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A Monte Carlo study of polarization structures in the Thomson-scattered line radiation
Thomson scattering is often invoked to explain broad wing features thatare seen in various objects including active galactic nuclei andsymbiotic stars. Despite the wavelength-independent scatteringcross-section of Thomson scattering, the line flux may exhibitwavelength-dependent linear degree of polarization, because variousparts of emission wings are contributed by photons with differentscattering numbers. Specifically, more scattered and hence more weaklypolarized photons tend to fill the farther wing parts from the linecentre, while the neighbourhood of the line centre is dominated byless-scattered photons with higher degree of polarization. Using a MonteCarlo technique, we investigate the polarization structure ofThomson-scattered line radiation. A detailed analysis of polarizationstructure formation is conducted by investigating the dependence of thepolarization and profile width on the scattering number for variousfinite electron scattering slabs. Significantly varying degree ofpolarization is obtained when the scattering medium has Thomson opticaldepth τTh >= 1. We present our high-resolutionspectrum of the symbiotic star V1016 Cyg obtained with the BohyunsanOptical Echelle Spectrograph (BOES) in order to fit the broad profilearound Hα by electron scattering wings adopting an oblatespheroidal geometry with Thomson optical depth τTh = 0.5and electron temperature Te = 6.2 × 104 K.Local maxima in the linear degree of polarization of Thomson-scatteredline radiation are expected to appear in the spectral regionscharacterized by the average scattering number ~=1.
| Searching for Balmer Self-Absorption in Planetary Nebulae
We analyse the distribution of 482 planetary nebulae (PNe) within theHα/Hβ - Hγ/Hβ and Hδ/Hγ -Hγ/Hβ planes. Whilst most sources appear to possess case Bline ratios, and normal reddening decrements, certain of them alsoappear to be affected by Balmer self-absorption. We are able to identify9 possible cases, and 11 probable examples where such absorption isimportant. Approximately half of these nebulae are compact, and probablyquite young. Most previous identifications of self-absorption are shownto be open to doubt, and there is no overlap at all between our presentcandidates, and those of previous analyses.
| Raman-scattered He II λλ4850, 6545 in the Young and Compact Planetary Nebula IC 5117
We report the discovery of He II λλ4850 and 6545Raman-scattered by atomic hydrogen in the high-resolution spectrum ofthe very compact and young planetary nebula IC 5117 obtained with theESPaDOnS (Echelle Spectropolarimetric Device for the Observation ofStars) installed on the 3.6 m Canada-France-Hawaii Telescope. Byadopting case B recombination atomic data and considering all thefine-structure emission components, we calculate the weighted averagesfor the line centers of the Raman-scattered features, as well as thosefor the emission lines He II λλ4859 and 6560 to determineredward velocity shifts of Δv4850=+29 km s-1and Δv6545=+24 km s-1. We compute the Ramanconversion efficiency by a Monte Carlo technique adopting a simplescattering geometry, in which the hot central star is surrounded by ahollow cylindrical H I region characterized by thickness T that is inturn parameterized by the H I column density. It is proposed that thecentral far-UV emission region is covered significantly by the neutralscattering region characterized by the H I column densityNHI~=4×1021 cm-2. The H I columndensity is higher by an order of magnitude than the value proposed from21 cm radio observations. We briefly discuss the importance of Ramanscattering as a probe of neutral material around a newly formed hotwhite dwarf and the mass-loss process occurring in the late stage ofstellar evolution.
| 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.
| 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).
| 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.
| The identification of PM 1-322 as a new young high-density planetary nebulae
We report spectroscopic observations of a new planetary nebula PM 1-322,which has not yet been identified. This object was found during anorthern survey of post-AGB and planetary nebulae candidates selectedfrom García-Lario et al. (1997, A&AS, 126, 479) andPreite-Martinez (1988, A&AS, 76, 317). Here we present the mainspectroscopic features including a list of the intensities of theidentified emission lines. The main spectral characteristic of PM 1-322is the presence of a very strong [O iii] λ4363 emission line(I[OIII]4363/Hβ ≃ 120) indicating high electron densities (logNe > 6.0). A comparison with similar objects stronglysuggests that PM 1-322 is a very young planetary nebula, although theposibility of a symbiotic nature can not be ruled out.Based on observations collected at the Centro Astronomico Hispano Aleman(CAHA) at Calar Alto, operated jointly by the Max-Planck Institutfür Astronomie and the Instituto de Astrofísica deAndalucía (CSIC).
| Unresolved Hα Enhancements at High Galactic Latitude in the WHAM Sky Survey Maps
We have identified 85 regions of enhanced Hα emission at|b|>10deg subtending approximately 1° or less on theWisconsin Hα Mapper (WHAM) sky survey. These high-latitude ``WHAMpoint sources'' have Hα fluxes of 10-11-10-9ergs cm-2 s-1, radial velocities within about 70km s-1 of the LSR, and line widths that range from less than20 to about 80 km s-1 (FWHM). Twenty-nine of theseenhancements are not identified with either cataloged nebulae or hotstars and appear to have kinematic properties that differ from thoseobserved for planetary nebulae. Another 14 enhancements are near hotevolved low-mass stars that had no previously reported detections ofassociated nebulosity. The remainder of the enhancements are catalogedplanetary nebulae and small, high-latitude H II regions surroundingmassive O and early B stars.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| 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]).
| Planetary nebula distances re-examined: an improved statistical scale
The distances of planetary nebulae (PNe) are still quite uncertain.Although observational estimates are available for a small proportion ofPNe, based on statistical parallax and the like, such distances are verypoorly determined for the majority of galactic PNe. In particular,estimates of so-called `statistical' distance appear to differ byfactors of ~2.7.We point out that there is a well-defined correlation between the 5-GHzluminosity of the sources, L5, and their brightnesstemperatures, TB. This represents a different trend to thoseinvestigated in previous statistical analyses, and permits us todetermine independent distances to a further 449 outflows. Thesedistances are shown to be closely comparable to those determined using aTB-R correlation, providing that the latter trend is taken tobe non-linear.This non-linearity in the TB-R plane has not been noted inprevious analyses, and is likely responsible for the broad (andconflicting) ranges of distance that have previously been published.Finally, we point out that there is a close accord between observedtrends within the L5-TB and TB-Rplanes, and the variation predicted through nebular evolutionarymodelling. This is used to suggest that observational biases areprobably modest, and that our revised distance scale is reasonablytrustworthy.
| On the O II Ground Configuration Energy Levels
The most accurate way to measure the energy levels for the O II2p3 ground configuration has been from the forbidden lines inplanetary nebulae. We present an analysis of modern planetary nebuladata that nicely constrain the splitting within the 2D termand the separation of this term from the ground4S3/2 level. We extend this method to H II regionsusing high-resolution spectroscopy of the Orion Nebula, covering all sixvisible transitions within the ground configuration. These data confirmthe splitting of the 2D term while additionally constrainingthe splitting of the 2P term. The energies of the2P and 2D terms relative to the ground(4S) term are constrained by requiring that all six linesgive the same radial velocity, consistent with independent limits placedon the motion of the O+ gas and the planetary nebula data.
| 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.
| On the centre shifts of Raman-scattered He II and Hα wings in symbiotic stars
Using a Monte Carlo technique, we investigate the centre shifts that areexpected to occur for the broad Hα wings and the HeIIλ6545feature as a function of the neutral hydrogen column density insymbiotic stars. These two features are proposed to be formed via Ramanscattering of ultraviolet (UV) continuum around the Lyβ and HeIIλ1025 emission line by neutral hydrogen. The strengths of thesetwo features are determined by a combination of various physicalparameters including the covering factor and the neutral hydrogen columndensity (NHI) of the scattering region. The branching ratioof Raman scattering to Rayleigh scattering for UV radiation aroundLyβ is a non-linearly increasing function of the wavelength, whichresults in enhanced Raman optical fluxes redward of the Hα linecentre as NHI increases. However, we find that the amount ofwing centre shift is quite small about 20 km s-1 asNHI increases from 1020 to 1021cm-2. Assuming that He IIλ1025 emission ischaracterized by a Gaussian profile, the Raman-scattered He II 6545feature exhibits near Gaussian profiles with the peak shifted redwardfor NHI < 1022 cm-2. The redwardcentre shift amounts to 1 Å for NHI~ 1020cm-2 and decreases as NHI increases up toNHI~ 1022 cm-2, above which no centreshift is observed. The redward peak shift is due to the fact that theincident emission profile is symmetric with respect to the HeIIλ1025 line centre whereas the Raman conversion rate isincreasing towards the line centre of Lyβ. We emphasize that thedetermination of NHI by locating the exact peak position ofthe He II 6545 feature will lift the degeneracy to allow a more accurateestimate of the covering factor of the neutral region, providing strongconstraints on the mass-loss process occurring in symbiotic stars.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| A Survey for Water Maser Emission toward Planetary Nebulae: New Detection in IRAS 17347-3139
We report on a water maser survey toward a sample of 27 planetarynebulae (PNe) using the Robledo de Chavela and Medicina single-dishantennas, as well as the Very Large Array (VLA). Two detections havebeen obtained: the already known water maser emission in K3-35, and anew cluster of masers in IRAS 17347-3139. This low rate of detections iscompatible with the short lifetime of water molecules in PNe (~100 yr).The water maser cluster at IRAS 17347-3139 are distributed on a ellipseof size ~=0.2"×0.1", spatially associated with compact 1.3 cmcontinuum emission (simultaneously observed with the VLA). From archiveVLA continuum data at 4.9, 8.4, and 14.9 GHz, a spectral indexα=0.76+/-0.03 (Sν~να) is derivedfor this radio source, which is consistent with either a partiallyoptically thick ionized region or an ionized wind. However, the latterscenario can be ruled out by mass-loss considerations, thus indicatingthat this source is probably a young PN. The spatial distribution andthe radial velocities of the water masers are suggestive of a rotatingand expanding maser ring, tracing the innermost regions of a torusformed at the end of the asymptotic giant branch phase. Given that the1.3 cm continuum emission peak is located near one of the tips of themajor axis of the ellipse of masers, we speculate on a possible binarynature of IRAS 17347-3139, where the radio continuum emission couldbelong to one of the components and the water masers would be associatedwith a companion.
| Raman-scattered He II λ6545 Line in the Symbiotic Star V1016 Cygni
We present a spectrum of the symbiotic star V1016 Cyg observed with the3.6 m Canada-France-Hawaii Telescope, in order to illustrate a method tomeasure the covering factor of the neutral scattering region around thegiant component with respect to the hot emission region around the whitedwarf component. In the spectrum, we find broad wings around Hαand a broad emission feature around 6545 Å that is blended withthe [N II] λ6548 line. These two features are proposed to beformed by Raman scattering by atomic hydrogen, where the incidentradiation is proposed to be UV continuum radiation around Lyβ inthe former case and the He II λ1025 emission line arising fromn=6-->n=2 transitions for the latter feature. We remove the Hαwings by a template Raman scattering wing profile and subtract the [NII] λ6548 line using the 3 times stronger [N II] λ6583feature in order to isolate the He II Raman-scattered 6545 Å line.We obtain the flux ratio F6545/F6560=0.24 of theHe II λ6560 emission line and the 6545 Å feature for V1016Cyg. Under the assumption that the He II emission from this object isisotropic, this ratio is converted to the ratioΦ6545/Φ1025=0.17 of the number of theincident photons and that of the scattered photons. This implies thatthe scattering region with H I column densityNHI>=1020 cm-2 covers 17% of theemission region. By combining the presumed binary period of ~100 yr forthis system we infer that a significant fraction of the slow stellarwind from the Mira component is ionized and that the scattering regionaround the Mira extends a few tens of AU, which is closely associatedwith the mass loss process of the Mira component. It is argued that theRaman-scattered He II λ6545 line is an important and useful toolto investigate the mass-loss process occurring in the late stage ofstellar evolution.
| The kinematics of the most oxygen-poor planetary nebula PN G 135.9+55.9
PN G 135.9+55.9 is a compact, high excitation nebula that has beenidentified recently as the most oxygen-poor halo planetary nebula. Givenits very peculiar characteristics and potential implications in therealms of stellar and Galactic evolution, additional data are needed tofirmly establish its true nature and evolutionary history. Here wepresent the first long-slit, high spectral resolution observations ofthis object in the lines of Hα and He Iilambda 4686. Theposition-velocity data are shown to be compatible with theinterpretation of PN G 135.9+55.9 being a halo planetary nebula. In bothemission lines, we find the same two velocity components thatcharacterize the kinematics as that of an expanding elliptical envelope.The kinematics is consistent with a prolate ellipsoidal model with axisratio about 2:1, a radially decreasing emissivity distribution, avelocity distribution that is radial, and an expansion velocity of 30 kms-1 for the bulk of the material. To fit the observed lineprofiles, this model requires an asymmetric matter distribution, withthe blue-shifted emission considerably stronger than the red-shiftedemission. We find that the widths of the two velocity components aresubstantially wider than those expected due to thermal motions, butkinematic structure in the projected area covered by the slit appears tobe sufficient to explain the line widths. The present data also rule outthe possible presence of an accretion disk in the system that could havebeen responsible for a fraction of the Hα flux, furthersupporting the planetary nebula nature of PN G 135.9+55.9.
| The relation between Zanstra temperature and morphology in planetary nebulae
We have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology.
| Very Large Telescope Echelle Spectrophotometry of the Planetary Nebula NGC 5307 and Temperature Variations
Echelle spectrophotometry of the planetary nebula NGC 5307 is presented.The data consist of Very Large Telescope Ultraviolet Visual EchelleSpectrograph observations in the 3100-10360 Å range. Electrontemperatures and densities have been determined using different lineintensity ratios. We determine the H, He, C, and O abundances based onrecombination lines; these abundances are almost independent of thetemperature structure of the nebula. We also determine the N, O, Ne, S,Cl, and Ar abundances based on collisionally excited lines; the ratiosof these abundances relative to that of H depend strongly on thetemperature structure of the nebula. From the O II/[O III] lineintensity ratios we find a t2=0.056+/-0.005. The chemicalcomposition of NGC 5307 is compared with those of the Sun and the OrionNebula. From the study of the relative intensities of the O IIrecombination lines of multiplet 1 in this and other nebulae, it isfound that for electron densities smaller than about 5000cm-3 collisional redistribution is not complete; this effecthas to be taken into account to derive the O abundances for those casesin which not all the lines of the multiplet are observed. From the O IIλ4649 versus Ne(Cl III) diagram we find a criticalelectron density of 1325 cm-3 for collisional redistributionof the O II lines of multiplet 1. Based on this diagram, we also arguethat the O II and the [O III] lines originate in the same regions. Wealso find that the radial velocities and the FWHM of the O II and [OIII] lines in NGC 5307 are similar, supporting the previous result.These two results imply that for NGC 5307 and probably for many othergaseous nebulae chemical inhomogeneities are not responsible for thelarge temperature fluctuations observed.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 68.C-0149(A).
| Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinates
We have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}
| Weak Emission-Line Central Stars of Planetary Nebulae
We present new spectroscopic data of 14 candidates for the weakemission-line type of central stars of planetary nebulae ([WELS]). Witha medium resolution of about 2 Å, we confirmed the main spectralcharacteristics of these stars but found that two objects of our sampleare in fact early-type [WC] ([WCE]) stars. The λ4650 and C IVλλ 5801, 5812 features are resolved, and they were used ina tentative classification. Based on the C IV lines the [WELS] stars ofour sample can be divided qualitatively into two main groups: clearemission and very weak or no emission. We compared mean equivalentwidths of carbon and oxygen lines of the [WELS] stars with those of the[WCE] and [WCL] types. The results are not conclusive but point in favorof an increase in temperature from [WCL] to [WCE], followed perhaps by adecrease from [WCE] to [WELS].Based on observations made with the 1.52 m telescope at the EuropeanSouthern Observatory (La Silla, Chile) under agreement with theObservatório Nacional, Brazil.
| Broad Hα Wings in Nebulae around Evolved Stars and in Young Planetary Nebulae
Eleven objects that have been reported as proto-planetary nebula or asyoung planetary nebulae that show very extended Hα wings arepresented. The extension of these wings is larger than 800 kms-1. Data for two symbiotic stars that show this samecharacteristic are also presented. Raman scattering is the mechanismthat best explains the wings in 10 of the PNe and in the two symbioticstars. In the PN IRAS 20462+3416 the wing profile can be explained byvery intense stellar wind.
| Optical spectrum of the planetary nebula M 2-24
We have obtained medium-resolution, deep optical long-slit spectra ofthe bulge planetary nebula (PN) M 2-24. The spectrumcovers the wavelength range from 3610-7330 Å. Over two hundredemission lines have been detected. The spectra show a variety of opticalrecombination lines (ORLs) from C, N, O and Ne ions. The diagnosticdiagram shows significant density and temperature variations across thenebula. Our analysis suggests that the nebula has a dense centralemission core. The nebula was thus studied by dividing it into tworegions: 1) a high ionization region characterized by an electrontemperature of Te=16 300 K and a density of logNe(cm-3) = 6.3; and 2) a low ionization regionrepresented by Te=11 400 K and logNe(cm-3) = 3.7. A large number of ORLs from C, N,O and Ne ions have been used to determine the abundances of theseelements relative to hydrogen. In general, the resultant abundances arefound to be higher than the corresponding values deduced fromcollisionally excited lines (CELs). This bulge PN is found to have largeenhancements in two alpha -elements, magnesium and neon.Full Table 2 is available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.126.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/545
| Quantitative classification of WR nuclei of planetary nebulae
We analyse 42 emission-line nuclei of Planetary Nebulae (PNe), in theframework of a large spectrophotometric survey of [WC] nuclei of PNeconducted since 1994, using low/medium resolution spectra obtained atESO and at OHP. We construct a grid of selected line-intensities(normalized to C Iv-5806 Å= 100) ordered by decreasing ionisationpotential going from 871 to 24 eV. In this grid, the stars appear tobelong clearly to prominent O (hot [WO1-4] types) or C (cooler [WC4-11]types) line-sequences, in agreement with the classification of massiveWR stars applied to Central Stars of Planetary Nebulae (CSPNe) byCrowther et al. \cite{crowther98} (CMB98). We propose 20 selected lineratios and the FWHM of C Iv and C Iii lines as classificationdiagnostics, which agree well with the 7 line ratios and the FWHMproposed by CMB98. This classification based on ionisation is related tothe evolution of the temperature and of the stellar wind, reflecting themass-loss history. In particular, inside the hot [WO4]-class, wediscover four stars showing very broad lines over the whole spectralrange. These stars possibly mark the transition from the initialmomentum-driven phase to the later energy-driven phase of the CSPNealong their evolution from the post-Asymptotic Giant Branch (post-AGB)phase through [WC] late, [WC4] and [WO]-types. The HR diagram and thediagram linking the terminal velocity and the temperature indicatehighly dispersed values of the stellar mass for our sample, around amean mass higher than for normal CSPNe. The distribution of the 42 starsalong the ionisation sequence shows 24% of [WO1-3], 21% of [WO4], 17% of[WC4] hot stars, and 26% of [WC9-11] cool stars. The [WC5-8] classesremain poorly represented (12%). This distribution is confirmed on thebasis of a large compilation of the 127 known emission-lines CSPNe,which represent about 5% of the known PNe.Based on observations obtained at the European Southern Observatory(ESO), La Silla (Chile), and at the Observatoire de Haute-Provence (OHP,France).Table \ref{liste} is only available in electronic form athttp://www.edpsciences.org
| Comparative Absorption and Emission Abundance Analyses of Nebulae: Ion Emission Densities for IC 418
Recent analyses of nebular spectra have resulted in discrepantabundances from CNO forbidden and recombination lines. We considerindependent methods of determining ion abundances for emission nebulae,comparing ion emission measures with column densities derived fromresonance absorption lines viewed against the central star continuum.Separate analyses of the nebular emission lines and the stellar UVabsorption lines yield independent abundances for ions, and their ratiocan be expressed in terms of a parameterem, the ``emission density'' for eachion. Adequate data for this technique are still scarce, but separateanalyses of spectra of the planetary nebula and central star of IC 418do show discrepant abundances for several ions, especially Fe II. Thediscrepancies are probably due to the presence of absorbing gas thatdoes not emit and/or to uncertain atomic data and excitation processes,and they demonstrate the importance of applying the technique ofcombining emission- and absorption-line data in deriving abundances fornebulae.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the STScI, which is operated by AURA, Inc., under NASAcontract NAS 5-26555, and on observations made at CTIO/NOAO, which isoperated by AURA, Inc., under cooperative agreement with the NSF.
| A spectroscopic study of the bipolar planetary nebula Mz 3
We have obtained a medium-resolution, deep optical long-slit spectrum ofthe bipolar planetary nebula Mz 3. The spectrum covers the wavelengthrange 3420-7400Å. Over 200 emission lines have been detected, manyof them permitted and forbidden transitions from Fe+,Fe2+ and Fe3+ and other iron-group elements. Thespectra have been used to determine nebular thermal and densitystructures and elemental abundances. The very rich and prominent [FeIII]emission lines observed in the optical spectrum of Mz 3 are found tooriginate exclusively from an unresolved emission region centred on thecentral star. The relative intensities of [FeIII] lines arising from thesame upper level are in good agreement with the theoretical predictions.The [FeIII] lines detected in Mz 3, arising from levels of differentexcitation energies and critical densities, provide powerful diagnostictools to probe the physical conditions in the central emitting region.We find that all the observed [FeIII] diagnostic line ratiosconsistently yield an electron temperature of Te= 11 000 Kand a density of logNe (cm-3) = 6.5. The lattervalue is close to the densities where the ratios of these [FeIII]diagnostic lines are most sensitive to density variations, suggestingthat the density in the central emission core could be even higher. Incontrast, all the other standard nebular density- andtemperature-diagnostic line ratios, all of lower critical densities thanthe [FeIII] lines and therefore only useful at densities<=106 cm-3, yield consistently lower electrondensities, with the resultant values correlating with their criticaldensities. This indicates that the central dense emission core has ahighly stratified density structure such that forbidden lines ofrelatively low critical densities are collisionally suppressed inhigh-density regions. Given the highly stratified thermal and densitystructures of the core emission region, no reliable elemental abundancescan be determined for this region, except possibly for iron, for whichwe find an abundance of approximately half the solar value. In contrastto the dense central core, emission from the extended bipolar lobes ofMz 3 are well represented by a mean electron temperature of 6800 K anddensity of 5200 cm-3. The elemental abundances derived forthis region, on a logarithmic scale where H = 12, are He = 10.93, C =8.41, N = 8.37, O = 8.50, Ne = 7.64, S = 7.15, Cl = 5.28 and Ar = 7.14.In all cases, the abundances are close to the average values deduced forGalactic planetary nebulae. In particular, there is no evidence of Heenrichment, as claimed in previous studies. The newly derivedheavy-element abundances are significantly higher than those publishedin the literature. In those earlier studies, as a result of thecontamination of emission from the dense central core, the averageelectron temperature in Mz 3 was significantly overestimated, resultingin grossly underestimated heavy-element abundances. However, Mz 3 doesseem to have a relatively high N/O abundance ratio, in line with theresult derived from the ISO observations of the far-infraredfine-structure lines.
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