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X-ray emission from planetary nebulae calculated by 1D spherical numerical simulations
We calculate the X-ray emission from both constant and time-evolvingshocked fast winds blown by the central stars of planetary nebulae (PNe)and compare our calculations with observations. Using sphericallysymmetric numerical simulations with radiative cooling, we calculate theflow structure and the X-ray temperature and luminosity of the hotbubble formed by the shocked fast wind. We find that a constant fastwind gives results that are very close to those obtained from theself-similar solution. We show that in order for a fast shocked wind toexplain the observed X-ray properties of PNe, rapid evolution of thewind is essential. More specifically, the mass-loss rate of the fastwind should be high early on when the speed is ~300-700 kms-1, and then it needs to drop drastically by the time the PNage reaches ~1000 yr. This implies that the central star has a veryshort pre-PN (post-asymptotic giant branch) phase.
| The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in theinterstellar medium. Data from FUSE were used to analyze 26 lines ofsight lying in both the galactic disk and halo, including lines toWolf-Rayet stars and through known supernova remnants. The equivalentwidths of the fluorine resonance lines at 951.871 and 954.827 Åwere measured or assigned upper limits and combined with a nitrogencurve of growth to obtain F I column densities. These column densitieswere then used to calculate fluorine depletions. Comparisons are made tothe previous study of F I by Federman and coworkers and implications forF I formation and depletion are discussed.
| Detailed Far-Ultraviolet to Optical Analysis of Four [WR] Stars
We present far-UV to optical analyses of four hydrogen-deficient centralstars of planetary nebulae: BD +30 3639, NGC 40, NGC 5315, and NGC 6905.Using the radiative transfer code CMFGEN, we determined new physicalparameters and chemical abundances for these stars. The results wereanalyzed in the context of the [WR]-->PG 1159 evolution via thetransformed radius-temperature (RT×T*) andH-R diagrams. NGC 5315 showed itself as an odd object among thepreviously analyzed central stars. Its temperature (~76 kK) isconsiderably lower than other early-type [WR] stars (~120-150 kK). Fromour models for NGC 5315 and NGC 6905, it is unclear if early-type [WR]stars have smaller C/He mass ratios than other spectral classes, asclaimed in the literature. A ratio of ~0.8 is found for NGC 6905. Weanalyzed FUSE spectra of these stars for the first time and identifiedphosphorus in the spectra of BD +30 3639, NGC 40, and NGC 5315 throughthe doublet transition P V λλ1118, 1128 (3p2Po-3s 2S). The Fe, Si, P, S, and Neabundances were analyzed in the context of the nucleosynthesis occurringin previous evolutionary phases. We found evidence for Fe deficiency inBD +30 3639 and NGC 5315, and we determined a solar Si abundance for BD+30 3639 and NGC 40. Regarding P, an oversolar abundance in the NGC 5315model was preferred. Upper limits for the S abundance were estimated. Wefound that Ne is overabundant in BD +30 3639. In the other stars, Ne isweak or undetectable. Our results are in agreement with theoreticalpredictions and show the usefulness of [WR] stars as test beds fornucleosynthesis calculations in the AGB and post-AGB phases.
| Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79northern Galactic planetary nebulae (PNe) whose morphological classeshave been uniformly determined. Ionic abundances and plasma diagnosticswere derived from selected optical line strengths in the literature, andelemental abundances were estimated with the ionization correctionfactor developed by Kingsbourgh & Barlow in 1994. We compare theelemental abundances to the final yields obtained from stellar evolutionmodels of low- and intermediate-mass stars, and we confirm that mostbipolar PNe have high nitrogen and helium abundance and are the likelyprogeny of stars with main-sequence mass greater than 3Msolar. We derive =0.27 and discuss the implication of such ahigh ratio in connection with the solar neon abundance. We determine theGalactic gradients of oxygen and neon and foundΔlog(O/H)/ΔR=-0.01 dex kpc-1 andΔlog(Ne/H)/ΔR=-0.01 dex kpc-1. These flat PNgradients are irreconcilable with Galactic metallicity gradientsflattening with time.
| X-Ray Emission from Planetary Nebulae. I. Spherically Symmetric Numerical Simulations
The interaction of a fast wind with a spherical asymptotic giant branch(AGB) wind is thought to be the basic mechanism for shapingpre-planetary nebulae (PPNs) and later planetary nebulae (PNs). Due tothe large speed of the fast wind, one expects extended X-ray emissionfrom these objects, but X-ray emission has only been detected in a smallfraction of PNs and only in one PPN. Using numerical simulations weinvestigate the constraints that can be set on the physical propertiesof the fast wind (speed, mass flux, opening angle) in order to producethe observed X-ray emission properties of PPNs and PNs. We combinenumerical hydrodynamic simulations including radiative cooling using thecode FLASH with calculations of the X-ray properties of the resultingexpanding hot bubble using the atomic database ATOMDB. In this firststudy, we compute X-ray fluxes and spectra using one-dimensional models.Comparing our results with analytical solutions, we find some agreementsand many disagreements. These disagreements, which are both qualitativeand quantitative in nature, argue for the necessity of using numericalsimulations for understanding the X-ray properties of PNs. We apply ourspherical models to the objects BD +30 3639 and NGC 40. We find that themodel values of the X-ray temperature and luminosity for these objectsare significantly higher than observed values and discuss severalmechanisms for resolving the discrepancies.
| 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.
| 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.
| X-ray emission by a shocked fast wind from the central stars of planetary nebulae
We calculate the X-ray emission from the shocked fast wind blown by thecentral stars of planetary nebulae (PNe) and compare with observations.Using spherically symmetric self-similar solutions, we calculate theflow structure and X-ray temperature for a fast wind slamming into apreviously ejected slow wind. We find that the observed X-ray emissionof six PNe can be accounted for by shocked wind segments that wereexpelled during the early-PN phase, if the fast wind speed is moderate,v2~ 400-600kms-1, and the mass-loss rate is a fewtimes 10-7Msolaryr-1. We find, asproposed previously, that the morphology of the X-ray emission is in theform of a narrow ring inner to the optical bright part of the nebula.The bipolar X-ray morphology of several observed PNe, which indicates animportant role of jets, rather than a spherical fast wind, cannot beexplained by the flow studied here.
| The structure of planetary nebulae: theory vs. practice
Context.This paper is the first in a short series dedicated to thelong-standing astronomical problem of de-projecting the bi-dimensional,apparent morphology of a three-dimensional mass of gas. Aims.Wefocus on the density distribution in real planetary nebulae (and alltypes of expanding nebulae). Methods. We introduce some basictheoretical notions, discuss the observational methodology, and developan accurate procedure for determining the matter radial profile withinthe sharp portion of nebula in the plane of the sky identified by thezero-velocity-pixel-column (zvpc) of high-resolution spectral images.Results. The general and specific applications of the method (andsome caveats) are discussed. Moreover, we present a series of evolutivesnapshots, combining illustrative examples of both model and trueplanetary nebulae. Conclusions. The zvpc radial-densityreconstruction - added to tomography and 3D recovery developed at theAstronomical Observatory of Padua (Italy) - constitutes a very usefultool for looking more closely at the spatio-kinematics, physicalconditions, ionic structure, and evolution of expanding nebulae.
| Planetary nebulae with emission-line central stars
The kinematic structure of a sample of planetary nebulae, consisting of23 [WR] central stars, 21 weak emission line stars (wels), and 57non-emission line central stars, is studied. The [WR] stars are shown tobe surrounded by turbulent nebulae, a characteristic shared by some welsbut almost completely absent from the non-emission line stars. Thefraction of objects showing turbulence for non-emission-line stars,wels, and [WR] stars is 7%, 24%, and 91%, respectively. The [WR] starsshow a distinct IRAS 12-micron excess, indicative of small dust grains,which is not found for wels. The [WR]-star nebulae are on average morecentrally condensed than those of other stars. On the age-temperaturediagram, the wels are located on tracks of both high and low stellarmass, while [WR] stars trace a narrow range of intermediate masses.Emission-line stars are not found on the cooling track. One group ofwels may form a sequence wels-[WO] stars with increasing temperature.For the other groups, both the wels and the [WR] stars appear torepresent several, independent evolutionary tracks. We find adiscontinuity in the [WR] stellar temperature distribution and suggestdifferent evolutionary sequences above and below the temperature gap.One group of cool [WR] stars has no counterpart among any other group ofPNe and may represent binary evolution. A prime factor distinguishingwels and [WR] stars appears to be stellar luminosity. We find noevidence for an increase in the nebular expansion velocity with time.
| Galactic Planetary Nebulae with Wolf-Rayet Nuclei III. Kinematical Analysis of a Large Sample of Nebulae
Expansion velocities (V_{exp}) of different ions and line widths at thebase of the lines are measured and analyzed for 24 PNe with [WC]-typenuclei (WRPNe), 9 PNe ionized by WELS (WLPNe) and 14 ordinary PNe. Acomparative study of the kinematical behavior of the sample clearlydemonstrates that WRPNe have on average 40-45% larger V_{exp}, andpossibly more turbulence than WLPNe and ordinary PNe. WLPNe havevelocity fields very much like the ones of ordinary PNe, rather than theones of WRPNe. All the samples (WRPNe, WLPNe and ordinary PNe) showexpansion velocities increasing with age indicators, for example is larger for low-density nebulae and also it is largerfor nebulae around high-temperature stars. This age effect is muchstronger for evolved WRPNe, suggesting that the [WC] winds have beenaccelerating the nebulae for a long time, while for non-WRPNe theacceleration seems to stop at some point when the star reaches atemperature of about 90,000 - 100,000. Non-WR nebulae reach a maximumV_{exp} ≤ 30 km s(-1) evolved WRPNe reach maximum V_{exp} about 40km s(-1) . For all kinds of objects (WRPNe and non-WRPNe) it is foundthat on average V_{exp}(N(+) ) is slightly larger than V_{exp}(O(++) ),indicating that the nebulae present acceleration of the external shells.
| 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.
| The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
We review the observed properties of extremely hot, hydrogen-deficientpost-asymptotic giant branch (AGB) stars of spectral type [WC] andPG1159. Their H deficiency is probably caused by a (very) latehelium-shell flash or an AGB final thermal pulse, laying bare interiorstellar regions that are usually kept hidden below the hydrogenenvelope. Thus, the photospheric elemental abundances of these starsallow us to draw conclusions about details of nuclear burning and mixingprocesses in the precursor AGB stars. We summarize the state of the artof stellar evolution models that simulate AGB evolution and theoccurrence of a late He-shell flash. We compare predicted elementalabundances to those determined by quantitative spectral analysesperformed with advanced non-LTE model atmospheres. Good qualitative andquantitative agreement is found. Future work can contribute to an evenmore complete picture of the nuclear processes in AGB stars.
| 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.
| X-Ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40
We present the results of Chandra X-Ray Observatory observations of theplanetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-typeWolf-Rayet central stars that are currently driving fast (~1000 kms-1), massive winds into denser, slow-moving (~10 kms-1) material ejected during recently terminated asymptoticgiant branch (AGB) evolutionary phases. Hence, these observationsprovide key tests of models of wind-wind interactions in PNs. In NGC 40,we detect faint, diffuse X-ray emission distributed within a partialannulus that lies nested within a ~40" diameter ring of nebulosityobserved in optical and near-infrared images. Hen 2-99 is not detected.The inferred X-ray temperature (TX~106 K) andluminosity (LX~2×1030 ergs s-1)of NGC 40 are the lowest measured thus far for any PN displaying diffuseX-ray emission. These results, combined with the ringlike morphology ofthe X-ray emission from NGC 40, suggest that its X-ray emission arisesfrom a ``hot bubble'' that is highly evolved and is generated by ashocked, quasi-spherical fast wind from the central star, as opposed toAGB or post-AGB jet activity. In contrast, the lack of detectable X-rayemission from Hen 2-99 suggests that this PN has yet to enter a phase ofstrong wind-wind shocks.
| Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio
Airborne and space-based low-resolution spectroscopy in the 1980sdiscovered tantalizing quantitative relationships between the gas phaseC/O abundance ratio in planetary nebulae (PNe) and the fractions oftotal far-infrared (FIR) luminosity radiated by the 7.7- and 11.3-μmbands (the C = C stretch and C-H bend, respectively), of polycyclicaromatic hydrocarbons (PAHs). Only a very small sample of nebulae wasstudied in this context, limited by airborne observations of the7.7-μm band, or the existence of adequate IRAS Low ResolutionSpectrometer data for the 11.3-μm band. To investigate these trendsfurther, we have expanded the sample of planetaries available for thisstudy using Infrared Space Observatory (ISO) low-resolution spectrasecured with the Short Wavelength Spectrometer and the Long WavelengthSpectrometer. The new sample of 43 PNe, of which 17 are detected in PAHemission, addresses the range from C/O = 0.2-13 with the objective oftrying to delineate the pathways by which carbon dust grains might haveformed in planetaries. For the 7.7-μm and 11.3-μm bands, weconfirm that the ratio of band strength to total infrared (IR)luminosity is correlated with the nebular C/O ratio. Expressed inequivalent width terms, the cut-on C/O ratio for the 7.7-μm band isfound to be 0.6+0.2-0.4, in good accord with thatfound from sensitive ground-based measurements of the 3.3-μ band.
| The distances of less-evolved planetary nebulae: a further test of statistical distance scales
It has recently been pointed out that a number of the methods used todetermine planetary nebulae (PNe) distances may be appreciably in error.Whilst the scales of Zhang (1995), Bensby & Lundstrom (2001) andothers are appropriate for higher radio brightness temperaturesTB, those of Phillips and Daub are more relevant whereTB is small.We note, in the following, that the absolute bolometric magnitudes ofless-evolved PNe are likely to be similar. The mean value of can therefore be used to constrain PNe distancesD, and confirm the distance scales for higher TB outflows. Wehave used this procedure to evaluate distances to a further 47 PNe, andwe find that the mean values of are consistent with those ofCahn, Kaler & Stanghellini (1992), Zhang (1995), Phillips et al.(2004) and van de Steene & Zijlstra (1995). They are, as expected,inconsistent with the lower TB scale of Phillips (2002a).
| Fluorine Abundances in Planetary Nebulae
We have determined fluorine abundances from the [F II] λ4789 and[F IV] λ4060 nebular emission lines for a sample of planetarynebulae (PNe). Our results show that fluorine is generally overabundantin PNe, thus providing new evidence for the synthesis of fluorine inasymptotic giant branch (AGB) stars. [F/O] is found to be positivelycorrelated with the C/O abundance ratio, in agreement with thepredictions of theoretical models of fluorine production in thermallypulsing AGB stars. A large enhancement of fluorine is observed in theWolf-Rayet PN NGC 40, suggesting that high mass-loss rates probablyfavor the survival of fluorine.
| Discovery of Ne VII in the Winds of Hot Evolved Stars
We show that a strong P Cygni feature seen in the far-UV spectra of somevery hot (Teff>~85 kK) central stars of planetary nebulae(CSPN), which has been previously identified as C III λ977,actually originates from Ne VII λ973. Using stellar atmospheremodels, we reproduce this feature seen in the spectra of two [WR] PG1159-type CSPN, A78 and NGC 2371, and in one PG 1159 CSPN, K1-16 (the``[WR]'' nomenclature denotes a CSPN with spectral features similar tothose of Wolf-Rayet stars). In the latter case, our analysis suggests anenhanced neon abundance. Strong neon features in CSPN spectra areimportant because an overabundance of this element is indicative ofprocessed material that has been dredged up to the surface from theintershell region in the ``born-again'' scenario, an explanation ofhydrogen-deficient CSPN. Our modeling indicates that the Ne VIIλ973 wind feature may be used to discern enhanced neon abundancesfor stars showing an unsaturated P Cygni profile, such as some PG 1159stars. We explore the potential of this strong feature as a winddiagnostic in stellar atmosphere analyses for evolved objects. For the[WR] PG 1159 objects, the line is present as a P Cygni line forTeff>~85 kK and becomes strong for100<~Teff<~155 kK when the neon abundance is solar, andcan be significantly strong beyond this range for higher neonabundances. When unsaturated, i.e., for very high Teff and/orvery low mass-loss rates, it is sensitive to M˙ and very sensitiveto the neon abundance. The Ne VII classification is consistent withrecent identification of this line, seen in absorption in many PG 1159spectra.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer and data from the Multimission Archive at theSpace Telescope Science Institute (MAST). FUSE is operated for NASA bythe Johns Hopkins University, under NASA contract NAS5-32985.
| The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edge
We infer the gas kinematics, diagnostics and ionic radial profiles,distance and central star parameters, nebular photo-ionization model,spatial structure and evolutionary phase of the Planetary Nebula NGC6741 by means of long-slit ESO NTT+EMMI high-resolution spectra at nineposition angles, reduced and analysed according to the tomographic and3-D methodologies developed at the Astronomical Observatory of Padua(Italy). NGC 6741 (distance≃2.0 kpc, age≃ 1400 yr, ionizedmass Mion≃ 0.06 Mȯ) is a dense(electron density up to 12 000 cm-3), high-excitation,almost-prolate ellipsoid (0.036 pc × 0.020 pc × 0.018 pc,major, intermediate and minor semi-axes, respectively), surrounded by asharp low-excitation skin (the ionization front), and embedded in aspherical (radius≃ 0.080 pc), almost-neutral, high-density (n(HI)≃ 7 ×103 atoms cm-3) halo containinga large fraction of the nebular mass (Mhalo≥ 0.20Mȯ). The kinematics, physical conditions and ionicstructure indicate that NGC 6741 is in a deep recombination phase,started about 200 years ago, and caused by the rapid luminosity drop ofthe massive (M*=0.66{-}0.68 Mȯ), hot (logT* ≃ 5.23) and faint (logL*/Lȯ ≃ 2.75) post-AGB star, which hasexhausted the hydrogen-shell nuclear burning and is moving along thewhite dwarf cooling sequence. The general expansion law of the ionizedgas in NGC 6741, Vexp(km s-1)=13 × R arcsec,fails in the innermost, highest-excitation layers, which move slowerthan expected. The observed deceleration is ascribable to the luminositydrop of the central star (the decreasing pressure of the hot-bubble nolonger balances the pressure of the ionized gas), and appears instriking contrast to recent reports inferring that acceleration is acommon property of the Planetary Nebulae innermost layers. A detailedcomparative analysis proves that the "U"-shaped expansion velocity fieldis a spurious, incorrect result due to a combination of: (a) simplisticassumptions (spherical shell hypothesis for the nebula); (b) unfitreduction method (emission profiles integrated along the slit); and (c)inappropriate diagnostic choice (λ4686 Å of He II, i.e. athirteen fine-structure components recombination line). Some generalimplications for the shaping mechanisms of Planetary Nebulae arediscussed.
| The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their Envelopes
The results of a study of the chemical compositions of Galacticplanetary nebulae taking into account two types of inhomogeneity in thenebular gas density in their envelopes are reported. New analyticalexpressions for the ionization correction factors have been derived andare used to determine the chemical compositions of the nebular gas inGalactic planetary nebulae. The abundances of He, N, O, Ne, S, and Arhave been found for 193 objects. The Y Z diagrams for various Heabundances are analyzed for type II planetary nebulae separately andjointly with HII regions. The primordial helium abundance Y p andenrichment ratio dY/dZ are determined, and the resulting values arecompared with the data of other authors. Radial abundance gradients inthe Galactic disk are studied using type II planetary nebulae.
| The FUSE Spectrum of the Planetary Nebula SwSt 1: Evidence for Inhomogeneities in the Gas and Dust
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofthe young, compact planetary nebula (PN) SwSt 1 along the line of sightto its central star HD 167362. We detect circumstellar absorption linesfrom several species against the continuum of the central star. Thephysical parameters of the nebula derived from the FUSE data differsignificantly from those found from emission lines. We derive anelectron density ne=8800+4800-2400cm-3 from the column density ratio of the excited S IIIfine-structure levels, which is at least a factor of 3 lower than allprior estimates. The gaseous iron abundance derived from the UV lines isquite high ([Fe/S]=-0.35+/-0.12), which implies that iron is notsignificantly depleted into dust. In contrast, optical and near-infraredemission lines indicate that Fe is more strongly depleted:[Fe/H]=-1.64+/-0.24 and [Fe/S]=-1.15+/-0.33. We do not detect nebularH2 absorption, to a limitN(H2)<7×1014 cm-2, at least 4orders of magnitude lower than the column density estimated frominfrared H2 emission lines. Taken together, the lack ofH2 absorption, low ne, and high gaseous Feabundance derived from the FUSE spectrum provide strong evidence thatdense structures (which can shield molecules and dust from thedestructive effects of energetic stellar photons) are not present alongthe line of sight to the central star. On the other hand, there issubstantial evidence for dust, molecular material, and dense gaselsewhere in SwSt 1. Therefore, we conclude that the nebula must have aninhomogeneous structure.We detect nebular absorption at 1040.94 and 1041.69 Å from the twoexcited fine-structure levels of neutral oxygen. These levels give riseto far-infrared emission lines at 63 and 145 μm, which are often usedto infer gas properties, particularly temperature, under the assumptionthat they are collisionally excited. We find that the O I fine-structurelevels in SwSt 1 have an inverted population ratio. This requires anonthermal excitation mechanism, which we identify as fluorescentexcitation by the stellar continuum. To the extent that fluorescenceaffects the level populations, the far-infrared [O I] line strengthscannot be directly used as diagnostics of density and temperature.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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.
| 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]).
| 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.
| 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.
| 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.
| 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.
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