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Mapping the Instability Domains of GW Vir Stars in the Effective Temperature-Surface Gravity Diagram
We present the results of a detailed stability study of models of GW Virpulsators. These are extremely hot, evolved, and compact stars that areon their way to the white dwarf cooling phase and that showmultiperiodic luminosity variations due to gravity-mode pulsationalinstabilities. Our nonadiabatic survey confirms that cyclic ionizationof carbon and oxygen is at the origin of the GW Vir phenomenon. We findthat the extent of the instability domain in the logg-Teffplane is a strong function of the C and O content in the envelopes ofthese stars. Given that GW Vir stars show important variations inatmospheric composition from one object to another, this implies thatthe notion of a blue edge for the GW Vir instability domain isnecessarily a ``fuzzy'' concept. We investigate the effects of varyingthe total mass, of adding hydrogen, and of changing the metallicity. Wesummarize our results in the form of extensive stability maps in thelogg-Teff diagram and produce tables and graphs showing theexpected ranges of excited periods under various conditions.
| 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
| Detailed Far-Ultraviolet to Optical Analysis of Four [WR] Stars
We present far-UV to optical analyses of four hydrogen-deficient centralstars of planetary nebulae: BD +30 3639, NGC 40, NGC 5315, and NGC 6905.Using the radiative transfer code CMFGEN, we determined new physicalparameters and chemical abundances for these stars. The results wereanalyzed in the context of the [WR]-->PG 1159 evolution via thetransformed radius-temperature (RT×T*) andH-R diagrams. NGC 5315 showed itself as an odd object among thepreviously analyzed central stars. Its temperature (~76 kK) isconsiderably lower than other early-type [WR] stars (~120-150 kK). Fromour models for NGC 5315 and NGC 6905, it is unclear if early-type [WR]stars have smaller C/He mass ratios than other spectral classes, asclaimed in the literature. A ratio of ~0.8 is found for NGC 6905. Weanalyzed FUSE spectra of these stars for the first time and identifiedphosphorus in the spectra of BD +30 3639, NGC 40, and NGC 5315 throughthe doublet transition P V λλ1118, 1128 (3p2Po-3s 2S). The Fe, Si, P, S, and Neabundances were analyzed in the context of the nucleosynthesis occurringin previous evolutionary phases. We found evidence for Fe deficiency inBD +30 3639 and NGC 5315, and we determined a solar Si abundance for BD+30 3639 and NGC 40. Regarding P, an oversolar abundance in the NGC 5315model was preferred. Upper limits for the S abundance were estimated. Wefound that Ne is overabundant in BD +30 3639. In the other stars, Ne isweak or undetectable. Our results are in agreement with theoreticalpredictions and show the usefulness of [WR] stars as test beds fornucleosynthesis calculations in the AGB and post-AGB phases.
| Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79northern Galactic planetary nebulae (PNe) whose morphological classeshave been uniformly determined. Ionic abundances and plasma diagnosticswere derived from selected optical line strengths in the literature, andelemental abundances were estimated with the ionization correctionfactor developed by Kingsbourgh & Barlow in 1994. We compare theelemental abundances to the final yields obtained from stellar evolutionmodels of low- and intermediate-mass stars, and we confirm that mostbipolar PNe have high nitrogen and helium abundance and are the likelyprogeny of stars with main-sequence mass greater than 3Msolar. We derive =0.27 and discuss the implication of such ahigh ratio in connection with the solar neon abundance. We determine theGalactic gradients of oxygen and neon and foundΔlog(O/H)/ΔR=-0.01 dex kpc-1 andΔlog(Ne/H)/ΔR=-0.01 dex kpc-1. These flat PNgradients are irreconcilable with Galactic metallicity gradientsflattening with time.
| A search for photometric variability of hydrogen-deficient planetary-nebula nuclei
Aims.We searched for photometric variability in a sample of hot,hydrogen-deficient planetary nebula nuclei (PNNi) with "PG 1159" or "OVI" spectral type, most of them embedded in a bipolar or ellipticalplanetary nebula envelope (PNe). These characteristics may indicate thepresence of a hidden close companion and an evolution affected byepisodes of interaction between them. Methods: .We obtainedtime-series photometry from a sample of 11 candidates using the NordicOptical Telescope (NOT) with the Andalucía Faint ObjectSpectrograph and Camera (ALFOSC), modified with our own control softwareto be able to observe in a high-speed multi-windowing mode. The datawere reduced on-line with the real time photometry (RTP) code, whichallowed us to detect periodic variable stars with small amplitudes fromCCD data in real time. We studied the properties of the observedmodulation frequencies to investigate their nature. Results: .Wereport the first detection of low-amplitude pulsations in the PNNiVV 47, NGC 6852, and Jn1. In addition, we investigated the photometric variability ofNGC 246. Time-series analysis shows that the powerspectra of VV 47, NGC 6852, andNGC 246 are variable on time scales of hours. Powerspectra from consecutive nights of VV 47 andNGC 6852 show significant peaks in differentfrequency regions. The same type of variability is present in NGC 246 in2 observing runs separated by 3 days. Changes are also found in thepower spectra of VV 47 and NGC 246during the same night. The VV 47 power spectra arepeculiar since they present modulation frequencies in a wide range from175 to 7600 μHz. This is different from the previously knownpulsating PNNi where no frequencies are found above ~3000 μHz. Thehigh-frequency modulation observed in VV 47 may bedue to g-modes triggered by the ɛ-mechanism, observed for thefirst time.
| Galactic Planetary Nebulae with Wolf-Rayet Nuclei III. Kinematical Analysis of a Large Sample of Nebulae
Expansion velocities (V_{exp}) of different ions and line widths at thebase of the lines are measured and analyzed for 24 PNe with [WC]-typenuclei (WRPNe), 9 PNe ionized by WELS (WLPNe) and 14 ordinary PNe. Acomparative study of the kinematical behavior of the sample clearlydemonstrates that WRPNe have on average 40-45% larger V_{exp}, andpossibly more turbulence than WLPNe and ordinary PNe. WLPNe havevelocity fields very much like the ones of ordinary PNe, rather than theones of WRPNe. All the samples (WRPNe, WLPNe and ordinary PNe) showexpansion velocities increasing with age indicators, for example is larger for low-density nebulae and also it is largerfor nebulae around high-temperature stars. This age effect is muchstronger for evolved WRPNe, suggesting that the [WC] winds have beenaccelerating the nebulae for a long time, while for non-WRPNe theacceleration seems to stop at some point when the star reaches atemperature of about 90,000 - 100,000. Non-WR nebulae reach a maximumV_{exp} ≤ 30 km s(-1) evolved WRPNe reach maximum V_{exp} about 40km s(-1) . For all kinds of objects (WRPNe and non-WRPNe) it is foundthat on average V_{exp}(N(+) ) is slightly larger than V_{exp}(O(++) ),indicating that the nebulae present acceleration of the external shells.
| 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.
| Detection of non-radial g-mode pulsations in the newly discovered PG 1159 star HE 1429-1209
We performed time-series photometry of the PG 1159-type star HE1429-1209, which was recently discovered in the ESO SPYsurvey. We show that the star is a low-amplitude (≈0.05 mag)non-radial g-mode pulsator with a period of 919 s. HE1429-1209 is among the hottest known post-AGB stars(Teff = 160 000 K) and, together with the known pulsatorRX J2117.1+3412, it defines empirically the blue edgeof the GW Vir instability strip in the HRD at high luminosities.
| 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.
| 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.
| UV Observations of Neutron Capture Elements in Planetary Nebulae
We present results from a search through existing Far UltravioletSpectroscopic Explorer (FUSE) and HST data for neutron capture speciesin planetary nebulae (PNe), which can be enriched by slow neutroncapture (the s-process) in the progenitor stars. Measurements of suchenrichments shed light on the s-process in AGB stars and the heavyelement enrichment of the interstellar medium. We derive Ge (Z=32)abundances relative to S or Fe from observations of Ge III lambda1088.46 for five PNe; four of these exhibit Ge abundances elevated by afactor of >3-10 above solar, depending on assumptions about depletioninto dust. In contrast, we find an approximately solar abundance for Gein IC 4776, and also in the ISM towards Abell 36 as derived from Ge IIlambda 1237.06. Another neutron-capture element, Ga (Z = 31), isprobably detected in SwSt 1 via Ga III lambda 1495.05, with a strengthindicating a greatly enhanced Ga abundance. The strongest evidence forenrichment of Ge is seen for PNe with H-deficient, C-rich Wolf-Rayetcentral stars. While the evolutionary path producing a [WR] central staris not well understood at present, these objects are likely to haveexperienced extensive mixing and dredge-up of nuclear-processedmaterial.
| 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
| 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.
| Revised Abundances and Ionizing Fluxes for [WC]-Type PN Central Stars Using Line Blanketed Models
Not Available
| WR Central Stars (invited review)
Not Available
| The WET Director's Report
This review summarizes several short contributions that I made in therole of WET (co-)Director. I review the progress of the WET since ourlast workshop in 1999. I discuss the diversification of our targets,the growth of the CCD component of our instrumentation, and theevolution of our funding situation. We also face new opportunities andnew challenges with respect to our changing and growing membership.
| The Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic Sample
Northern Galactic planetary nebulae (PNs) are studied to disclosepossible correlations between the morphology of the nebulae and theevolution of the central stars (CSs). To this end, we have built thebest database available to date, accounting for homogeneity andcompleteness. We use updated statistical distances and an updatedmorphological classification scheme, and we calculate Zanstratemperatures for a large sample of PNs. With our study we confirm thatround, elliptical, and bipolar PNs have different spatial distributionswithin the Galaxy, with average absolute distances to the Galactic planeof 0.73, 0.38, and 0.21 kpc, respectively. We also find evidence thatthe distributions of the CS masses are different across thesemorphological groups, although we do not find that CSs hosted by bipolarPNs are hotter, on average, than CSs within round and elliptical PNs.Our results are in broad agreement with previous analyses, indicatingthat round, elliptical, and bipolar PNs evolve from progenitors indifferent mass ranges and might belong to different stellar populations,as also indicated by the helium and nitrogen abundances of PNs ofdifferent morphology.
| A New Look at the Evolution of Wolf-Rayet Central Stars of Planetary Nebulae
On the basis of recent observational evidence and new theoreticalresults, we construct a speculative scenario for the evolution ofWolf-Rayet central stars of planetary nebulae. Although single-starevolutionary calculations have succeeded recently in reproducing thecomposition of these objects, it is clear from the latest infraredobservations that a new perspective has to be adopted; the simultaneouspresence of carbon- and oxygen-rich dust (double-dust chemistry), whilebeing a rare phenomenon for H-rich central stars, is found around thevast majority of cool Wolf-Rayet central stars. This correlation betweenWolf-Rayet characteristics and double-dust chemistry points to a commonmechanism. Within the binary evolution framework established by Soker,two scenarios are proposed, responsible for the majority (80%-85%) andminority (15%-20%) of Wolf-Rayet central stars. In the first scenario,proposed here for the first time, a low-mass main-sequence star, browndwarf, or planet spirals into the asymptotic giant branch star, inducingextra mixing, hence a chemistry change, and terminating the asymptoticgiant branch evolution. In the second scenario, previously proposed, aclose binary companion is responsible for the formation of a disk aroundeither the binary or the companion. This long-lived disk harbors theO-rich dust. Both models are speculative, although they are supported byseveral observations and recent theoretical results.
| 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.
| 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 .
| A Bi-Component Spectral Analysis of 4 Planetary Nebulae
Presented here is the analysis of four planetary nebulae which showwell-resolved Doppler splitting of their emission lines into distinctspectral components. The red, (R), and blue, (B), components of thespectral lines have been separately analyzed to obtain the reddening,the electron temperature, the electron density, and the ionic and totalabundances for each object within two areas of each nebula along theline of sight. The derived physical conditions for the R and Bcomponents were then compared to see if any variations occurred alongthe line of sight within these objects, indicating the presence ofphysical imhomogeneities along the line of sight. For the centralspectrum of NGC 6309, a variation of 1600K in T[e][O III] is found,along with suggested variations in n[e][O II], c(H β ) and N/Hbetween the R and B components. For the central position of NGC 6751,T[e][O III] varies by 1500K between the R and B components. However,despite taking these T[e] variations found for NGC 6309 and NGC 6751into account when calculating O2 +/H+ ratios, discrepancies still existbetween ratios derived from the recombination vs. the collisional lines.It is finally noted that the expansion velocities for both thecollisional and recombination lines of O2 + are the same, indicatingthat the same volume of emission is traced by these lines.
| 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.
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