4361 with the VIMOS instrumentattached to VLT-UT3. By co-adding a large number of spaxels, we reach anemission-line detection limit of 5×10-18 ergscm-2 s-1 arcsec-2. In the case of NGC3242, we succeed in determining some properties of the halo. The radialsurface brightness profile in [O III] implies increasing mass lossbefore the formation of the planetary nebula. Traces of the mysterious``rings'' are clearly visible. We find for the first time an apparenttemperature gradient across a halo: from about 16,000 K close to theshell/halo transition to 20,000 K at the halo's outer edge. No lineemission is seen in the suspected halo region of NGC 4361 down to thesensitivity limit.

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.

G313.3+00.3: A New Planetary Nebula Discovered by the Australia Telescope Compact Array and the Spitzer Space Telescope
We present a new planetary nebula, first identified in images from theAustralia Telescope Compact Array, although not recognized at that time.Recent observations with the Spitzer Space Telescope during the GLIMPSELegacy program have rediscovered the object. The high-resolution radioand infrared images enable the identification of the central star or itswind, the recognition of the radio emission as thermal, and the probablepresence of polycyclic aromatic hydrocarbons in and around the source.These lead to the conclusion that G313.3+00.3 is a planetary nebula.This object is of particular interest because it was discovered solelythrough radio and mid-infrared imaging, without any optical (ornear-infrared) confirmation, and acts as a proof of concept for thediscovery of many more highly extinguished planetary nebulae.G313.3+00.3 is well resolved by both the instruments with which it wasidentified and suffers extreme reddening due to its location in theScutum-Crux spiral arm.

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.

The evolution of planetary nebulae. II. Circumstellar environment and expansion properties
We investigate and discuss the expansion properties of planetary nebulaeby means of 1D radiation-hydrodynamics models computed for differentinitial envelope configurations and central star evolutionary tracks. Inparticular, we study how the expansion depends on the initial densitygradient of the circumstellar envelope and show that it is possible toderive information on the very last mass-loss episodes during the star'sfinal evolution along and off the asymptotic giant branch. To facilitatethe comparison of the models with real objects, we have also computedobservable quantities like surface brightness and emission-lineprofiles. With the help of newly acquired high-resolution emission-lineprofiles for a sample of planetary nebulae we show that models withinitial envelopes based on the assumption of a stationary wind outflowfail to explain the observed expansion speeds of virtually all of theobserved planetary nebulae. Instead it must be assumed that during thevery last phase of evolution along the final asymptotic giant branchevolution the mass-loss rate increases in strength, resulting in a muchsteeper slope of the circumstellar radial density distribution. Underthese conditions, the expansion properties of the nebular gas differconsiderably from the self-similar solutions found for isothermalconditions. Furthermore, the mass loss must remain at a rather highlevel until the stellar remnant begins to evolve quickly towards thecentral star regime. Current theoretical computations of dust-drivenmass-loss which are restricted to rather low temperatures cannot beapplied during the star's departure from the asymptotic giant branch.Based on observations obtained at the 3.5 μm NTT and the 1.2 μmCAT telescope of the European Southern Observatory, La Silla, and at the2.6 μm NOT telescope operated on the island of La Palma by NOTSA, inthe Spanish Observatorio del Roque de Los Muchachos of the InstitutodeAstrofísica de Canarias.Dedicated to Prof. V. Weidemann on the occasion of his 80th birthday,October 3, 2004.

Unresolved Hα Enhancements at High Galactic Latitude in the WHAM Sky Survey Maps
We have identified 85 regions of enhanced Hα emission at|b|>10deg subtending approximately 1° or less on theWisconsin Hα Mapper (WHAM) sky survey. These high-latitude ``WHAMpoint sources'' have Hα fluxes of 10-11-10-9ergs cm-2 s-1, radial velocities within about 70km s-1 of the LSR, and line widths that range from less than20 to about 80 km s-1 (FWHM). Twenty-nine of theseenhancements are not identified with either cataloged nebulae or hotstars and appear to have kinematic properties that differ from thoseobserved for planetary nebulae. Another 14 enhancements are near hotevolved low-mass stars that had no previously reported detections ofassociated nebulosity. The remainder of the enhancements are catalogedplanetary nebulae and small, high-latitude H II regions surroundingmassive O and early B stars.

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.

2D Ionization Structure of Haloes of Wind Driven Nebulae .
The study of the nebular environments of stars provides clues to theirrecent evolution: the history of stellar winds and ejecta is written inthe extended nebulae around massive (Wolf-Rayet Ring Nebulae (WRNe)) andintermediate mass stars (Planetary Nebulae (PNe)). Fast winds canproduce shock excitation of the interstelar medium in the haloes of PNeand WR nebulae producing an extra source of heating to their thermalbalance. We have explored the impact on the interstellar medium of thiseffect by searching for shock-heated gas in the 2D ionization structureof haloes, derived from deep multi-filter imaging of a sample ofwind-driven nebulae. In this work, we present results for candidateregions where fotoionization does not appear to be the main ionizationmechanism.

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]).

Chemical abundances of planetary nebulae from optical recombination lines - II. Abundances derived from collisionally excited lines and optical recombination lines
In 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.

Chemical abundances of planetary nebulae from optical recombination lines - I. Observations and plasma diagnostics
We have obtained deep optical spectra of medium resolution for a sampleof 12 Galactic planetary nebulae (PNe). Optical recombination lines(ORLs) from carbon, nitrogen and oxygen have been detected in 11 of themand neon ORLs in nine of them. All spectra were obtained by scanning along slit across the nebular surface, yielding relative line intensitiesfor the entire nebula that are suitable for comparison with integratedline fluxes measured in other wavelength regions using space-bornefacilities, such as the Infrared Space Observatory (ISO) and theInternational Ultraviolet Explorer (IUE). For 11 PNe, ISO infraredspectra between 2.4 and 197 μm are available, most of them taken byourselves, plus a Kuiper Airborne Observatory (KAO) infrared spectrum ofNGC 6210. IUE ultraviolet (UV) spectra are available for all nebulaeexcept one in our sample. The UV, optical and infrared spectra have beencombined to study nebular thermal and density structures and todetermine elemental abundances.We have determined UV to optical extinction curves towards these PNe byexamining observed fluxes of HI and HeII recombination lines, radiofree-free continuum flux density, and UV to optical nebular continua.For 11 PNe in our sample, the derived optical reddening curves are foundto be consistent with the standard Galactic extinction law for atotal-to-selective extinction ratio, R≡A(V)/EB-V= 3.1.However, the optical extinction curve towards Hu 1-2 yields R= 2.0. TheUV extinction towards Hu 1-2 and NGC 6572 is also found to be muchsteeper than the standard Galactic reddening law. In contrast, the UVextinction curve along the sight lines towards NGC 6210 is found to bemuch shallower, although in the latter case the uncertainties involvedare quite large.Electron temperatures and densities have been derived using a variety ofdiagnostic ratios of collisionally excited lines (CELs) in the UV,optical and infrared. The results show clear stratifications, both intemperature and density. Lines emitted by ions formed in regions ofhigher ionization degree yield higher temperatures than lines arisingfrom regions of lower ionization degree, while densities deduced fromratios of infrared diagnostic CELs of low critical densities, such asthe [OIII] 88-μm/52-μm ratio, are systematically lower than thosederived from UV and optical diagnostic lines, which in general have muchhigher critical densities than the infrared fine-structure lines.Electron temperatures have also been derived from the ratio of thenebular continuum Balmer discontinuity to H 11 for 11 PNe. For four ofthese, the Balmer jump temperatures are more than 1000 K lower thanvalues derived from the [OIII] optical collisionally excited diagnosticline ratio. With a difference of 3580 K, NGC 40 has the lowest Balmerjump temperature relative to the [OIII] optical forbidden-linetemperature. High-order Balmer line decrements have been used todetermine electron densities. The results are consistent with valuesderived from forbidden-line density-diagnostics.

Physical Structure of Planetary Nebulae. III. The Large and Evolved NGC 1360
NGC 1360 is a large planetary nebula (PN) without an obvious shellmorphology. We have analyzed Hα images and high-dispersion echellespectra of NGC 1360 in order to construct spatio-kinematic models and todetermine its density distribution. The best-fit model indicates thatNGC 1360 is a prolate ellipsoidal shell whose major axis is twice aslong as its minor axis and is tilted by 60° with respect to the lineof sight. The large kinematic age of the shell, ~10,000 yr, and the lowdensity of the nebula, <=130 H atom cm-3, imply that NGC1360 is an evolved PN and has begun to merge with the interstellarmedium. The observed morphology and surface brightness profiles of NGC1360 can be described well as a thick shell with a Gaussian radialdensity profile without a sharp inner edge, indicating a lack of ongoingcompression by a fast stellar wind. The fast, low-ionization emissionregions observed in NGC 1360 near the end of its major axis expandfaster than the shell and are younger than the nebular shell.

Physical Structure of Planetary Nebulae. II. NGC 7662
We present a study of the structure and kinematics of the triple-shellplanetary nebula NGC 7662 based on long-slit echelle spectroscopicobservations and Hubble Space Telescope archival narrowband images. Thestructure of the main nebula consists of a central cavity surrounded bytwo concentric shells. The bright inner shell has a nonnegligiblethickness and the highest density in the nebula. The outer shell isfilled with nebular material whose density profile shows a~r-1 drop-off in the inner regions of the shell and flattensin the outermost regions. Both the inner and the outer shells can bedescribed as prolate ellipsoids: the inner shell is more elongated,while the outermost layer of the outer shell expands faster. Thephysical structure of NGC 7662 is qualitatively consistent with thepredictions of hydrodynamical simulations based on the interactingstellar wind models for planetary nebulae with a 0.605 Msolarcentral star. Models with a higher asymptotic giant branch wind velocityare needed for detailed comparisons.

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.

STIS observations of FLIERs in NGC 7662
We observed the planetary nebula NGC 7662 with STIS on board the HST toinvestigate the physical conditions in the microstructures within thenebula at high spatial resolution and to closely examine the physicalconditions in FLIERs. We aim to reveal spectroscopic differences insmall scale structures moving at about the same velocity as thesurrounding nebular gases, called SLOWERs. Shocks are expected to play arole in FLIERs but not in SLOWERs. The physical conditions in the twotypes of microstructures have been determined better than ever. In bothtypes of substructures, the electron density shows a moderate increaseinward, while the electron temperature becomes a little lower. Theionization increases from the central parts of the FLIERs to theiredges. No particular increase of the line emission typical of shockexcitation, such as the [SII] 6725 Å doublet, is seen at theinterface between the ambient gas and the microcondensations. Thus, weobserve no evidence for shock effects in the FLIERs. We estimate thatthe thickness of the post-shock layer is too small to be detected, inspite of the brightness of the nebula. For the same reason thedeterminations of abundances within the FLIERs done assuming thecontribution of photoionization followed by ion-electron collisionexcitation to be dominant in producing the observed emission lines,might be considered to be acceptable. The geometrical complexity ofmicrostructures composed of sub and subsub-structures coupled with theneed to take into account charge-exchange reaction effects, prevent usfrom drawing a firm conclusion on the nitrogen overabundance suggestedin the literature for the FLIERs relative to the surrouding ambient gas.We however argue that our HST observations do not support thisenhancement.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprogram GO-8128.

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.

Flux Ratio [Nev] 14.3/24.3 as a Test of Collision Strengths
From 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.

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Observation and Astrometry data

Right ascension:23h25m53.60s
Apparent magnitude:9

Catalogs and designations:
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NGC 2000.0NGC 7662

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