Contents
Images
Upload your image
DSS Images Other Images
Related articles
Chemical compositions and plasma parameters of planetary nebulae with Wolf-Rayet and wels type central stars
Aims.Chemical compositions and other properties of planetary nebulaearound central stars of spectral types [WC], [WO], and wels are comparedwith those of “normal” central stars, in order to clarifythe evolutionary status of each type and their interrelation. Methods:We use plasma diagnostics to derive from optical spectra the plasmaparameters and chemical compositions of 48 planetary nebulae. We alsoreanalyze the published spectra of a sample of 167 non-WR PN. Theresults as well as the observational data are compared in detail withthose from other studies of the objects in common. Results: We confirmthat [WC], [WO] and wels nebulae are very similar to those“normal” PN: the relation between [N II] and [O III]electron temperatures, abundances of He, N, O, Ne, S and Ar, and thenumber of ionizing photons show no significant differences. However,some differences are observed in their infrared (IRAS) properties. welsnebulae appear bluer than [WR] PN. The central star's spectral type isclearly correlated with electron density, temperature and excitationclass of the nebula, [WC] nebulae tend to be smaller than the othertypes. All this corroborates the view of an evolutionary sequence fromcool [WC 11] central stars inside dense, low excitation nebulae towardshot [WO 1] stars with low density, high excitation nebulae. The wels PN,however, appear to be a separate class of objects, not linked to WRPN byevolution: nebular excitation, electron temperature and density, and thenumber of ionizing photons all cover the whole range found in the othertypes. Their lower mean N/O ratio and slightlylower He/H suggestprogenitor stars less massive than for the other PN types. Furthermore,the differences between results of different works are dominated by thedifferences in observational data rather than differences in theanalysis methods.Based on observations obtained at the European Southern Observatory(ESO), La Silla, Chile. Table 3 and Appendices are only available inelectronic form at http://www.aanda.org Table with fluxes andintensities is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/265
| 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.
| The mean properties of planetary nebulae as a function of Peimbert class
Planetary nebulae are known to possess a broad range of abundances, andthese (with other characteristics) have been used to define five classesof outflow. Peimbert Type I sources, for instance, possess high N and Heabundances, filamentary structures, and low mean scaleheights above theGalactic plane, whilst those of Type III have much lower abundances,high peculiar velocities, and belong to the Galactic thick disc. Apartfrom some rather ill-defined indications, however, very little is knownconcerning their mean physical, spatial, structural, kinematic andthermal characteristics.We have performed a comprehensive study of all of these properties, andfind evidence for strong variations between the various Peimbertclasses. Certain of these differences are consistent with Type I sourceshaving the highest progenitor masses, although it seems that thesenebulae also possess the lowest rms densities and 5-GHz brightnesstemperatures. The latter results are in conflict with a range of recentmodelling.
| 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.
| Evolution from AGB to planetary nebula in the MSX survey
We investigate the evolution of oxygen- and carbon-rich AGB stars,post-AGB objects, and planetary nebulae using data collected mainly fromthe MSX catalogue. Magnitudes and colour indices are compared with thosecalculated from a grid of synthetic spectra that describe the post-AGBevolution beginning at the onset of the superwind. We find that carbonstars and OH/IR objects form two distinct sequences in the (K-[8.3])×([8.3]-[14.7]) MSX colour diagram. OH/IR objects are distributedin two groups: the bluest ones are crowded near [14.7]-[21.3]≃ 1and [8.3]-[14.7]≃ 2, and a second, redder group is spread over alarge area in the diagram, where post-AGB objects and planetary nebulaeare also found. High mass-loss rate OH/IR objects, post-AGB stars, andplanetary nebulae share the same region in the (K-[8.3])×([8.3]-[14.7]) and [14.7]-[21.3]×([8.3]-[14.7]) colour-colourdiagrams. This region in the diagram is clearly separated from a bluerone where most OH/IR stars are found. We use a grid of models ofpost-AGB evolution, which are compared with the data. The gap in thecolour-colour diagrams is interpreted as the result of the rapidtrajectory in the diagram of the stars that have just left the AGB.Based on results obtained by the MSX survey.Tables 1 to 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/565
| The Use of Line Excitation Mapping to Investigate Planetary Nebula Morphologies
It is now well established that circular, elliptical, and bipolarplanetary nebulae (PNe) possess differing physical and spatialcharacteristics. Not only are their structures quite distinct, but theyalso appear to possess differing Zanstra temperatures, abundances,expansion velocities, brightness temperatures, and scale heights abovethe Galactic plane. We report here a further sensitive way in which thedifferences between these outflows may be illustrated. We shall notethat the nebulae posses varying ranges of emission line ratio, and thatthis is likely to arise as a consequence of their differing progenitormasses. Similarly, we point out the potential of line-ratio mapping foranalyzing other morphological classes, as well as for establishing theuniqueness of their populations, and their relative progenitor masses.Thus we find that sources containing jets often possess low lineemission intensities relative to hydrogen, suggesting that theirprogenitors may have low overall masses. This conclusion is supported bya variety of independent observational evidence. We also show thatirregular sources appear to represent a distinct class of outflows. Itis likely that their mean progenitor masses are greater than those ofelliptical PNe, but less than thoseof the bipolar outflows.
| 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.
| The distances of Type I planetary nebulae
The distances D of planetary nebulae (PNe) are still extremelyuncertain. Although a variety of methods have been used to evaluate thisparameter, these are often in conflict, and subject to large random andsystematic errors. It is therefore important to evaluate D using as manyindependent procedures as possible. We outline here one further way inwhich this parameter may be assessed. It is noted that where the nebularmass range is narrow, then one might expect observed PNe radii to beroughly similar. This, where it occurs, would also result in acorrelation between their angular diameters Θ, and distances D.We find that just such a trend occurs for Type I nebulae, and we employthis to determine distances to a further 44 such outflows. Our meanvalues of D appear similar to those of Zhang [ApJS 98 (1995) 659],implying a relatively long PNe distance scale.
| 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.
| The 3-D ionization structure and evolution of NGC 7009 (Saturn Nebula)
Tomographic and 3-D analyses for extended, emission-line objects areapplied to long-slit ESO NTT + EMMI high-resolution spectra of theintriguing planetary nebula NGC 7009, covered at twelve position angles.We derive the gas expansion law, the diagnostics and ionic radialprofiles, the distance and the central star parameters, the nebularphoto-ionization model and the spatial recovery of the plasma structureand evolution. The Saturn Nebula (distance≃1.4 kpc, age≃6000yr, ionized mass≃0.18 Mȯ) consists of severalinterconnected components, characterized by different morphology,physical conditions, excitation and kinematics. We identify four``large-scale'', mean-to-high excitation sub-systems (the internalshell, the main shell, the outer shell and the halo), and as many``small-scale'' ones: the caps (strings of low-excitation knots withinthe outer shell), the ansae (polar, low-excitation, likely shockedlayers), the streams (high-excitation polar regions connecting the mainshell with the ansae), and an equatorial, medium-to-low excitationpseudo-ring within the outer shell. The internal shell, the main shell,the streams and the ansae expand at Vexp≃4.0 × Rarcsec km s-1, the outer shell, the caps and the equatorialpseudo-ring at Vexp≃3.15 × R arcsec kms-1, and the halo at Vexp≃10 kms-1. We compare the radial distribution of the physicalconditions and the line fluxes observed in the eight sub-systems withthe theoretical profiles coming from the photo-ionization code CLOUDY,inferring that all the spectral characteristics of NGC 7009 areexplainable in terms of photo-ionization by the central star, a hot (log T* ≃4.95) and luminous ( logL*/Lȯ≃3.70) 0.60-0.61Mȯ post-AGB star in the hydrogen-shell nuclear burningphase. The 3-D shaping of the Saturn Nebula is discussed within anevolutionary scenario dominated by photo-ionization and supported by thefast stellar wind: it begins with the superwind ejection (firstisotropic, then polar deficient), passes through the neutral, transitionphase ({lasting} ≃3000 yr), the ionization start (occurred≃2000 yr ago), and the full ionization of the main shell(≃1000 yr ago), at last reaching the present days: the wholenebula is optically thin to the UV stellar flux, except the caps (meanlatitude condensations in the outer shell, shadowed by the main shell)and the ansae (supersonic ionization fronts along the major axis).Based on observations made with: ESO Telescopes at the La SillaObservatories (program ID 65.I-0524), and the NASA/ESA Hubble SpaceTelescope, obtained from the data archive at the Space TelescopeInstitute. Observing programs: GO 6117 (P.I. Bruce Balick), GO 6119(P.I. Howard Bond) and GO 8390 (P.I. Arsen Hajian). STScI is operated bythe association of Universities for Research in Astronomy, Inc. underthe NASA contract NAS 5-26555. We extensively apply the photo-ionizationcode CLOUDY, developed at the Institute of Astronomy of the CambridgeUniversity (Ferland et al. 1998).
| Characteristics of Planetary Nebulae with [WC] Central Stars
We have analyzed the plasma diagnostics (electron densities andtemperatures and abundance ratios), and the kinematics of a large sampleof planetary nebulae around [WC] stars by means of high resolutionspectra. The results have been compared with characteristics ofplanetary nebulae around WELS and non-WR central stars. We find that theproportion of nitrogen rich nebulae is larger in WRPNe than innon-WRPNe. None of the 9 nebulae around WELS in our sample showsN-enrichment. WRPNe have larger expansion velocities and/or largerturbulence than non-WRPNe demonstrating that the mechanical energy ofthe massive [WC] stellar wind largely affects the kinematical behaviorof nebulae. A weak relation between stellar temperature and expansionvelocities has been found for all kind of nebulae, indicating that oldernebulae expand faster. The effect is more important for WRPNe. Thiscould be useful in testing the evolutionary sequence [WC]-late ->[WC]-early, proposed for [WC] stars.
| 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}
| 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
| Abundances of [WC] Central Stars of PN and the Double Dust Chemistry Problem (invited review)
Not Available
| WR Central Stars (invited review)
Not Available
| 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
| Infrared properties of planetary nebulae with [WR] central stars
We have gathered from the literature near and mid infrared (photometricand spectroscopic) data for Galactic planetary nebulae, with specialattention to planetary nebulae surrounding Wolf-Rayet type central stars([WR] PN). These data have been analyzed to obtain insight into thedust-properties of [WR] PN and their evolutionary status. We have foundthat a sizeable fraction of [WR] PN seems to contain hot dust (1000-2000K), probably located in the winds of the central stars. The mean dusttemperature is shown to decrease with decreasing [WC] spectral type.This is in line with suggestions that the [WC] sequence is anevolutionary sequence from late to early types. [WR] PN in differentdiagrams, when compared to those of non-[WR] PN, suggest that [WR] PNform a homogeneous class of planetary nebulae. There is an unusuallylarge proportion of [WR] PN showing PAH features in their spectra.
| Helium contamination from the progenitor stars of planetary nebulae: The He/H radial gradient and the ΔY / ΔZ enrichment ratio
In this work, two aspects of the chemical evolution of 4He inthe Galaxy are considered on the basis of a sample of disk planetarynebulae (PN). First, an application of corrections owing to thecontamination of 4He from the evolution of the progenitorstars shows that the He/H abundance by number of atoms is reduced by0.012 to 0.015 in average, leading to an essentially flat He/H radialdistribution. Second, a determination of the helium to heavy elementenrichment ratio using the same corrections leads to values in the range2.8 < ΔY / ΔZ < 3.6 for Y p = 0.23 and 2.0< ΔY / ΔZ < 2.8 for Y p = 0.24, in goodagreement with recent independent determinations and theoretical models.
| The distance scale of planetary nebulae
By collecting distances from the literature, a set of 73 planetarynebulae with mean distances of high accuracy is derived. This sample isused for recalibration of the mass-radius relationship, used by manystatistical distance methods. An attempt to correct for a statisticalpeculiarity, where errors in the distances influences the mass-radiusrelationship by increasing its slope, has been made for the first time.Distances to PNe in the Galactic Bulge, derived by this new method aswell as other statistical methods from the last decade, are then usedfor the evaluation of these methods as distance indicators. In order ofachieving a Bulge sample that is free from outliers we derive newcriteria for Bulge membership. These criteria are much more stringentthan those used hitherto, in the sense that they also discriminateagainst background objects. By splitting our Bulge sample in two, onewith optically thick (small) PNe and one with optically thin (large)PNe, we find that our calibration is of higher accuracy than most othercalibrations. Differences between the two subsamples, we believe, aredue to the incompleteness of the Bulge sample, as well as the dominanceof optical diameters in the ``thin'' sample and radio diameters in the``thick'' sample. Our final conclusion is that statistical methods givedistances that are at least as accurate as the ones obtained from manyindividual methods. Also, the ``long'' distance scale of Galactic PNe isconfirmed.
| Planetary Nebulae with [WR] nuclei
The first results on an extensive program for analyzing the nebular andstellar characteristics of planetary nebulae ionized by a [WR] centralstar are presented. Spacially resolved spectrophotometric data are usedfor the analysis .
| Galactic planetary nebulae with Wolf-Rayet nuclei. II. A consistent observational data set
We present high resolution spectrophotometric data for a sample of 34planetary nebulae with [WC] spectral type central stars (WRPNe) in ourGalaxy. The observed objects cover a wide range in stellarcharacteristics: early and late [WC] type stars, as well asweak-emission line stars (WELS). Physical conditions in the nebulae(electron density and temperatures) have been obtained from variousdiagnostic line ratios, and chemical abundances have been derived withthe usual empirical scheme. Expansion velocities were estimated in aconsistent manner from the line profiles for most objects of the sample.A statistical study was developed for the derived data in order to findfundamental relationships casting some light on the evolutionary statusof WRPNe. We found evidence for a strong electron temperature gradientin WRPNe which is related to nebular excitation. Such a gradient is notpredicted in simple photoionization models. Abundance ratios indicatethat there seems to be no preferential stellar mass for the Wolf-Rayetphenomenon to occur in the nucleus of a planetary nebula. Two objects, M1-25 and M 1-32, were found to have a very small Ne/O ratio, a propertydifficult to understand. We reexamined the relation between the nebularproperties of the WRPNe and the spectral types of the central stars. Ourdata confirm the trend found by other authors of the electron densitydecreasing with decreasing spectral type, which was interpreted asevidence that [WC] stars evolve from late to early [WC] types. On theother hand, our data on the expansion velocities do not show theincrease of expansion velocity with decreasing spectral type, that onemight expect in such a scenario. Two objects with very late [WC] typecentral stars, K 2-16 and PM 1-188, do not follow the general densitysequence, being of very low density for their spectral types. We suggestthat the stars either underwent a late helium flash (the ``born again''scenario) or that they have had a particularly slow evolution from theAGB. The 6 WELS of our sample follow the same density vs. [WC]-typerelation as the bona fide WRPNe, but they tend to have smaller expansionvelocities. Considerations about the evolutionary status of WELS mustawait the constitution of a larger observational sample. The analysis ofthe differences between the WRPNe in the Magellanic Clouds (distributionof [WC] spectral types, N/O ratios) and in the Galaxy indicates thatmetallicity affects the [WR] phenomenon in central stars of planetarynebulae. Based on data obtained at the Observatorio AstronómicoNacional, SPM, B.C., México Tables 2 and 3 are only available athttp://www.edpsciences.org
| Low-Ionization Structures in Planetary Nebulae: Confronting Models with Observations
Around 50 planetary nebulae (PNs) are presently known to possess``small-scale'' low-ionization structures (LISs) located inside oroutside their main nebular bodies. We consider here the different kindsof LISs (jets, jetlike systems, symmetrical and nonsymmetrical knots)and present a detailed comparison of the existing model predictions withthe observational morphological and kinematical properties. We find thatnebulae with LISs appear indistinctly spread among all morphologicalclasses of PNs, indicating that the processes leading to the formationof LISs are not necessarily related to those responsible for theasphericity of the large-scale morphological components of PNs. We showthat both the observed velocities and locations of most nonsymmetricalsystems of LISs can be reasonably well reproduced assuming either fossilcondensations originated in the asymptotic giant branch (AGB) wind or insitu instabilities. The jet models proposed to date (hydrodynamical andmagnetohydrodynamical interacting winds or accretion disk collimatedwinds) appear unable to account simultaneously for several keycharacteristics of the observed high-velocity jets, such as theirkinematical ages and the angle between the jet and the symmetry axes ofthe nebulae. The linear increase in velocity observed in several jetsfavors magnetohydrodynamical confinement compared to pure hydrodynamicalinteracting wind models. On the other hand, we find that the formationof jetlike systems characterized by relatively low expansion velocities(similar to those of the main shells of PNs) cannot be explained by anyof the existing models. Finally, the knots that appear in symmetricaland opposite pairs of low velocity could be understood as the survivalof fossil (symmetrical) condensations formed during the AGB phase or asstructures that have experienced substantial slowing down by the ambientmedium.
| The infrared [WC] stars
A number of late [WC] stars have unique infrared properties, not foundamong the non-[WC] planetary nebulae, and together define a class ofIR-[WC] stars. They have unusual IRAS colours, resembling stars in theearliest post-AGB evolution and possibly related to PAH formation. Mostor all show a double chemistry, with both a neutral (molecular)oxygen-rich and an inner carbon-rich region. Their dense nebulaeindicate recent evolution from the AGB, suggesting a fatal-thermal-pulse(FTP) scenario. Although both the colours and the stellarcharacteristics predict fast evolution, it is shown that this phase mustlast for 10^4 yr. The morphologies of the nebulae are discussed. For oneobject in Sgr, the progenitor mass (1.3 M_solar) is known. The stellartemperatures of the IR-[WC] stars appear much higher in low metallicitysystems (LMC, Sgr). This may be indicative of an extended `pseudo'photosphere. It is proposed that re-accretion of ejected gas may slowdown the post-AGB evolution and so extend the life time of the IR-[WC]stars.
| Statistics of planetary nebulae with [WR] central stars
The status of planetary nebulae with Wolf-Rayet type central stars ([WR]PN) remains one of the most important problems in the investigation ofplanetary nebulae. We cannot claim to understand the evolution of lowand intermediate-mass stars without answering the question how [WR] PNare created. Analyzing the statistical properties of the wholepopulation of [WR] PN and comparing them to other planetary nebulae(non-[WR] PN) brings important information on their origin andevolutionary status. In this article I will summarize our results ofthis type of studies and show what limits they put on the possibleevolutionary routes of [WR] PN creation.
| Abundances of [WC] central stars and their planetary nebulaee
We review elemental abundances derived for planetary nebula (PN) WCcentral stars and for their nebulae. Uncertainties in the abundances of[WC] stars are still too large to enable an abundance sequence to beconstructed. In particular it is not clear why the hotter [WCE] starshave C and O abundances which are systematically lower than those oftheir supposed precursors, the [WCL] stars. This abundance differencecould be real or it may be due to unaccounted-for systematic effects inthe analyses. Hydrogen might not be present in [WC] star winds asoriginally suggested, since broad pedestals observed at the base ofnebular lines can plausibly be attributed to high velocity nebularcomponents. It is recommended that stellar abundance analyses should becarried out with non-LTE model codes, although recombination lineanalyses can provide useful insights. In particular, C II dielectronicrecombination lines provide a unique means to determine electrontemperatures in cool [WC] star winds. We then compare the abundancesfound for PNe which have [WC] central stars with those that do not.Numerous abundance analyses of PNe have been published, but comparisonsbased on non-uniform samples and methods are likely to lack reliability.Nebular C/H ratios, which might be expected to distinguish between PNearound H-poor and H-rich stars, are rather similar for the two groups,with only a small tendency towards larger values for nebulae aroundH-deficient stars. Nebular abundances should be obtained withphotoionization models using the best-fitting non-LTE model atmospherefor the central star as the input. Heavy-metal line blanketing stillneeds to be taken into consideration when modeling the central star, asits omission can significantly affect the ionizing fluxes as well as theabundance determinations. We discuss the discrepancies between nebularabundances derived from collisionally excited lines and those derivedfrom optical recombination lines, a phenomenon that may have links withthe presence of H-deficient central stars.
| Spectral analyses of WR-type central stars of planetary nebulae
So far, the evolution of post-AGB stars is not fully understood. Inparticular the formation of hydrogen-deficient and hydrogen-free CentralStars of Planetary Nebulae (CSPN) is unsettled. New evolution models,which allow for the consistent treatment of the physics of late thermalpulses, promise new insights to the formation of these stars. In thispaper we summarize the results of non-LTE analyses of CSPN with wind. Bycomparing these results with the predictions of the new evolutionmodels, open questions concerning the evolution of the stars might beanswered. In addition we discuss the driving mechanism of the winds ofWolf-Rayet CSPN. New models, which account for millions of iron lines,support the assumptions that these winds are driven by radiation.
| Evolutionary status of hydrogen-deficient central stars of planetary nebulae
The observational data for the planetary nebulae with hydrogen-deficientcentral stars are analysed. We show that the general evolutionarysequence is: late-[WC], early-[WC], PG 1159. An analysis of the observeddistributions of nebular parameters leads to a conclusion that theplanetary nebulae with hydrogen-deficient nuclei are not different fromthe population of other planetary nebulae in the Galaxy. In particularthe proportion of the H-deficient stars among young nebulae is the sameas in the whole population. We have made a detailed comparison of theobserved parameters with theoretical modelling of the late He-shellflash (born again AGB) scenario. Our finding is that the [WC] nuclei arenot formed in a late He-shell flash. This scenario can, however, giveorigin to some PG 1159 objects. There are five objects known which havepresumably suffered from a late He-shell flash. The observed parametersof their nebulae imply that these stars will not become typical [WC]objects. Thus most of hydrogen-deficient central stars (at least [WC])evolve directly from the AGB as do the other planetary nebula nuclei. Wediscuss implications of this result.
| Distances of Galactic Planetary Nebulae Based on a Relationship Between the Central Star Mass and the N/O Abundance
In this paper, we propose a method to determine distances of Galacticplanetary nebulae on the basis of a relationship between the centralstar mass and the nebular N/O abundance ratio. This relationship is usedin combination with some basic parameters of the central stars, such asthe lambda 5480 flux, surface gravity and visual magnitude in order toobtain distances to a sample of a hundred Galactic planetary nebulae.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
