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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.
| Detection of HCO+ Emission toward the Planetary Nebula K3-35
We report the detection, for the first time, ofHCO+(J=1-->0) emission, as well as marginal CO(J=1-->0)emission, toward the planetary nebula (PN) K3-35 as a result of amolecular survey carried out toward this source. We also report newobservations of the previously detected CO(J=2-->1) and water maseremission, as well as upper limits for the emission of the SiO,H13CO+, HNC, HCN, HC3OH,HC5N, CS, HC3N, 13CO, CN, andNH3 molecules. From the ratio of CO(J=2-->1) toCO(J=1-->0) emission we have estimated the kinetic temperature of themolecular gas, obtaining a value of ~=20 K. Using this result we haveestimated a molecular mass for the envelope of ~=0.017 Msolarand an HCO+ abundance relative to H2 of6×10-7, similar to the abundances found in other PNe.K3-35 is remarkable because it is one of the two PNe reported to exhibitwater maser emission, which is present in the central region, as well asat a distance of ~=5000 AU from the center. The presence of molecularemission provides some clues that could help in understanding thepersistence of water molecules in the envelope of K3-35. TheHCO+ emission could be arising in dense molecular clumps,which may provide the shielding mechanism that protects water moleculesin this source.
| Further 2MASS mapping of hot dust in planetary nebulae
We have used 2 Micron All Sky Survey (2MASS) mapping results toinvestigate the distribution of hot dust continua in 12 planetarynebulae (PNe). The nature of this emission is unclear, but it ispossible that where the continuum is extended, as is the case for M 1-12and NGC 40, then the grains concerned may be very small indeed. Theabsorption of individual photons by such grains may lead to sharp spikesin temperature, as has previously discussed for several other suchoutflows. Other sources (such as MaC 1-4, He 2-25, B1 2-1 and K 3-15)appear to be relatively compact, and the high temperatures observed areunderstandable in terms of more normal heating processes. It is possiblethat the grains in these cases are experiencing high radiant fluxlevels.Finally, it is noted that whilst the core of M 2-2 appears to show hotgrain emission, this is less the case for its more extended envelope.The situation may, in this case, be similar to that of NGC 2346, inwhich much of the emission is located within an unresolved nucleus.Similarly, it is noted that in addition to hot dust and gas thermalcontinua, the emission in the interior of NGC 40 may be enhanced throughrotational-vibrational transitions of H2, and/or the2p3P0-2s3S transition of HeI.
| A Spectrophotometric Survey of K-Band Emission Lines in Planetary Nebulae
We present observations of 16 planetary nebulae (PNs) in the 2 μm (Kband) spectral region, obtained with a long-slit near-infraredspectrometer at McDonald Observatory. In general, the strongest featuresin our spectra are recombination lines of H I, He I, and (in some cases)He II. Half the sample shows emission from vibrationally excitedH2. Some of the observed PNs (e.g., M 1-13) displayH2 line ratios characteristic of shocked, thermalized gas,while others (e.g., BD +30 3639) have ratios intermediate between pureradiative (UV) and shock excitation, consistent with either acombination of the mechanisms or UV illumination of dense material. Ourspectra of J900 and M 1-13 confirm that published narrowband imagestrace the H2 emission, and we find that the H2emission in SwSt 1 has a larger spatial extent than previously reported.In IC 5117, SwSt 1, and NGC 40 we detect the [Kr III] 2.199 μm lineidentified by Dinerstein in 2001, with strengths indicating that kryptonis enriched relative to the solar abundance, most markedly so in NGC 40.We also detect several lines from the 3G term of [Fe III] inVy 2-2, SwSt 1, and marginally in Cn 3-1. The [Kr III] and [Fe III]lines fall near in wavelength to H2 transitions, which areoften used as diagnostics for UV excitation because they arise fromhigher vibrationally excited levels (v=2, 3). For moderate spectralresolving power, R<=600, these lines may be blended with, or evenmistaken for, the corresponding H2 lines, leading tomisinterpretation of the H2 emission. The strength of boththe Kr and Fe nebular emission lines can be enhanced by specialcircumstances, Kr because of nucleosynthetic self-enrichment in theprogenitor star and Fe due to inefficient initial dust condensation orpartial destruction of the dust after formation, causing a largerfraction of the elemental iron to reside in the gas phase.
| Raman-scattered He II λλ4850, 6545 in the Young and Compact Planetary Nebula IC 5117
We report the discovery of He II λλ4850 and 6545Raman-scattered by atomic hydrogen in the high-resolution spectrum ofthe very compact and young planetary nebula IC 5117 obtained with theESPaDOnS (Echelle Spectropolarimetric Device for the Observation ofStars) installed on the 3.6 m Canada-France-Hawaii Telescope. Byadopting case B recombination atomic data and considering all thefine-structure emission components, we calculate the weighted averagesfor the line centers of the Raman-scattered features, as well as thosefor the emission lines He II λλ4859 and 6560 to determineredward velocity shifts of Δv4850=+29 km s-1and Δv6545=+24 km s-1. We compute the Ramanconversion efficiency by a Monte Carlo technique adopting a simplescattering geometry, in which the hot central star is surrounded by ahollow cylindrical H I region characterized by thickness T that is inturn parameterized by the H I column density. It is proposed that thecentral far-UV emission region is covered significantly by the neutralscattering region characterized by the H I column densityNHI~=4×1021 cm-2. The H I columndensity is higher by an order of magnitude than the value proposed from21 cm radio observations. We briefly discuss the importance of Ramanscattering as a probe of neutral material around a newly formed hotwhite dwarf and the mass-loss process occurring in the late stage ofstellar evolution.
| Infrared photometry and evolution of mass-losing AGB stars. I. Carbon stars revisited
As part of a reanalysis of galactic Asymptotic Giant Branch (AGB) starsat infrared (IR) wavelengths, we discuss a sample (357) of carbon starsfor which mass loss rates, near-IR photometry and distance estimatesexist. For 252 sources we collected mid-IR fluxes from the MSX (6C) andthe ISO-SWS catalogues. Most stars have spectral energy distributions upto 21 μm, and some (1/3) up to 45 μm. This wide wavelengthcoverage allows us to obtain reliable bolometric magnitudes. Theproperties of our sample are discussed with emphasis on ~70 stars withastrometric distances. We show that mid-IR fluxes are crucial toestimate the magnitude of stars with dusty envelopes. We construct HRdiagrams and show that the luminosities agree fairly well with modelpredictions based on the Schwarzschild's criterion, contrary to what iswidely argued in the literature. A problem with the brightness of Cstars does not appear to exist. From the relative number of Mira andSemiregular C-variables, we argue that the switch between these classesis unlikely to be connected to thermal pulses. The relevance of the twopopulations varies with the evolution, with Miras dominating the finalstages. We also analyze mass loss rates, which increase for increasingluminosity, but with a spread that probably results from a dependence ona number of parameters (like e.g. different stellar masses and differentmechanisms powering stellar winds). Instead, mass loss rates are wellmonitored by IR colours, especially if extended to 20 μm and beyond,where AGB envelopes behave like black bodies. From these colours theevolutionary status of various classes of C stars is discussed.
| Hot dust haloes in planetary nebulae
We 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 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.
| Dust Extinction in Compact Planetary Nebulae
The effects of dust extinction on the departure from axisymmetry in themorphology of planetary nebulae (PNs) are investigated through acomparison of the radio free-free emission and hydrogen recombinationline images. The dust extinction maps from five compact PNs are derivedusing high-resolution (~0.1") Hα and radio maps of the HST andVLA. These extinction maps are then analyzed by an ellipsoidal shellionization model including the effects of dust extinction to infer thenebulae's intrinsic structure and orientation in the sky. This methodprovides a quantitative analysis of the morphological structure of PNsand represents a step beyond qualitative classification of morphologicaltypes of PNs.
| Absolute spectrophotometry of northern compact planetary nebulae
We present medium-dispersion spectra and narrowband images of sixnorthern compact planetary nebulae (PNe): BoBn 1,DdDm 1, IC 5117, M1-5, M 1-71, and NGC6833. From broad-slit spectra, total absolute fluxes andequivalent widths were measured for all observable emission lines. Highsignal-to-noise emission line fluxes of Hα, Hβ, [Oiii],[Nii], and HeI may serve as emission line flux standards for northernhemisphere observers. From narrow-slit spectra, we derive systemicradial velocities. For four PNe, available emission line fluxes weremeasured with sufficient signal-to-noise to probe the physicalproperties of their electron densities, temperatures, and chemicalabundances. BoBn 1 and DdDm 1,both type IV PNe, have an Hβ flux over three sigma away fromprevious measurements. We report the first abundance measurements ofM 1-71. NGC 6833 measured radialvelocity and galactic coordinates suggest that it is associated with theouter arm or possibly the galactic halo, and its low abundance([O/H]=1.3× 10-4) may be indicative of low metallicitywithin that region.
| 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.
| Physical conditions in Photo-Dissociation Regions around Planetary Nebulae
We 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
| Planetary nebula distances re-examined: an improved statistical scale
The 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.
| 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.
| 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.
| A Survey for Water Maser Emission toward Planetary Nebulae: New Detection in IRAS 17347-3139
We report on a water maser survey toward a sample of 27 planetarynebulae (PNe) using the Robledo de Chavela and Medicina single-dishantennas, as well as the Very Large Array (VLA). Two detections havebeen obtained: the already known water maser emission in K3-35, and anew cluster of masers in IRAS 17347-3139. This low rate of detections iscompatible with the short lifetime of water molecules in PNe (~100 yr).The water maser cluster at IRAS 17347-3139 are distributed on a ellipseof size ~=0.2"×0.1", spatially associated with compact 1.3 cmcontinuum emission (simultaneously observed with the VLA). From archiveVLA continuum data at 4.9, 8.4, and 14.9 GHz, a spectral indexα=0.76+/-0.03 (Sν~να) is derivedfor this radio source, which is consistent with either a partiallyoptically thick ionized region or an ionized wind. However, the latterscenario can be ruled out by mass-loss considerations, thus indicatingthat this source is probably a young PN. The spatial distribution andthe radial velocities of the water masers are suggestive of a rotatingand expanding maser ring, tracing the innermost regions of a torusformed at the end of the asymptotic giant branch phase. Given that the1.3 cm continuum emission peak is located near one of the tips of themajor axis of the ellipse of masers, we speculate on a possible binarynature of IRAS 17347-3139, where the radio continuum emission couldbelong to one of the components and the water masers would be associatedwith a companion.
| The relation between Zanstra temperature and morphology in planetary nebulae
We have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology.
| Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinates
We have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}
| 13C isotope effects on infrared bands of quenched carbonaceous composite (QCC)
We investigate carbon isotope effects on the infrared bands of alaboratory analogue of carbonaceous dust, the quenched carbonaceouscomposite (QCC), synthesized from a plasma gas of methane with various12C/13C ratios. Peak shifts to longer wavelengthsdue to the substitution of 12C by 13C are clearlyobserved in several absorption bands. The shifts are almost linearlyproportional to the 13C fraction. New features associatedwith 13C are not seen, indicating that the infrared bands inthe QCC are not very localized vibration modes but come from vibrationsassociated with rather large carbon structures. An appreciable peakshift (Delta lambda ~ 0.23-0.26 mu m per 13C fraction) isdetected in the 6.2 mu m band, which is attributed to a carbon-carbonvibration. A peak shift (Delta lambda ~ 0.16-0.18 mu m per13C fraction) in an out-of-plane bending mode of aromatic C-Hat 11.4 mu m is also observed, while only a small shift (Delta lambda< 0.015 mu m per 13C fraction) is detected in the 3.3 mu mband, which arises from a C-H stretching mode. The present experimentsuggests that peak shifts in the unidentified infrared (UIR) bands,particularly in the 6.2 mu m band, should be detectable in celestialobjects with low 12C/13C ratios (<10). Theisotopic shifts seen in the QCC are discussed in relation to thevariations in the UIR band peaks observed in post-asymptotic giantbranch stars and planetary nebulae. The observed peak shift pattern ofthe UIR bands is qualitatively in agreement with the isotopic shifts inthe QCC except for the 7.7 mu m band complex although the observedshifts in the UIR bands are larger than those inferred from derivedisotope ratios for individual objects. The poor quantitative agreementmay be attributed partly to large uncertainties in the derived12C/13C, to possible spatial variations of theisotope abundance within the object, and to combinations of othereffects, such as hetero-atom substitutions. The present investigationsuggests that part of the observed variations in the UIR band peaks maycome from the isotopic effects.
| The Local Group Census: Planetary nebulae in IC 10, Leo A and Sextans A
In the framework of our narrow-band survey of the Local Group galaxies,we present the results of the search for planetary nebulae (PNe) in thedwarf irregular galaxies IC 10, LeoA and Sextans A. Using the standardon-band/off-band technique, sixteen new candidate PNe have beendiscovered in the closest starburst galaxy, IC 10.The optical size of this galaxy is estimated to be much larger thanpreviously thought, considering the location of the new PNe in an areaof 3.6 kpc x 2.7 kpc. We also confirm the results of previous studiesfor the other two dwarf irregular galaxies, with the detection of onecandidate PN in Leo A and another one inSextans A. We review the number of planetary nebulaediscovered in the Local Group to date and their behaviour withmetallicity. We suggest a possible fall in the observed number of PNewhen [Fe/H] << -1.0, which might indicate that below this pointthe formation rate of PNe is much lower than for stellar populations ofnear Solar abundances. We also find non-negligible metallicity effectson the [O III] luminosity of the brightest PN of a galaxy.Based on observations obtained at the 2.5 m INT telescope operated onthe island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de Los Muchachos of the Instituto de Astrofisicade Canarias.
| The relation between elemental abundances and morphology in planetary nebulae
An investigation of the variation of elemental abundances with planetarynebula morphology is of considerable interest, since it has a bearingupon how such sources are formed, and from which progenitors they areejected. Recent advances in morphological classification now enable usto assess such trends for a statistically significant number of sources.We find, as a result, that the distribution N[log(X/H)] of sources withrespect to elemental abundance (X/H) varies between the differingmorphologies. Circular sources tend to peak towards low abundancevalues, whilst bipolar nebulae (BPNe) peak towards somewhat highervalues. This applies for most elemental species, although it is perhapsleast apparent for oxygen. In contrast, elliptical sources appear todisplay much broader functions N[log(X/H)], which trespass upon thedomains of both circular and elliptical planetary nebulae (PNe).We take these trends to imply that circular sources derive fromlower-mass progenitors, bipolar sources from higher-mass stars, and thatelliptical nebulae derive from all masses of progenitor, high and low.Whilst such trends are also evident in values of mean abundance, they are much less clear. Only in the cases of He/H, N/H,Ne/H and perhaps Ar/H is there evidence for significant abundancedifferences.Certain BPNe appear to possess low abundance ratios He/H and Ar/H, andthis confirms that a few such outflows may arise from lower-massprogenitors. Similarly, we note that ratios are quite modestin elliptical planetary nebulae, and not much different from those forcircular and bipolar PNe; a result that conflicts with the expectationsof at least one model of shell formation.
| Emission Lines of [Cl II] in the Optical Spectra of Gaseous Nebulae
Recent R-matrix calculations of electron impact excitation rates amongthe 3s23p4 levels of Cl II are used to derive thenebular emission-line intensity ratios R1=I(6161.8Å)/I(8578.7 Å) and R2=I(6161.8 Å)/I(9123.6Å) as a function of electron temperature (Te) anddensity (Ne). The ratios are found to be very sensitive tochanges in Te but not Ne for densities lower than105 cm-3. Hence, they should, in principle,provide excellent optical Te diagnostics for planetarynebulae. The observed values of R1 and R2 for theplanetary nebulae NGC 6741 and IC 5117, measured from spectra obtainedwith the Hamilton echelle spectrograph on the 3 m Shane Telescope, implytemperatures in excellent agreement with those derived from otherdiagnostic lines formed in the same region of the nebula as [Cl II].This provides some observational support for the accuracy of the [Cl II]line ratio calculations and hence the atomic data on which they arebased. The [Cl II] 8578.7 and 9123.6 Å lines are identified forthe first time (to our knowledge) in a high-resolution spectrum of thesymbiotic star RR Telescopii, obtained with the University CollegeLondon Echelle Spectrograph on the 3.9 m Anglo-Australian Telescope.However, the 6161.8 Å feature is unfortunately too weak to beidentified in the RR Telescopii observations, consistent with itspredicted line strength.
| The chemistry of compact planetary nebulae
We report high-sensitivity millimetre observations of several molecularspecies (13CO, HCN, HNC, CN, HCO+ andN2H+) in a sample of compact planetary nebulae.Some species such as HCO+ and CN are particularly abundantcompared to envelopes around AGB stars or even interstellar clouds. Wehave estimated the following average values for the column densitiesratios: CN/HCN ~ 2.6, HCO+/HCN ~ 0.5, and HNC/HCN ~ 0.4.Thus, the chemical composition of the molecular envelopes in thesecompact PNe appears somewhat intermediate between the composition ofproto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe (such asthe Ring, M4-9, or the Helix). From observations of the CO isotopomers,we have estimated that the 12C/13C ratio is in therange 10 la 12C/13C <~ 40. These values arebelow those expected from standard asymptotic giant branch models andsuggest non-standard mixing processes. The observed molecular abundancesare compared to very recent modelling work, and we conclude that theobservations are well explained, in general terms, by time-dependentgas-phase chemical models in which the ionization rate is enhanced byseveral orders of magnitude with respect to the average interstellarvalue. Thus, our observations confirm that the chemistry in the neutralshells of PNe is essentially governed by the high energy radiation fromthe hot central stars. The complexity of the chemical processes isincreased by numerous factors linked to the properties of the centralstar and the geometry and degree of clumpiness of the envelope. Severalaspects of the PN chemistry that remains to be understood are discussedwithin the frame of the available chemical models. Based on observationscarried out with the IRAM 30m telescope. IRAM is supported by INSU/CNRS(France), MPG (Germany) and IGN (Spain).
| HST Observations of Young Planetary Nebulae
Not Available
| Mid-infrared spectroscopy of protoplanetary and planetary nebulae
We present medium-resolution (R~ 600), mid-infrared (7.5-14 μm)spectra of 15 young planetary nebulae. Linestrengths for observedforbidden transitions are presented, and numerous broad emissionfeatures from silicates, polyaromatic hydrocarbons (PAHs) and siliconcarbide (SiC) are observed.
| The carrier of the ``30'' mu m emission feature in evolved stars. A simple model using magnesium sulfide
We present 2-45 mu m spectra of a large sample of carbon-rich evolvedstars in order to study the ``30'' mu m feature. We find the ``30'' mu mfeature in a wide range of sources: low mass loss carbon stars, extremecarbon-stars, post-AGB objects and planetary nebulae. We extract theprofiles from the sources by using a simple systematic approach to modelthe continuum. We find large variations in the wavelength and width ofthe extracted profiles of the ``30'' mu m feature. We modelled the wholerange of profiles in a simple way by using magnesium sulfide (MgS) dustgrains with a MgS grain temperature different from the continuumtemperature. The systematic change in peak positions can be explained bycooling of MgS grains as the star evolves off the AGB. In severalsources we find that a residual emission excess at ~ 26 mu m can also befitted using MgS grains but with a different grains shape distribution.The profiles of the ``30'' mu m feature in planetary nebulae arenarrower than our simple MgS model predicts. We discuss the possiblereasons for this difference. We find a sample of warm carbon-stars withvery cold MgS grains. We discuss possible causes for this phenomenon. Wefind no evidence for rapid destruction of MgS during the planetarynebula phase and conclude that the MgS may survive to be incorporated inthe ISM. Based on observations obtained with ISO, an ESA project withinstruments funded by ESA Member states (especially the PI countries:France, Germany, The Netherlands and the United Kingdom) with theparticipation of ISAS and NASA. Appendix A (Figs. A.1 and A.2) is onlyavailable in electronic form at http://www.edpsciences.org
| High resolution near-infrared spectro-imaging of NGC 7027
We present near-infrared spectro-imaging of the young planetary nebulaNGC 7027 between 2.10 and 2.20 mu m with high spatial (0.5'')and high spectral (8.7 km s-1) resolution. The observations,made using BEAR at the CFHT, reveal the detailed morphology andkinematics of the ionized nebula (in the Brgamma line and 16 otheratomic lines) and the surrounding molecular envelope (in the 1-0 S(1)line of H2). The observations show that the ionized gas formsan elongated ( ~ 6 arcsec x 12 arcsec , PA = 32degr ), limb-brightenedshell with an expansion velocity of 19.5 km s-1 along theline of sight. The shell is composed of numerous small condensations andhas nearly parallel sides with flattened ends that are not well matchedby a uniform ellipsoidal model. Low level Brgamma emission is detectedat high red- and blue-shifted velocities (+/- 55 km s-1)along a bipolar axis at PA = 60degr that deviates significantly fromthat of the main nebula. The H2 emission is distributed atthe periphery of the ionized gas, in a limb-brightened, bi-conical shell(~ 10arcsec x 13arcsec , PA = 28degr ) with enhanced emission at theequator and complex structure at the ends of the major axis. TheH2 emission traces the inner edge of the extended molecularenvelope seen in CO, and its distribution and intensity are well matchedby model predictions of a high-density (nH ~106-105 cm-3) photo-dissociationregion. The kinematic structure of the H2 emission reveals aremarkable series of lobes and openings in the molecular shell. Thesefeatures are point symmetric about the center, and the most prominentpair aligns with the high velocity, bipolar emission seen in Brgamma.These observations demonstrate recent activity by collimated outflows inNGC 7027, with a multiple, bipolar geometry. The interaction of theoutflows with the surrounding envelope has significantly affected themorphology of the developing nebula and its environment, and theirpresence in this well-studied archetype underscores the generalimportance of outflows in the early shaping history of planetarynebulae. Based on observations collected at the Canada-France-HawaiiTelescope, operated by the National Research Council of Canada, theCentre National de la Recherche Scientifique de France, and theUniversity of Hawaii.
| Study of electron density in planetary nebulae. A comparison of different density indicators
We present a comparison of electron density estimates for planetarynebulae based on different emission-line ratios. We have considered thedensity indicators [O Ii]lambda 3729/lambda 3726, [S Ii]lambda6716/lambda 6731, [Cl Iii]lambda 5517/lambda 5537, [Ar Iv]lambda4711/lambda 4740, C Iii]lambda 1906/lambda 1909 and [N I]lambda5202/lambda 5199. The observational data were extracted from theliterature. We have found systematic deviations from the densityhomogeneous models, in the sense that: Ne(ion {N}i) <~Ne(ion {O}{ii}) < Ne(ion {S}{ii}, ion {C}{iii},ion {Cl}{iii} or ion {Ar}{iv}) and Ne(ion {S}{ii}) ~Ne(ion {C}{iii}) ~ Ne(ion {Cl}{iii}) ~Ne(ion {Ar}{iv}). We argue that the lower [O Ii] densityestimates are likely due to errors in the atomic parameters used.
| Spectroscopic Observation of the Planetary Nebula IC 4846
A high-dispersion optical (3700-10100 Å) spectrum was secured withthe Hamilton echelle spectrograph at the coudé focus of the 120inch (3 m) Shane telescope at Lick Observatory, with the goal ofderiving chemical abundances for the compact planetary nebula IC 4846.We also remeasured the UV spectra from the IUE archive. Diagnosticsindicate that the entire planetary nebula may be represented by anelectron density Nɛ~=9000 cm-3 andelectron temperature Tɛ~=10,500 K. However,diagnostics seem to imply that the [O II] and [S II] zone electrontemperatures are higher than the [O III] radiating strata. The electrondensity of the [O II] and [S II] zone might be higher as well (for whichNɛ~=20,000 cm-3). The photoionizationmodel, which represents most of the observed line intensities, seems toconfirm this physical condition, although the actual situation may bemuch more complicated. On the basis of the semiempirical ionizationcorrection method and the photoionization model calculation, we derivedthe chemical abundances in this planetary nebula. Our optical data werecompared with the image-tube scanner data from Aller & Czyzak.Apparently, there is a long-term spectral line variation in IC 4846.Chemical abundances derived in the present study agree, within 30%, withthose previously estimated in Aller & Czyzak, except for carbon; theC, N, O, and Ne ratios are lower than both the average planetary nebulaand the solar abundance. IC 4846 is a metal-deficient planetary nebulathat may have evolved from a Population II progenitor.
| Optical Spectrum of the Compact Planetary Nebula IC 5117
High-resolution spectroscopic data of the very compact planetary nebulaIC 5117 were obtained in optical wavelengths of 3700-10050 Å withthe Hamilton Echelle Spectrograph at the Lick Observatory and have beenanalyzed along with the International Ultraviolet Explorer UV archivedata. Although a diagnostic diagram shows significant density andtemperature fluctuations, our analysis indicates that the nebular gasmay be represented by a homogeneous shell of extremely high-density gas,Nɛ~90,000 cm-3. The average electrontemperatures, e.g., indicated by the [O III] diagnostics, are around12,000 K. We construct a photoionization model to represent most of theobserved line intensities and the physical condition of this compactnebulosity. Based on the semiempirical ionization correction approachand model indications, we derived the elemental abundances; He, C, N, O,Ne, and Ar appear to be normal or marginally depleted compared to theaverage planetary nebula, while the remaining elements S, Cl, and Kappear to be enhanced. IC 5117 is perhaps a very young compact planetarynebula, slightly more evolved than the other well-known compactplanetary nebula IC 4997. The central stellar temperature is likely tobe around 120,000 K, evolved from a C-rich asymptotic giant branchprogenitor.
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