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An Atlas of [N II] and [O III] Images and Spectra of Planetary Nebulae
We present an atlas of Hubble Space Telescope images and ground-based,long-slit, narrowband spectra centered on the 6584 Å line of [NII] and the 5007 Å line of [O III]. The spectra were obtained fora variety of slit positions across each target (as shown on the images)in an effort to account for nonspherical nebular geometries in a robustmanner. We have extended the prolate ellipsoidal shell model originallydevised by Aaquist, Zhang, and Kwok to generate synthetic images, aswell as long-slit spectra. Using this model, we have derived basicparameters for the subsample of PNe that present ellipsoidal appearancesand regular kinematic patterns. We find differences between ourparameters for the target PNe as compared to those of previous studies,which we attribute to increased spatial resolution for our image dataand the inclusion of kinematic data in the model fits. The data andanalysis presented in this paper can be combined with detections ofnebular angular expansion rates to determine precise distances to the PNtargets.
| HST and VLT observations of the symbiotic star Hen 2-147. Its nebular dynamics, its Mira variable and its distance
Aims.We investigate the dynamics of the nebula around the symbiotic starHen 2-147, determine its expansion parallax, and compare it with thedistance obtained via the period-luminosity relation for its Miravariable. Methods: A combination of multi-epoch HST images and VLTintegral field high-resolution spectroscopy is used to study the nebulardynamics both along the line of sight and in the plane of the sky. Theseobservations allow us to build a 3D spatio-kinematical model of thenebula, which, together with the measurement of its apparent expansionin the plane of the sky over a period of 3 years, provides the expansionparallax for the nebula. Additionally, SAAO near-infrared photometryobtained over 25 years is used to determine the Mira pulsation periodand derive an independent distance estimation via the period-luminosityrelationship for Mira variables. Results: The geometry of the nebula isfound to be that of a knotty annulus of ionized gas inclined to theplane of sky and expanding with a velocity of ~90 km s-1. Astraightforward application of the expansion parallax method provides adistance of 1.5 ± 0.4 kpc, which is a factor of two lower thanthe distance of 3.0 ± 0.4 kpc obtained from the period-luminosityrelationship for the Mira (which has a pulsation period of 373 days).The discrepancy is removed if, instead of expanding matter, we areobserving the expansion of a shock front in the plane of the sky. Thisshock interpretation is further supported by the broadening of thenebular emission lines.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc. under NASAcontract No. NAS5-26555; on observations obtained at the 8 m VLTtelescope of the European Southern Observatory in Chile; and onobservations made at the South African Astronomical Observatory. Themovie (Fig. 3) is only available in electronic form athttp://www.aanda.org Table 2 is only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/481
| 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.
| Photospheric opacity and over-expanded envelopes of asymptotic giant branch stars
I suggest that the behavior of the photospheric opacity in oxygen-rich(similar to solar abundance) upper asymptotic giant branch stars maycause these stars to substantially expand for a few thousand years. Iterm this process over-expansion. This may occur when the photospheric(effective) temperature drops to Tp 3000 K, and becausethe opacity sharply increases as temperature further decreases down toTp 2000 K. The much higher opacity implies a much lowerphotospheric density, which stabilizes the envelope structure. As massloss proceeds, the star eventually contracts to become a post-asymptoticgiant branch star. Some possible outcomes of the over-expanded phase arediscussed: (1) The over-expanded phase may be connected to the formationof semi-periodic concentric arcs (rings; shells); (2) the over-expandedphase may be related to the positive correlation between the mass lossrate and the transition to axisymmetric mass loss geometry; and (3) anover-expanded asymptotic giant branch star, which doubles its radius, issomewhat more likely to swallow a low mass companion.
| 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 abundance discrepancy - recombination line versus forbidden line abundances for a northern sample of galactic planetary nebulae
We present deep optical spectra of 23 galactic planetary nebulae, whichare analysed in conjunction with archival infrared and ultravioletspectra. We derive nebular electron temperatures based on standardcollisionally excited line (CEL) diagnostics as well as the hydrogenBalmer jump and find that, as expected, the Balmer jump almost alwaysyields a lower temperature than the [OIII] nebular-to-auroral lineratio. We also make use of the weak temperature dependence of helium andOII recombination line ratios to further investigate the temperaturestructure of the sample nebulae. We find that, in almost every case, thederived temperatures follow the relation , which is the relationpredicted by two-component nebular models in which one component is coldand hydrogen-deficient. Te(OII) may be as low as a fewhundred Kelvin, in line with the low temperatures found for thehydrogen-deficient knots of Abell 30 by Wesson, Liu and Barlow.Elemental abundances are derived for the sample nebulae from both CELsand optical recombination lines (ORLs). ORL abundances are higher thanCEL abundances in every case, by factors ranging from 1.5 to 12. Fiveobjects with O2+ abundance discrepancy factors greater than 5are found. DdDm 1 and Vy 2-2 are both found to have a very largeabundance discrepancy factor of 11.8.We consider the possible explanations for the observed discrepancies.From the observed differences between Te(OIII) andTe(BJ), we find that temperature fluctuations cannot resolvethe abundance discrepancies in 22 of the 23 sample nebulae, implyingsome additional mechanism for enhancing ORL emission. In the oneambiguous case, the good agreement between abundances derived fromtemperature-insensitive infrared lines and temperature-sensitive opticallines also points away from temperature fluctuations being present. Theobserved recombination line temperatures, the large abundancediscrepancies and the generally good agreement between infrared andoptical CEL abundances all suggest instead the existence of a coldhydrogen-deficient component within the `normal' nebular gas. The originof this component is as yet unknown.
| 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.
| 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.
| Radial-Velocity Survey of Central Stars of Southern Planetary Nebulae .
We have monitored selected southern-hemisphere planetary-nebula nuclei(PNNi) in order to search for radial-velocity (RV) variations. Theobservations have been carried out regularly since early 2003 with theSMARTS Consortium 1.5-m telescope and Cassegrain spectrograph at CerroTololo Inter-American Observatory, Chile. This study is a followup to anearlier survey of northern PNNi made by \citet{demarco04}, whichsuggested that there is a high incidence of RV variability among PNNi.If the variations are due to motion in binary orbits, the fraction ofclose binaries among PNNi must be very high, suggesting that mostplanetary nebulae are ejected through binary-star processes, such ascommon-envelope interactions. We presente here the results of thesouthern portion of our RV survey. Preliminary results indicate that thefraction of variable RVs is also very high among southern PNNi.
| 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.
| 12C/13C Ratio in Planetary Nebulae from the IUE Archives
We investigated the abundance ratio of 12C/13C inplanetary nebulae by examining emission lines arising from C III2s2p3Po2,1,0-->2s21S0.Spectra were retrieved from the International Ultraviolet Explorerarchives, and multiple spectra of the same object were co-added toachieve improved signal-to-noise ratio. The 13C hyperfinestructure line at 1909.6 Å was detected in NGC 2440. The12C/13C ratio was found to be ~4.4+/-1.2. In allother objects, we provide an upper limit for the flux of the 1910Å line. For 23 of these sources, a lower limit for the12C/13C ratio was established. The impact on ourcurrent understanding of stellar evolution is discussed. The resultinghigh-signal-to-noise ratio C III spectrum helps constrain the atomicphysics of the line formation process. Some objects have the measured1907/1909 Å flux ratio outside the low-electron densitytheoretical limit for 12C. A mixture of 13C with12C helps to close the gap somewhat. Nevertheless, someobserved 1907/1909 Å flux ratios still appear too high to conformto the currently predicted limits. It is shown that this limit, as wellas the 1910/1909 Å flux ratio, are predominantly influenced byusing the standard partitioning among the collision strengths for themultiplet1S0-3PoJaccording to the statistical weights. A detailed calculation for thefine-structure collision strengths between these individual levels wouldbe valuable.
| Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IV. Synthesis and the Sulfur Anomaly
We have compiled a large sample of O, Ne, S, Cl, and Ar abundances thathave been determined for 85 Galactic planetary nebulae in a consistentand homogeneous manner using spectra extending from 3600 to 9600Å. Sulfur abundances have been computed using the near-IR lines of[S III] λλ9069, 9532 along with [S III] temperatures. Wefind average values, expressed logarithmically with a standarddeviation, of log(S/O)=-1.91+/-0.24, log(Cl/O)=-3.52+/-0.16, andlog(Ar/O)=-2.29+/-0.18, numbers consistent with previous studies of bothplanetary nebulae and H II regions. We also find a strong correlationbetween [O III] and [S III] temperatures among planetary nebulae. Inanalyzing abundances of Ne, S, Cl, and Ar with respect to O, we find atight correlation for Ne-O, and loose correlations for Cl-O and Ar-O.All three trends appear to be colinear with observed correlations for HII regions. S and O also show a correlation, but there is a definiteoffset from the behavior exhibited by H II regions and stars. We suggestthat this S anomaly is most easily explained by the existence ofS+3, whose abundance must be inferred indirectly when onlyoptical spectra are available, in amounts in excess of what is predictedby model-derived ionization correction factors in PNe. Finally for thedisk PNe, abundances of O, Ne, S, Cl, and Ar all show gradients whenplotted against Galactocentric distance. The slopes are statisticallyindistinguishable from one another, a result which is consistent withthe notion that the cosmic abundances of these elements evolve inlockstep.
| Indications of a Large Fraction of Spectroscopic Binaries among Nuclei of Planetary Nebulae
Previous work indicates that about 10% of planetary-nebula nuclei (PNNi)are photometrically variable short-period binaries with periods of hoursto a few days. These systems have most likely descended fromcommon-envelope (CE) interactions in initially much wider binaries.Population-synthesis studies suggest that these very close pairs couldbe the short-period tail of a much larger post-CE binary population withperiods of up to a few months. We have initiated a radial-velocity (RV)survey of PNNi with the WIYN 3.5 m telescope and Hydra spectrograph,which is aimed at discovering these intermediate-period binaries. Wepresent initial results showing that 10 out of 11 well-observed PNNihave variable RVs, suggesting that a significant binary population maybe present. However, further observations are required because we haveas yet been unable to fit our sparse measurements with definite orbitalperiods and because some of the RV variability might be due tovariations in the stellar winds of some of our PNNi.
| The relation between Zanstra temperature and morphology in planetary nebulae
We have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology.
| Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinates
We have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}
| 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.
| Angular dimensions of planetary nebulae
We have measured angular dimensions of 312 planetary nebulae from theirimages obtained in Hα (or Hα + [NII]). We have appliedthree methods of measurements: direct measurements at the 10% level ofthe peak surface brightness, Gaussian deconvolution and second-momentdeconvolution. The results from the three methods are compared andanalysed. We propose a simple deconvolution of the 10% levelmeasurements which significantly improves the reliability of thesemeasurements for compact and partially resolved nebulae. Gaussiandeconvolution gives consistent but somewhat underestimated diameterscompared to the 10% measurements. Second-moment deconvolution givesresults in poor agreement with those from the other two methods,especially for poorly resolved nebulae. From the results of measurementsand using the conclusions of our analysis we derive the final nebulardiameters which should be free from systematic differences between small(partially resolved) and extended (well resolved) objects in our sample.Table 1 is only available in electronic form athttp://www.edpsciences.org
| 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
| The relation between elemental abundances and morphology in planetary nebulae
An investigation of the variation of elemental abundances with planetarynebula morphology is of considerable interest, since it has a bearingupon how such sources are formed, and from which progenitors they areejected. Recent advances in morphological classification now enable usto assess such trends for a statistically significant number of sources.We find, as a result, that the distribution N[log(X/H)] of sources withrespect to elemental abundance (X/H) varies between the differingmorphologies. Circular sources tend to peak towards low abundancevalues, whilst bipolar nebulae (BPNe) peak towards somewhat highervalues. This applies for most elemental species, although it is perhapsleast apparent for oxygen. In contrast, elliptical sources appear todisplay much broader functions N[log(X/H)], which trespass upon thedomains of both circular and elliptical planetary nebulae (PNe).We take these trends to imply that circular sources derive fromlower-mass progenitors, bipolar sources from higher-mass stars, and thatelliptical nebulae derive from all masses of progenitor, high and low.Whilst such trends are also evident in values of mean abundance, they are much less clear. Only in the cases of He/H, N/H,Ne/H and perhaps Ar/H is there evidence for significant abundancedifferences.Certain BPNe appear to possess low abundance ratios He/H and Ar/H, andthis confirms that a few such outflows may arise from lower-massprogenitors. Similarly, we note that ratios are quite modestin elliptical planetary nebulae, and not much different from those forcircular and bipolar PNe; a result that conflicts with the expectationsof at least one model of shell formation.
| Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical properties
We present a comprehensive observational study of haloes aroundplanetary nebulae (PNe). Deep Hα+[NII] and/or [OIII] narrow-bandimages have been obtained for 35 PNe, and faint extended haloes havebeen newly discovered in the following 10 objects: Cn 1-5, IC 2165, IC2553, NGC 2792, NGC 2867, NGC 3918, NGC 5979, NGC 6578, PB 4, andpossibly IC 1747. New deep images have also been obtained of other knownor suspected haloes, including the huge extended emission around NGC3242 and Sh 2-200. In addition, the literature was searched, andtogether with the new observations an improved data base containing some50 PN haloes has been compiled.The halo sample is illustrated in an image atlas contained in thispaper, and the original images are made available for use by thescientific community at http://www.ing.iac.es/~rcorradi/HALOES/.The haloes have been classified following the predictions of modernradiation-hydrodynamical simulations that describe the formation andevolution of ionized multiple shells and haloes around PNe. According tothe models, the observed haloes have been divided into the followinggroups: (i) circular or slightly elliptical asymptotic giant branch(AGB) haloes, which contain the signature of the last thermal pulse onthe AGB; (ii) highly asymmetrical AGB haloes; (iii) candidaterecombination haloes, i.e. limb-brightened extended shells that areexpected to be produced by recombination during the late post-AGBevolution, when the luminosity of the central star drops rapidly by asignificant factor; (iv) uncertain cases which deserve further study fora reliable classification; (v) non-detections, i.e. PNe in which no halois found to a level of <~10-3 the peak surface brightnessof the inner nebulae.We discuss the properties of the haloes: detection rate, morphology,location of the central stars in the Hertzsprung-Russell diagram, sizes,surface brightness profiles, and kinematical ages. Among the mostnotable results, we find that, as predicted by models, ionized AGBhaloes are a quite common phenomenon in PNe, having been found in 60 percent of elliptical PNe for which adequately deep images exist. Another10 per cent show possible recombination haloes. In addition, using thekinematical ages of the haloes and inner nebulae, we conclude that mostof the PNe with observed AGB haloes have left the AGB far from a thermalpulse, at a phase when hydrogen burning is the dominant energy source.We find no significant differences between the AGB haloes ofhydrogen-poor and hydrogen-rich central stars.
| 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.
| Abundances of [WC] Central Stars of PN and the Double Dust Chemistry Problem (invited review)
Not Available
| Introductory Review (invited review)
Not Available
| The Dynamical Evolution of the Circumstellar Gas around Low- and Intermediate-Mass Stars. II. The Planetary Nebula Formation
We have studied the effect of the mass of the central star (CS) on thegas evolution during the planetary nebula (PN) phase. We have performednumerical simulations of PN formation using CS tracks for six stellarcore masses corresponding to initial masses from 1 to 5Msolar. The gas structure resulting from the previousasymptotic giant branch (AGB) evolution is used as the startingconfiguration. The formation of multiple shells is discussed in thelight of our models, and the density, velocity, and Hα emissionbrightness profiles are shown for each stellar mass considered. We havecomputed the evolution of the different shells in terms of radius,expansion velocity, and Hα peak emissivity. We find that theevolution of the main shell is controlled by the ionization front ratherthan by the thermal pressure provided by the hot bubble during the earlyPN stages. This effect explains why the kinematical ages overestimatethe age in young CSs. At later stages in the evolution and for low-massprogenitors the kinematical ages severely underestimate the CS age.Large (up to 2.3 pc), low surface brightness shells (less than 2000times the brightness of the main shell) are formed in all of our models(with the exception of the 5 Msolar model). These PN haloscontain most of the ionized mass in PNe, which we find is greatlyunderestimated by the observations because of the low surface brightnessof the halos.
| Enigmatic Low-Velocity Jet-Like Features in Planetary Nebulae
We are developing a project aimed at studying the physical properties,origin and evolution of low-ionization structures in planetary nebulae.Within this project we have identified a number of pairs of highlycollimated low-ionization jet-like features (Gonçalves et al.2001). In spite of being very similar to real jets, they have theintriguing property of possessing expansion velocities which are verylow, or at least not significantly different from, that of the shells inwhich they are embedded. In this contribution we discuss our data onthese fake jets (Corradi et al. 1997, 1999) and compare them withexisting theoretical models for the formation of collimated structuresin PNe. These enigmatic jet-like systems are not easily accounted forwithin the theoretical scenarios that deal with collimated features inPNe.
| Gas temperature and excitation classes in planetary nebulae
Empirical methods to estimate the elemental abundances in planetarynebulae usually use the temperatures derived from the [O III] and [N II]emission-line ratios, respectively, for the high- and low-ionizationzones. However, for a large number of objects these values may not beavailable. In order to overcome this difficulty and allow a betterdetermination of abundances, we discuss the relationship between thesetwo temperatures. Although a correlation is not easily seen when asample of different PNe types is used, the situation is improved whenthey are gathered into excitation classes. From [OII]/[OIII] andHeII/HeI line ratios, we define four excitation classes. Then, usingstandard photoionization models which fit most of the data, a linearrelation between the two temperatures is obtained for each of the fourexcitation classes. The method is applied to several objects for whichonly one temperature can be obtained from the observed emission linesand is tested by recalculation of the radial abundance gradient of theGalaxy using a larger number of PNe. We verified that our previousgradient results, obtained with a smaller sample of planetary nebulae,are not changed, indicating that the temperature relation obtained fromthe photoionization models are a good approximation, and thecorresponding statistical error decreases as expected. Tables 3-5, 7 and9 are only available in electronic form at http://www.edpsciences.org
| Hubble Space Telescope Expansion Parallaxes of the Planetary Nebulae NGC 6578, NGC 6884, NGC 6891, and IC 2448
We have combined two epochs of Hubble Space Telescope WFPC2 imaging datawith ground-based expansion velocities to determine distances to threeplanetary nebulae (NGC 6578, NGC 6884, and IC 2448). We used twovariants of the expansion parallax technique-a gradient method and amagnification method-to determine the distances. The results from thetwo methods agree to within the errors. A fourth nebula was included inthe study (NGC 6891), but the expansion was too small to determine thedistance, and only a lower limit was obtained. This is the second paperin a series that will examine at least 24 nebulae in total. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by AURA, Inc.,under NASA contract NAS 5-26555.
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