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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.
| Chemical abundances for Hf 2-2, a planetary nebula with the strongest-known heavy-element recombination lines
We present high-quality optical spectroscopic observations of theplanetary nebula (PN) Hf2-2. The spectrum exhibits many prominentoptical recombination lines (ORLs) from heavy-element ions. Analysis ofthe HI and HeI recombination spectrum yields an electron temperature of~900 K, a factor of 10 lower than given by the collisionally excited[OIII] forbidden lines. The ionic abundances of heavy elements relativeto hydrogen derived from ORLs are about a factor of 70 higher than thosededuced from collisionally excited lines (CELs) from the same ions, thelargest abundance discrepancy factor (adf) ever measured for a PN. Bycomparing the observed OIIλ4089/λ4649 ORL ratio totheoretical value as a function of electron temperature, we show thatthe OII ORLs arise from ionized regions with an electron temperature ofonly ~630 K. The current observations thus provide the strongestevidence that the nebula contains another previously unknown componentof cold, high-metallicity gas, which is too cool to excite anysignificant optical or ultraviolet CELs and is thus invisible via suchlines. The existence of such a plasma component in PNe provides anatural solution to the long-standing dichotomy between nebular plasmadiagnostics and abundance determinations using CELs on the one hand andORLs on the other.
| Chemical abundances in seven galactic planetary nebulae
An observational study of chemical abundances in the galactic planetarynebulae NGC 1535, NGC 2438,NGC 2440, NGC 3132, NGC3242, NGC 6302, and NGC7009 based on long-slit spectra of high signal-to-noise ratioin the 3100 to 6900 Å range is presented. We determined the N, O,Ne, S, and Cl abundances from collisionally excited lines and the He andO++ abundances from recombination lines. TheO++/H+ estimates derived from recombination linesare about a factor of four and two higher than those derived fromforbidden lines for NGC 7009 and NGC3242, respectively. Spatial profiles ofO++/H+ abundance from O II permitted lines andfrom [O III] forbidden lines were obtained for the planetary nebulaNGC 7009. The differences betweenO++/H+ derived from recombination and fromforbidden lines present smooth variations along the nebular surface ofNGC 7009, with the differences decreasing from thecenter to the edges of the nebula. If these abundance differences areexplained by the presence of electron temperature fluctuations,quantified by the parameter t2, a value of aboutt2=0.09 is required for NGC 3242 andNGC 7009.
| Electron temperature fluctuations in planetary nebulae
An observational study of the spatial variation of the electrontemperature and density in 10 galactic planetary nebulae is presented.The data consist of long-slit spectra of high signal-to-noise ratio inthe 3100 to 6900 Å range. Electron temperatures were determinedfrom the [O III](λ 4959 + λ 5007)/λ 4363 and [NII](λ 6548 + λ 6583)/λ 5755 ratios and from theBalmer discontinuity. Electron densities were estimated from the [SII]λ 6716/λ 6731, [Cl III]λ 5517/λ 5537, and[Ar IV]λ 4711/λ 4740 ratios. Electron temperaturevariations of low amplitude were found across the nebular surface in theplanetary nebulae studied. The temperature distribution across eachnebula presents a variance relative to the mean corresponding to 0.0003≤ t2s(Bal) ≤ 0.0078, 0.0003 ≤t2s(N II) ≤ 0.0097, and 0.0011 ≤t2s(O III) ≤ 0.0050. A systematic spatialvariation of electron density has been detected in most of objects(NGC 1535, NGC 2438, NGC2440, NGC 3132, NGC3242, NGC 6302, NGC6563, and NGC 7009). The remaining objects(NGC 6781 and NGC 6853) have notshown any significant electron density dependence on position.NGC 2438, NGC 6563, NGC6781, and NGC 6853 are in general the mostdiffuse and probably evolved objects studied here, with low meandensities in the range Ne(S II) ≈ 95-158~cm-3.An anti-correlation between temperature and density was found forNGC 2438 and NGC 3132, with theelectron temperature increasing with the decrease of electron densityand a correlation between temperature and density was found forNGC 2440, NGC 3242, NGC6302, and NGC 7009, with the electrontemperature increasing with the increase of electron density. Theserelationships seem to be associated with the structure of the nebula.The nebulae in which the correlation between temperature and density ispresent are ring shaped. The anti-correlation between temperature anddensity is found in bipolar planetary nebulae that are denser in thecentre of the nebula.
| Helium Recombination Lines as a Probe of Abundance and Temperature Problems
The paper presents a simplified formula to determine an electrontemperature, Te(He I), for planetary nebulae (PNe) using theHe I λ7281/λ6678 line flux ratio. In our previous studiesof Te(He I) (Zhang et al. 2005), we used the He I lineemission coefficients given by Benjamin et al. (1999). Here we examinethe results of using more recent atomic data presented by Porter et al.(2005). A good agreement is shown, suggesting that the effect ofuncertainties of atomic data on the resultant Te(He I) isnegligible. We also present an analytical formula to derive electrontemperature using the He I discontinuity at 3421Å. Our analysisshows that Te(He I) values are significantly lower thanelectron temperatures deduced from the Balmer jump of H I recombinationspectra, Te(H I),and that inferred from the collisionallyexcited [O III] nebular-to-auroral forbidden line flux ratio,Te([O III]). In addition, Te(H I) covers a widerrange of values than either Te(He I) or Te([OIII]). This supports the two-abundance nebular model withhydrogen-deficient material embedded in diffuse gas of a "normal"chemical composition (i.e. ~ solar).
| Oxygen Recombination Line Abundances in Gaseous Nebulae
The determination of the heavy element abundances from giantextragalactic H II regions has been generally based on collisionallyexcited lines. We will discuss the reasons to study the characteristicsof recombination lines, and then use these lines to determine chemicalabundances. Of these lines the oxygen (specifically the O II) lines arethe most important; and, of them, the lines of multiplet 1 of O II arethe most accessible. It has often been assumed that by measuring theintensity of a single line within a multiplet the intensities of all thelines in the multiplet can be determined; in recent studies we havefound that the intensity ratios of lines within a multiplet can dependon density; we will present empirical density-intensity relationshipsfor multiplet 1 based on recent observations of H II regions andplanetary nebulae. From observations of H II regions we find that thecritical density for collisional redistribution of the multiplet 1 O IIrecombination lines amounts to 2800+/-500 cm-3. We point out that theO/H recombination abundances of H II regions in the solar vicinity arein excellent agreement with the O/H solar value, while the abundancesderived from collisionally excited lines are not. We present acalibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8range based on O recombination lines.
| Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio
Airborne and space-based low-resolution spectroscopy in the 1980sdiscovered tantalizing quantitative relationships between the gas phaseC/O abundance ratio in planetary nebulae (PNe) and the fractions oftotal far-infrared (FIR) luminosity radiated by the 7.7- and 11.3-μmbands (the C = C stretch and C-H bend, respectively), of polycyclicaromatic hydrocarbons (PAHs). Only a very small sample of nebulae wasstudied in this context, limited by airborne observations of the7.7-μm band, or the existence of adequate IRAS Low ResolutionSpectrometer data for the 11.3-μm band. To investigate these trendsfurther, we have expanded the sample of planetaries available for thisstudy using Infrared Space Observatory (ISO) low-resolution spectrasecured with the Short Wavelength Spectrometer and the Long WavelengthSpectrometer. The new sample of 43 PNe, of which 17 are detected in PAHemission, addresses the range from C/O = 0.2-13 with the objective oftrying to delineate the pathways by which carbon dust grains might haveformed in planetaries. For the 7.7-μm and 11.3-μm bands, weconfirm that the ratio of band strength to total infrared (IR)luminosity is correlated with the nebular C/O ratio. Expressed inequivalent width terms, the cut-on C/O ratio for the 7.7-μm band isfound to be 0.6+0.2-0.4, in good accord with thatfound from sensitive ground-based measurements of the 3.3-μ band.
| Integrated spectrum of the planetary nebula NGC 7027
We present deep optical spectra of the archetypal young planetary nebula(PN) NGC 7027, covering a wavelength range from 3310to 9160 Å. The observations were carried out by uniformly scanninga long slit across the entire nebular surface, thus yielding averageoptical spectra for the whole nebula. A total of 937 emission featuresare detected. The extensive line list presented here should provevaluable for future spectroscopic analyses of emission line nebulae. Theoptical data, together with the archival IUE and ISO spectra, are usedto probe the temperature and density structures and to determine theelemental abundances from lines produced by different excitationmechanisms. Electron temperatures have been derived from the hydrogenrecombination Balmer jump (BJ), from ratios of He I opticalrecombination lines (ORLs) and from a variety of diagnostic ratios ofcollisionally excited lines (CELs). Electron densities have beendetermined from the intensities of high-order H I Balmer lines and of HeII Pfund lines, as well as from a host of CEL diagnostic ratios. CEL andORL diagnostics are found to yield compatible results. Adoptingrespectively electron temperatures of Te = 12 600 and 15 500K for ions with ionization potentials lower or higher than 50 eV and aconstant density of N_e=47 000 cm-3, elemental abundanceshave been determined from a large number of CELs and ORLs. TheC2+/H^+, N2+/H^+, O2+/H+ andNe2+/H+ ionic abundance ratios derived from ORLsare found to be only slightly higher than the corresponding CEL values.We conclude that whatever mechanism is causing the BJ/CEL temperaturediscrepanies and the ORL/CEL abundance discrepancies that have beenobserved in many PNe, it has an insignificant effect on this brightyoung compact PN. The properties of the central star are also discussed.Based on the integrated spectrum and using the energy-balance method, wehave derived an effective temperature of 219 000 K for the ionizingstar. Finally, we report the first detection in the spectrum of thisbright young PN of Raman-scattered O VI features at 6830 and 7088Å, pointing to the existence of abundant neutral hydrogen aroundthe ionized regions.
| The abundance discrepancy - recombination line versus forbidden line abundances for a northern sample of galactic planetary nebulae
We present deep optical spectra of 23 galactic planetary nebulae, whichare analysed in conjunction with archival infrared and ultravioletspectra. We derive nebular electron temperatures based on standardcollisionally excited line (CEL) diagnostics as well as the hydrogenBalmer jump and find that, as expected, the Balmer jump almost alwaysyields a lower temperature than the [OIII] nebular-to-auroral lineratio. We also make use of the weak temperature dependence of helium andOII recombination line ratios to further investigate the temperaturestructure of the sample nebulae. We find that, in almost every case, thederived temperatures follow the relation , which is the relationpredicted by two-component nebular models in which one component is coldand hydrogen-deficient. Te(OII) may be as low as a fewhundred Kelvin, in line with the low temperatures found for thehydrogen-deficient knots of Abell 30 by Wesson, Liu and Barlow.Elemental abundances are derived for the sample nebulae from both CELsand optical recombination lines (ORLs). ORL abundances are higher thanCEL abundances in every case, by factors ranging from 1.5 to 12. Fiveobjects with O2+ abundance discrepancy factors greater than 5are found. DdDm 1 and Vy 2-2 are both found to have a very largeabundance discrepancy factor of 11.8.We consider the possible explanations for the observed discrepancies.From the observed differences between Te(OIII) andTe(BJ), we find that temperature fluctuations cannot resolvethe abundance discrepancies in 22 of the 23 sample nebulae, implyingsome additional mechanism for enhancing ORL emission. In the oneambiguous case, the good agreement between abundances derived fromtemperature-insensitive infrared lines and temperature-sensitive opticallines also points away from temperature fluctuations being present. Theobserved recombination line temperatures, the large abundancediscrepancies and the generally good agreement between infrared andoptical CEL abundances all suggest instead the existence of a coldhydrogen-deficient component within the `normal' nebular gas. The originof this component is as yet unknown.
| Abundance gradients in a sample of barred spiral galaxies
We used photoionization models in order to reproduce the observedgradients of emission-line ratios for H II regions located in the normalspiral galaxy M 101 and in three barred spiralgalaxies, namely NGC 1365, NGC925, and NGC 1073. The behavior of thefollowing nebular parameters across the disk of these galaxies wasdetermined: temperature of the ionizing star (T_eff), ionizationparameter (U), and the abundance ratios O/H, N/O, and S/O. Our O/Hpredictions were found to be consistent with some empirical abundancedeterminations, but are overestimated by a factor of 0.1-0.4 dex whencompared to the direct abundance determinations. NGC1073 seems to be overabundant in nitrogen compared to otherspiral galaxies. No gradient of S/O was found in the studied galaxies,and we derived a positive T_eff gradient of triangle T_eff/triangle R =(400 ± 112) K kpc-1 and U ranging from -3.0 to -2.3.The N/O vs. O/H diagram in general is consistent with chemical evolutionmodels that assume that the nitrogen synthesis has both a primary and asecondary component compared to oxygen. However, a very strong N/Odependence on the O/H at high abundance was found.
| The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their Envelopes
The results of a study of the chemical compositions of Galacticplanetary nebulae taking into account two types of inhomogeneity in thenebular gas density in their envelopes are reported. New analyticalexpressions for the ionization correction factors have been derived andare used to determine the chemical compositions of the nebular gas inGalactic planetary nebulae. The abundances of He, N, O, Ne, S, and Arhave been found for 193 objects. The Y Z diagrams for various Heabundances are analyzed for type II planetary nebulae separately andjointly with HII regions. The primordial helium abundance Y p andenrichment ratio dY/dZ are determined, and the resulting values arecompared with the data of other authors. Radial abundance gradients inthe Galactic disk are studied using type II planetary nebulae.
| Helium recombination spectra as temperature diagnostics for planetary nebulae
Electron temperatures derived from the HeI recombination line ratios,designated Te(HeI), are presented for 48 planetary nebulae(PNe). We study the effect that temperature fluctuations inside nebulaehave on the Te(HeI) value. We show that a comparison betweenTe(HeI) and the electron temperature derived from the Balmerjump of the HI recombination spectrum, designated Te(HI),provides an opportunity to discriminate between the paradigms of achemically homogeneous plasma with temperature and density variations,and a two-abundance nebular model with hydrogen-deficient materialembedded in diffuse gas of a `normal' chemical composition (i.e.~solar), as the possible causes of the dichotomy between the abundancesthat are deduced from collisionally excited lines and those deduced fromrecombination lines. We find that Te(HeI) values aresignificantly lower than Te(HI) values, with an averagedifference of = 4000 K. Theresult is consistent with the expectation of the two-abundance nebularmodel but is opposite to the prediction of the scenarios of temperaturefluctuations and/or density inhomogeneities. From the observeddifference between Te(HeI) and Te(HI), we estimatethat the filling factor of hydrogen-deficient components has a typicalvalue of 10-4. In spite of its small mass, the existence ofhydrogen-deficient inclusions may potentially have a profound effect inenhancing the intensities of HeI recombination lines and thereby lead toapparently overestimated helium abundances for PNe.
| Recombination Line versus Forbidden Line Abundances in Planetary Nebulae
Recombination lines (RLs) of C II, N II, and O II in planetary nebulae(PNs) have been found to give abundances that are much larger in somecases than abundances from collisionally excited forbidden lines (CELs).The origins of this abundance discrepancy are highly debated. We presentnew spectroscopic observations of O II and C II recombination lines forsix planetary nebulae. With these data we compare the abundances derivedfrom the optical recombination lines with those determined fromcollisionally excited lines. Combining our new data with publishedresults on RLs in other PNs, we examine the discrepancy in abundancesderived from RLs and CELs. We find that there is a wide range in themeasured abundance discrepancyΔ(O+2)=logO+2(RL)-logO+2(CEL),ranging from approximately 0.1 dex (within the 1 σ measurementerrors) up to 1.4 dex. This tends to rule out errors in therecombination coefficients as a source of the discrepancy. Most RLsyield similar abundances, with the notable exception of O II multipletV15, known to arise primarily from dielectronic recombination, whichgives abundances averaging 0.6 dex higher than other O II RLs. Wecompare Δ(O+2) against a variety of physical propertiesof the PNs to look for clues as to the mechanism responsible for theabundance discrepancy. The strongest correlations are found with thenebula diameter and the Balmer surface brightness; high surfacebrightness, compact PNs show small values of Δ(O+2),while large low surface brightness PNs show the largest discrepancies.An inverse correlation of Δ(O+2) with nebular densityis also seen. A marginal correlation of Δ(O+2) is foundwith expansion velocity. No correlations are seen with electrontemperature, He+2/He+, central star effectivetemperature and luminosity, stellar mass-loss rate, or nebularmorphology. Similar results are found for carbon in comparing C II RLabundances with ultraviolet measurements of C III].
| Search for radiative pumping lines of OH masers. I. The 34.6 m absorption line towards 1612 MHz OH maser sources
The 1612 MHz hydroxyl maser in circumstellar envelopes has long beenthought to be pumped by 34.6 μm photons. Only recently, the InfraredSpace Observatory has made possible spectroscopic observations whichenable the direct confirmation of this pumping mechanism in a few cases.To look for the presence of this pumping line, we have searched theInfrared Space Observatory Data Archive and found 178 spectra with dataaround 34.6 μm for 87 galactic 1612 MHz masers. The analysisperformed showed that the noise level and the spectral resolution of thespectra are the most important factors affecting the detection of the34.6 μm absorption line. Only 5 objects from the sample (3 redsupergiants and 2 galactic center sources) are found to show clear 34.6μm absorption (all of them already known) while two additionalobjects only tentatively show this line. The 3 supergiants show similarpump rates and their masers might be purely radiatively pumped. The pumprates of OH masers in late type stars are found to be about 0.05, only1/5 of the theoretical value of 0.25 derived by Elitzur (1992). We havealso found 16 maser sources which, according to the analysis assumingElitzur's pump rate, should show the 34.6 μm absorption line but donot. These non-detections can be tentatively explained by far-infraredphoton pumping, clumpy nature of the OH masing region or a limb-fillingemission effect in the OH shell.
| The OH 1612 MHz maser pump rates of stellar, interstellar and post-AGB OH masers
(Pseudo) radiative pumprate of OH 1612 MHz masers is defined for asample of 44 OH/IR sources (infrared sources with OH 1612 MHz maser),irrespective of the real maser pumping mechanisms. The correlationbetween the (pseudo) maser pumprates and the evolutionary status of themaser sources reveals that the radiative pumprates of stellar OH masersare nearly fixed, which agrees with the theoretical prediction forradiatively pumped OH maser. The (pseudo) radiative pumprates ofinterstellar OH masers are not only very small but also varying broadlyover two orders of magnitude, which is argued to be the manifestation ofvarying number of quiet absorbing OH cloudlets and/or various OH maserpumping mechanisms and/or competitive gain between mainline and 1612 MHzOH masers and/or anisotropy of the maser emission. The radiativepumprates of post-AGB OH masers very possibly decrease with increasingIRAS C32 color indices and distribute in an interim regionbetween the stellar and interstellar OH masers in the pumprate-colordiagram.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| A reexamination of electron density diagnostics for ionized gaseous nebulae
We present a comparison of electron densities derived from opticalforbidden line diagnostic ratios for a sample of over a hundred nebulae.We consider four density indicators, the [O II]λ3729/λ3726, [S II] λ6716/λ6731, [Cl III]λ5517/λ5537 and [Ar IV] λ4711/λ4740 doubletratios. Except for a few H II regions for which data from the literaturewere used, diagnostic line ratios were derived from our own high qualityspectra. For the [O II] λ3729/λ3726 doublet ratio, we findthat our default atomic data set, consisting of transition probabilitiesfrom Zeippen (\cite{zeippen1982}) and collision strengths from Pradhan(\cite{pradhan}), fit the observations well, although at high electrondensities, the [O II] doublet ratio yields densities systematicallylower than those given by the [S II] λ6716/λ6731 doubletratio, suggesting that the ratio of transition probabilities of the [OII] doublet, A(λ3729)/A(λ3726), given by Zeippen(\cite{zeippen1982}) may need to be revised upwards by approximately 6per cent. Our analysis also shows that the more recent calculations of[O II] transition probabilities by Zeippen (\cite{zeippen1987a}) andcollision strengths by McLaughlin & Bell (\cite{mclaughlin}) areinconsistent with the observations at the high and low density limits,respectively, and can therefore be ruled out. We confirm the earlierresult of Copetti & Writzl (\cite{copetti2002}) that the [O II]transition probabilities calculated by Wiese et al. (\cite{wiese}) yieldelectron densities systematically lower than those deduced from the [SII] λ6716/λ6731 doublet ratio and that the discrepancy ismost likely caused by errors in the transition probabilities calculatedby Wiese et al. (\cite{wiese}). Using our default atomic data set for [OII], we find that Ne([O II]) Ne([S II]) ≈Ne([Cl III])< Ne([Ar IV]).
| Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418
We 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.
| The populations of planetary nebulae in the direction of the Galactic bulge. Chemical abundances and Wolf-Rayet central stars
We have observed 44 planetary nebulae (PNe) in the direction of theGalactic bulge, and merged our data with published ones. We havedistinguished, in the merged sample of 164 PNe, those PNe most likely toprtain physically to the Galactic bulge and those most likely to belongto the Galactic disk. We have determined the chemical composition of allthe 164 objects in a coherent way. We looked for stellar emissionfeatures and discovered 14 new [WR] stars and 15 new weak emission linecentral stars. The analyzed data led us to the following conclusions:(1) the spectral type distribution of [WR] stars is very different inthe bulge and in the disk of the Galaxy. However, the observeddistributions are strongly dependent on selection effects. (2) Theproportion of [WR] PNe is significantly larger in the bulge than in thedisk. (3) The oxygen abundances in [WR] stars do no appear to besignificantly affected by nucleosynthesis and mixing in the progenitors.(4) The O/H gradient of the Galactic disk PNe population flattens in themost internal parts of the Galaxy. (5) The median oxygen abundance inthe bulge PN population is larger by 0.2 dex than in the disk populationseen in the direction of the bulge. (6) Bulge PNe with smaller O/H tendto have smaller radial velocities. (7) The oxygen abundance distributionof bulge PNe is similar in shape to that of the metallicity distributionof bulge giants, but significantly narrower. (8) The location ofSB 32 (PN G 349.7-09.1) in the(Vlsr, lII) diagram and its low oxygen abundanceargues that it probably belongs to the halo population.Based on observations made at the South African AstronomicalObservatory.Tables 1-3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http: / /cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/427/231
| Observations and three-dimensional photoionization modelling of the Wolf-Rayet planetary nebula NGC 1501
Deep optical spectra of the high-excitation planetary nebula NGC 1501and its W04 central star are presented. A recombination line abundanceanalysis of the emission-line spectrum of the central star yields He:C:Omass fractions of 0.36:0.48:0.16, similar to those of PG 1159 stars. Adetailed empirical analysis of the nebular collisionally excited line(CEL) and optical recombination line (ORL) spectrums are presented,together with fully three-dimensional photoionization modelling of thenebula. We found very large ORL-CEL abundance discrepancy factors (ADFs)for O2+ (32) and Ne2+ (33). The mean value of~5100 K for the Te derived from HeI recombination linesratios is 6000 K, lower than the value of 11100 K implied by the [OIII]line ratio. This result indicates the existence of a second,low-temperature nebular component, which could account for the observedORL emission. Electron temperature fluctuations (t2) cannotaccount for the high ADFs found from our optical spectra of this nebula.A three-dimensional photoionization model of NGC 1501 was constructedusing the photoionization code MOCASSIN, based on our new spectroscopicdata and using the three-dimensional electron density distributiondetermined from long-slit echellograms of the nebula by Ragazzoni et al.The central star ionizing radiation field is approximated by a modelatmosphere, calculated using the Tübingen non-local thermodynamicequilibrium model atmosphere package, for abundances typical of the W04nucleus of NGC 1501 and PG 1159 stars. The nebular emission-linespectrum was best reproduced using a central star model with aneffective temperature of Teff= 110 kK and a luminosity ofL*= 5000Lsolar. The initial models showed higherdegrees of ionization of heavy elements than indicated by observations.We investigated the importance of the missing low-temperaturedielectronic recombination rates for third-row elements and haveestimated upper limits to their rate coefficients.Our single-phase, three-dimensional photoionization model heavilyunderpredicts the optical recombination line emission. We conclude thatthe presence of a hydrogen-deficient, metal-rich component is necessaryto explain the observed ORL spectrum of this object. The existence ofsuch knots could also provide a softening of the radiation field, viathe removal of ionizing photons by absorption in the knots, therebyhelping to alleviate the overionization of the heavy elements in ourmodels.
| 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.
| Searching for radiative pumping lines of OH masers: II. The 53.3 μm absorption line towards 1612 MHz OH maser sources
This is the second paper in a series aiming at searching for infraredpumping lines for galactic 1612 MHz OH masers. Our paper I is devoted tothe 34.6 μm absorption lines in ISO SWS spectra towards a largesample of galactic OH/IR sources. This paper analyzes the 53.3 μmline in the ISO LWS spectra towards a similar sample of OH/IR sources. Asearch with position radius of 1 arcmin in ISO Data Archive (IDA)results in 137 LWS spectra covering 53.3 μm associated with 47galactic OH/IR sources and 4 ones associated with megamasers Arp 220 andNGC 253. (These two magamasers are included for comparison purposeonly.) Ten of these galactic OH/IR sources are found to show and another5 ones tentatively show the 53.3 μm absorption while another 7sources (our group U1 and U2 sources) highly probably do not show thisline. The source class is found to be correlated with the type ofspectral profile: red supergiants (RSGs) and AGB stars tend to showstrong blue-shifted filling emission in their 53.3 μm absorption lineprofiles while H II regions tend to show a weak red-shifted fillingemission in the line profile. GC sources and megamasers mainly showsymmetrical profile in the line core while megamasers tend to show anadditional absorption tail on the blue side of the line profile. It isargued that the filling emission might be the manifestation of anunresolved half emission half absorption profile of the 53.3 μmdoublet which might be produced by the transitions among the two levels:2Π1/2(J=3/2) and2Π1/2(J=5/2) and their closely related levels.The 53.3 to 34.6 μm equivalent width (EW) ratio is close to unity forRSGs but much larger than unity for GC sources and megamasers while H IIregions only show the 53.3 μm line. The pump rate defined as maser toIR photon flux ratio is approximately 5% for RSGs. The pump rates of GCsources are three order of magnitude smaller. Both the large 53.3 to34.6 μm EW ratio and the small pump rate of the GC OH masers reflectthat the two detected `pumping lines' in these sources are actually ofinterstellar origin. The pump rate of Arp 220 is 32% - much larger thanthat of RSGs, which indicates that the contribution of other pumpingmechanisms to this megamaser is important. A handful of non-detectionsof the 34.6 or 53.3 μm line or both can be explained partly by thegenuinely weakness of the OH masers and partly by some other mechanismsweakening the IR pumping lines, such as clumpy OH shell or limb fillingemission.
| A deep survey of heavy element lines in planetary nebulae - II. Recombination-line abundances and evidence for cold plasma
In 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 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.
| Chemical Abundances of the Galactic H II Region NGC 3576 Derived from Very Large Telescope Echelle Spectrophotometry
We present echelle spectrophotometry of the Galactic H II region NGC3576. The data have been taken with the VLT UVES echelle spectrograph inthe 3100-10400 Å range. We have measured the intensities of 458emission lines, 344 are permitted lines of H0,He0, C+, N0, N+,N++, O0, O+, Ne+,S++, Si0, Si+, Ar0, andAr+; some of them are produced by recombination and othersmainly by fluorescence. Electron temperatures and densities have beendetermined using different continuum and line intensity ratios. We havederived He+, C++, O+, O++,and Ne++ ionic abundances from pure recombination lines. Wehave also derived abundances from collisionally excited lines for alarge number of ions of different elements. Remarkably consistentestimations of t2 have been obtained by comparing Balmer andPaschen with [O III] temperatures, and O++ andNe++ ionic abundances obtained from collisionally excited andrecombination lines. The chemical composition of NGC 3576 is comparedwith those of other Galactic H II regions and with the one from the Sun.A first approach to the gas-phase Galactic radial abundance gradient ofC as well as of the C/O ratio has been made.Based on observations collected at the European Southern Observatory,Chile, proposal number ESO 68.C-0149(A).
| Me 1-1: A PN containing a cool star
We report the detection of a cool stellar component at the center of theplanetary nebula (PN) Me 1-1 and present optical spectra of the system.From measurements of nebular emission lines, we have derived electrontemperature, density and chemical composition. Heavy elementalabundances deduced from collisionally excited lines (CELs) are comparedwith those derived from optical recombination lines (ORLs). The electrontemperature and density deduced from the nebular analysis were used tocalculate the nebular continuum emission, which was then subtracted fromthe observed spectrum in order to obtain the spectrum of the coolstellar component apparent in the observed spectrum. We calculate B andV magnitudes of the cool companion and obtain a color index of B-V=1.20.By comparing the spectrum of the cool star with standard spectra inPickles's 1998 Stellar Flux Library, we find that the cool component hasthe spectral type of a K3-4 bright giant. Our analysis suggests that Me1-1 is probably a yellow symbiotic system.
| Physical conditions in the planetary nebula NGC 6543
We present analysis of spectra and Hubble Space Telescope (HST) imagesof NGC 6543, the Cat's Eye Nebula. Using HST WFPC2 images taken innarrow-band filters isolating Hα and Hβ, we have produced anextinction map towards the nebula, and find that c(Hβ) is low, at~0.1, and exhibits only very small spatial variations across the surfaceof the nebula. Using filters isolating [OIII] emission lines, we haveproduced a temperature map of NGC 6543, accounting carefully forcontamination due to continuum emission and other lines within thefilter bandpasses. We find that the temperature over most of the nebulais between 7000 and 9000K, with higher values seen towards the edge ofthe bright core. The value of t2A, the temperaturefluctuation parameter, is low at around 0.005 in the central regions.We have obtained long-slit scanning spectra of NGC 6453, which, whencombined with the total Hβ flux, give integrated fluxes for all thelines in our spectra. We determine ionic and total abundances within thenebula from both recombination lines and forbidden lines, and we findthat recombination lines yield a higher abundance by a factor ofapproximately three. A temperature fluctuation parameter of 0.053 wouldresolve the discrepancy, but several factors argue against temperaturefluctuations as the main cause: the value of t2 required isinconsistent with the results derived from the temperature map; theagreement of the temperatures derived from the hydrogen Balmer jump and[OIII] line diagnostics also implies a low value of t2; andthe agreement of abundances derived from temperature-insensitiveinfrared fine-structure lines and temperature-sensitive opticalforbidden lines is inconsistent with the temperature fluctuationscenario.We argue that a more likely explanation for the observed temperature andabundance discrepancies is the presence within the nebula of somehydrogen-deficient material at low temperatures. Weaklytemperature-dependent line ratios of HeI and OII imply lowertemperatures than those found from forbidden-line temperaturediagnostics and the hydrogen Balmer jump, as is expected in the presenceof cold hydrogen-deficient material.
| 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.
| The distances of Type I planetary nebulae
The distances D of planetary nebulae (PNe) are still extremelyuncertain. Although a variety of methods have been used to evaluate thisparameter, these are often in conflict, and subject to large random andsystematic errors. It is therefore important to evaluate D using as manyindependent procedures as possible. We outline here one further way inwhich this parameter may be assessed. It is noted that where the nebularmass range is narrow, then one might expect observed PNe radii to beroughly similar. This, where it occurs, would also result in acorrelation between their angular diameters Θ, and distances D.We find that just such a trend occurs for Type I nebulae, and we employthis to determine distances to a further 44 such outflows. Our meanvalues of D appear similar to those of Zhang [ApJS 98 (1995) 659],implying a relatively long PNe distance scale.
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