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Abell 43: longest period planetary nebula nucleus variable
Context: Most PG 1159 stars are hydrogen deficient post-AGB stars ontheir way to the white dwarf cooling sequence. However, a fraction ofthem show small amounts of atmospheric hydrogen and are known as thehybrid PG 1159 stars. Both the normal and hybrid PG 1159 stars mayreside at the centre of a Planetary Nebula. Some PG 1159 stars exhibitnon-radial g-mode pulsations, providing us with a means to constraintheir fundamental physical parameters, internal structure, andevolutionary status; asteroseismology is the technique of using globalstellar pulsations to probe the star's interior. Aims: Modelcalculations demonstrate that the PG 1159 instability strip is driven bythe partial ionization of carbon and oxygen through theκ-mechanism. The range of excited frequencies in models issensitive to the details of the chemical composition in the drivingzone. Hydrogen content plays a crucial role in deciding the range ofexcited frequencies for models of hybrid PG 1159 pulsators. The range ofperiods observed in all PG 1159 pulsators can then be compared againstthe range predicted by theoretical models, which are based onobservations of their chemical composition and evolutionary status. Ouraim is to test the validity of the predicted range of pulsation periodsfor the hybrid PG 1159 variables, Abell 43 and NGC 7094. Methods: Toachieve this goal, we acquired continuous high speed CCD photometry ofthe hybrid PG 1159 star Abell 43 using the 2.6 m Nordic OpticalTelescope and the 3.5 m telescope at Apache Point Observatory. We alsoobserved NGC 7094, a hybrid PG 1159 star with atmospheric parameterssimilar to Abell 43. We analyzed our data to obtain the pulsationspectra of the two stars. Results: We detected six significantpulsations in the light curves of Abell 43 with periods between 2380 sand 6075 s. We were also fortunate to detect low amplitude pulsations inour NGC 7094 data with periods between 2000 s and 5000 s. The observedrange of periods for both stars is consistent with the theoreticalpredictions. Conclusions: The range of periods observed in both hybridPG 1159 stars, Abell 43 and NGC 7094, agree with the theoretical modelthat the pulsations are driven by the κ-mechanism induced by thepartial ionization of carbon and oxygen.Based on observations madewith the Nordic Optical Telescope, operated on the island of La Palmajointly by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto deAstrofisica de Canarias.
| Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfs
Context: We report the first discovery of argon in hot evolved stars andwhite dwarfs. We have identified the Ar VII 1063.55 Å line in someof the hottest known (T eff = 95 000-110 000 K) central starsof planetary nebulae and (pre-) white dwarfs of various spectral type. Aims: We determine the argon abundance and compare it to theoreticalpredictions from stellar evolution theory as well as from diffusioncalculations. Methods: We analyze high-resolution spectra taken withthe Far Ultraviolet Spectroscopic Explorer. We use non-LTEline-blanketed model atmospheres and perform line-formation calculationsto compute synthetic argon line profiles. Results: We find a solarargon abundance in the H-rich central star NGC 1360 and in theH-deficient PG 1159 star PG 1424+535. This confirms stellar evolutionmodeling that predicts that the argon abundance remains almostunaffected by nucleosynthesis. For the DAO-type central star NGC 7293and the hot DA white dwarfs PG 0948+534 and RE J1738+669 we find argonabundances that are up to three orders of magnitude smaller thanpredictions of calculations assuming equilibrium of radiative levitationand gravitational settling. For the hot DO white dwarf PG 1034+001 thetheoretical overprediction amounts to one dex. Conclusions: Our resultsconfirm predictions from stellar nucleosynthesis calculations for theargon abundance in AGB stars. The argon abundance found in hot whitedwarfs, however, is another drastic example that the current state ofequilibrium theory for trace elements fails to explain the observationsquantitatively.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| High-resolution ultraviolet spectroscopy of PG 1159-035 with HST and FUSE
Context: PG 1159-035 is the prototype of the PG 1159 spectral classwhich consists of extremely hot hydrogen-deficient (pre-) white dwarfs.It is also the prototype of the GW Vir variables, which are non-radialg-mode pulsators. The study of PG 1159 stars reveals insight intostellar evolution and nucleosynthesis during AGB and post-AGB phases. Aims: We perform a quantitative spectral analysis of PG 1159-035focusing on the abundance determination of trace elements. Methods: Wehave taken high-resolution ultraviolet spectra of PG 1159-035 with theHubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer.They are analysed with non-LTE line blanketed model atmospheres. Results: We confirm the high effective temperature with high precision({T_eff} = 140 000 ± 5000 K) and the surface gravity of log g =7. For the first time we assess the abundances of silicon, phosphorus,sulfur, and iron. Silicon is about solar. For phosphorus we find anupper limit of solar abundance. A surprisingly strong depletion ofsulfur (2% solar) is discovered. Iron is not detected, suggesting anupper limit of 30% solar. This coincides with the Fe deficiency found inother PG 1159 stars. We redetermine the nitrogen abundance and find itto be lower by one dex compared to previous analyses. Conclusions: .Thesulfur depletion is in contradiction with current models of AGB starintershell nucleosynthesis. The iron deficiency confirms similar resultsfor other PG 1159 stars and is explained by the conversion of iron intoheavier elements by n-capture in the s-processing environment of theprecursor AGB star. However, the extent of the iron depletion isstronger than predicted by evolutionary models. The relatively lownitrogen abundance compared to other pulsating PG 1159 stars weakens therole of nitrogen as a distinctive feature of pulsators and non-pulsatorsin the GW Vir instability strip.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 NAS5-26666. Based on observations made with the NASA-CNES-CSAFar Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985.
| Do Most Planetary Nebulae Derive from Binaries? I. Population Synthesis Model of the Galactic Planetary Nebula Population Produced by Single Stars and Binaries
We present a population synthesis calculation to derive the total numberof planetary nebulae (PNs) in the Galaxy that descend from single starsand stars in binary systems. Using the most recent literature results onGalactic and stellar formation and stellar evolution, we predict thetotal number of Galactic PNs with radii <0.9 pc to be(4.6+/-1.3)×104. We do not claim this to be thecomplete population, since there can be visible PNs with radii largerthan this limit. However, by taking this limit, we make our predictedpopulation inherently comparable to the observationally based value ofPeimbert and Jacoby (8000+/-2000 objects). Our prediction is discrepantwith the observations at the 2.9 σ level, a disagreement that weargue is meaningful in view of our specific treatment of theuncertainty. We conclude that it is likely that only a subset of thestars thought to be capable of making a visible PN actually do. In thesecond paper in this series, an argument will be presented that the bulkof the Galactic PN population might be better explained if only binariesproduce PNs. The predicted local PN formation rate density from singlestars and binaries is (1.1+/-0.5)×10-12 PNsyr-1 pc-3, lower than recent estimates(2.1×10-12 PNs yr-1 pc-3), whichare based on local PN counts and the PN distance scale, but more in linewith the white dwarf (WD) birthrate densities[(1.0+/-0.25)×10-12 WDs yr-1pc-3]. The predicted PN birthrate density will be reviseddown if we assume that only binaries make PNs, implying that the PNdistance scale has to be revised to larger values.
| New nonadiabatic pulsation computations on full PG 1159 evolutionary models: the theoretical GW Virginis instability strip revisited
Aims.We reexamine the theoretical instability domain of pulsating PG1159 stars (GW Vir variables). Methods: .We performed an extensiveg-mode stability analysis on PG 1159 evolutionary models with stellarmasses ranging from 0.530 to 0.741 Mȯ, for which thecomplete evolutionary stages of their progenitors from the ZAMS, throughthe thermally pulsing AGB and born-again phases to the domain of the PG1159 stars have been considered. Results: .We found thatpulsations in PG 1159 stars are excited by the κ-mechanism due topartial ionization of carbon and oxygen, and that no compositiongradients are needed between the surface layers and the driving region,much in agreement with previous studies. We show, for the first time,the existence of a red edge of the instability strip at highluminosities. We found that all of the GW Vir stars lay within ourtheoretical instability strip. Our results suggest a qualitative goodagreement between the observed and the predicted ranges of unstableperiods of individual stars. Finally, we found that generally theseismic masses (derived from the period spacing) of GW Vir stars aresomewhat different from the masses suggested by evolutionary trackscoupled with spectroscopy. Improvements in the evolution during thethermally pulsing AGB phase and/or during the core helium burning stageand early AGB could help to alleviate the persisting discrepancies.
| Precise radial velocities with BOES. Detection of low-amplitude pulsations in the K-giant α Arietis
We present the first results from a high-precision radial velocity studyof the K2 III giant star α Ari. Observationswere acquired over 6 nights in 2004 using the new high-resolutionspectrograph BOES (Bohyunsan Observatory Echelle Spectrograph) of the1.8-m telescope. A high radial-velocity precision was achieved by usingthe high-resolution (R=90 000) mode of BOES and an iodine gas absorptioncell. The radial velocity measurements made during JD = 2 452 948{-}2452 950 show coherent, low-amplitude variations with a period ofP1=0.571 days (or aliases at 0.445 or 0.821 days), and anamplitude of 18.9 m s-1. Observations of τ Cet over thissame interval are constant to within 3 m s-1. Aftersubtracting the contribution of the 0.57-day period, we find evidencefor a second period, P2 = 0.190 days. Observations made on asecond run during JD = 2 452 975{-}2 452 981 show that the radialvelocity variations are indeed present, but on shorter time-scales andwith a lower amplitude. Two probable periods fit the radial velocitymeasurements from the second run reasonably well: 0.185 days or an aliasof 0.256 days. The shorter period coincides with the secondary one foundin the earlier measurements. We conclude that, similar to other K-giantpulsating stars, α Ari shows unstable acousticpulsations or mode switching on time scales of tens of days. Thecalculated pulsation constants for the dominant 0.571-day period isconsistent with third-overtone pulsations, while the secondaryperiodicity found in both data runs corresponds to a high overtone(n≥ 12).
| 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.
| 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.
| The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
We review the observed properties of extremely hot, hydrogen-deficientpost-asymptotic giant branch (AGB) stars of spectral type [WC] andPG1159. Their H deficiency is probably caused by a (very) latehelium-shell flash or an AGB final thermal pulse, laying bare interiorstellar regions that are usually kept hidden below the hydrogenenvelope. Thus, the photospheric elemental abundances of these starsallow us to draw conclusions about details of nuclear burning and mixingprocesses in the precursor AGB stars. We summarize the state of the artof stellar evolution models that simulate AGB evolution and theoccurrence of a late He-shell flash. We compare predicted elementalabundances to those determined by quantitative spectral analysesperformed with advanced non-LTE model atmospheres. Good qualitative andquantitative agreement is found. Future work can contribute to an evenmore complete picture of the nuclear processes in AGB stars.
| AGB star intershell abundances inferred from analyses of extremely hot H-deficient post-AGB stars
The hydrogen-deficiency in extremely hot post-AGB stars of spectralclass PG1159 is probably caused by a (very) late helium-shell flash or aAGB final thermal pulse that consumes the hydrogen envelope, exposingthe usually-hidden intershell region. Thus, the photospheric elementabundances of these stars allow to draw conclusions about details ofnuclear burning and mixing processes in the precursor AGB stars. Wecompare predicted element abundances to those determined by quantitativespectral analyses performed with advanced non-LTE model atmospheres. Agood qualitative and quantitative agreement is found for many species(He, C, N, O, Ne, F, Si) but discrepancies for others (P, S, Fe) pointat shortcomings in stellar evolution models for AGB stars.
| The nature of the driving mechanism in the pulsating hybrid PG 1159 star Abell 43
We extend our previous pulsational stability analyses of PG 1159 starsby modeling the hybrid PG 1159 type star Abell 43. Weshow that the standard κ-mechanism due to the ionization of C andO in the envelope of this H-rich PG 1159 star is perfectly able to driveg-mode pulsations. Thus, contrary to a recent suggestion, there is noneed to invoke any new or exotic mechanism to explain the pulsationalinstabilities observed in this particular star. Our expected instabilityband for l=1 modes extends in period from ~2604 s to ~5529 s, which isconsistent with the available photometric observations of Abell 43. Wealso suggest that efforts to detect luminosity variations in its siblingNGC 7094 be pursued.
| Evolution of Asymptotic Giant Branch Stars
The current status of modeling the evolution and nucleosynthesis ofasymptotic giant branch (AGB) stars is reviewed. The principles of AGBevolution have been investigated in recent years leading to improved andrefined models, for example with regard to hot-bottom burning or thethird dredge-up. The postprocessing s-process model yields quantitativeresults that reproduce many observations. However, these and most otherprocesses in AGB stars are intimately related to the physics of stellarmixing. Mixing in AGB stars is currently not well-enough understood foraccurate yield predictions. Several constraints and methods areavailable to improve the models. Some regimes of AGB evolution have notyet been studied in sufficient detail. These include the super-AGB starsand AGB stars at extremely low or ultra low metallicity.
| Abell 43, a second pulsating ``hybrid-PG 1159'' star
We report observations of the planetary nebula nucleus Abell 43,obtained at the 2.5 m Nordic Optical Telescope, which show that it is apulsator. Abell 43, a “hybrid-PG 1159” type star, is thesecond pulsator of this class, after HS 2324+3944. From the limited dataset acquired, we find that Abell 43 exhibits at least two periods of2600 s and 3035 s, the longest ones observed up to now in PG 1159 and“hybrid-PG 1159” pulsators. This strongly suggests that thevariations are due to non-radial g-mode pulsations and cannot be aconsequence of binarity. This discovery raises puzzling questionsregarding the excitation mechanism in this H rich, C and O poor“hybrid-PG 1159” since the C and O abundances are too low totrigger the instability through the κ-mechanism induced by thepartial ionization of C and O, a mechanism invoked to explain theinstability in the PG 1159 stars and in the previously known“hybrid-PG 1159” pulsator HS 2324+3944.
| Fluorine in extremely hot post-AGB stars: Evidence for nucleosynthesis
We have discovered lines of highly ionized fluorine (ion{F}{v} andion{F}{vi}) in the far-UV spectra of extremely hot (T_eff = 85 000-150000 K) post-AGB stars. Our sample comprises H-rich central stars ofplanetary nebulae as well as H-deficient PG1159 stars. We performednon-LTE calculations and find strong F overabundances (up to10-4 by mass, i.e., 250 times solar) in a number of PG1159stars, while F is essentially solar in the H-rich stars. Since PG1159stars are believed to exhibit intershell matter of the preceding AGBphase on their surface, their chemical analyses allow for a directinsight into nucleosynthesis processes during the AGB phase. The high Fabundances in PG1159 stars confirm the conclusion from abundancedeterminations in giants, that F is synthesized in AGB stars and thatthe F enrichment in the intershell must be very high.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| The distances of highly evolved planetary nebulae
The central stars of highly evolved planetary nebulae (PNe) are expectedto have closely similar absolute visual magnitudes MV. Thisenables us to determine approximate distances to these sources where oneknows their central star visual magnitudes, and levels of extinction. Wefind that such an analysis implies values of D which are similar tothose determined by Phillips; Cahn, Kaler & Stanghellin; Acker, andDaub. However, our distances are very much smaller than those of Zhang;Bensby & Lundstrom, and van de Steene & Zijlstra. The reasonsfor these differences are discussed, and can be traced to errors in theassumed relation between brightness temperature and radius.Finally, we determine that the binary companions of such stars can be nobrighter than MV~ 6mag, implying a spectral type of K0 orlater in the case of main-sequence stars.
| Identification of neon in FUSE and VLT spectra of extremely hot hydrogen-deficient (pre-) white dwarfs
One of the strongest absorption lines observed in far-ultraviolet FUSEspectra of many PG 1159 stars remained unidentified up to now. We showthat this line, located at 973.3 Å, stems from Ne VII. We alsopresent new optical high-resolution spectra of PG 1159 stars, obtainedwith the ESO VLT, which cover the Ne VII 3644 Å line and a newlyidentified Ne VII multiplet in the 3850-3910 Å region. We comparethe observed neon lines with NLTE models and conclude a substantial neonoverabundance in a number of objects. Although a detailed analysis isstill to be performed in order to compare quantitatively the abundanceswith evolutionary theory predictions, this corroborates the idea thatthe PG 1159 stars and their immediate progenitors, the [WC]-type nucleiof planetary nebulae, display intershell matter of their precursor AGBstars. Possibly as the consequence of a late He-shell flash, H-deficientand (s-processed) Fe-depleted matter, that is strongly enriched by3α-processed elements (C, O, Ne), is dredged up to the surface.Hence, a detailed study of the element abundance patterns in thesepeculiar stars gives the unique possibility to probe mixing andnucleosynthesis processes in the precursor AGB stars.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.Based on observations at the Paranal Observatory of the EuropeanSouthern Observatory for programs Nos. 165.H-0588(A), 167.D-0407(A), and69.D-0719(A).
| 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 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
| FUSE Observations of the Central Star of Abell 78
FUSE high resolution spectra of two PG1159 type central stars (K1-16 andNGC 7094) have revealed an unexpected iron deficiency of at least 1 or 2dex (Miksa et al. 2002). Here we present early results of FUSEspectroscopy of the CSPN Abell 78. It is shown that iron is stronglydeficient in this star, too.
| WR Central Stars (invited review)
Not Available
| Temperature Scale and Iron Abundances of Very Hot Central Stars of Planetary Nebulae (invited review)
The determination of effective temperatures of very hot central stars(Teff>70000K) by model atmosphere analyses of optical H and He lineprofiles is afflicted with considerable uncertainty, primarily due tothe lack of neutral helium lines. Ionization balances of metals,accessible only with UV lines, allow more precise temperature estimates.The potential of iron lines is pointed out. At the same time iron andother metal abundances, hardly investigated until today, may be derivedfrom UV spectra. We describe recent HST spectroscopy performed for thispurpose. A search for iron lines in FUV spectra of the hottestH-deficient central stars (PG1159-type, Teff>100000K) taken with FUSEwas unsuccessful. The derived deficiency is interpreted in terms of irondepletion due to n-capture nucleosynthesis in intershell matter, whichis now exposed at the stellar surface as a consequence of a late Heshell flash.
| 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.
| What Are the Hot R Coronae Borealis Stars?
We investigate the evolutionary status of four stars: V348 Sgr, DY Cen,and MV Sgr in the Galaxy and HV 2671 in the LMC. These stars have incommon random deep declines in visual brightness, which arecharacteristic of R Coronae Borealis (RCB) stars. RCB stars aretypically cool hydrogen-deficient supergiants. The four stars studied inthis paper are hotter (Teff=15-20 kK) than the majority ofRCB stars (Teff=5000-7000 K). Although these are commonlygrouped together as the hot RCB stars they do not necessarily share acommon evolutionary history. We present new observational data and anextensive collection of archival and previously published data that isreassessed to ensure internal consistency. We find temporal variationsof various properties on different timescales that will eventually helpus to uncover the evolutionary history of these objects. DY Cen and MVSgr have typical RCB helium abundances, which exclude any currentlyknown post-asymptotic giant branch (post-AGB) evolutionary models.Moreover, their carbon and nitrogen abundances present us with furtherproblems for their interpretation. V348 Sgr and HV 2671 are in generalagreement with a born-again post-AGB evolution, and their abundances aresimilar to Wolf-Rayet central stars of planetary nebulae (PNs). Thethree Galactic stars in the sample have circumstellar nebulae, whichproduce forbidden line radiation (for HV 2671 we have no information).V348 Sgr and DY Cen have low-density, low-expansion velocity nebulae(resolved in the case of V348 Sgr), while MV Sgr has a higher density,higher expansion velocity nebula. All three stars, on the other hand,have split emission lines, which indicate the presence of an equatorialbulge but not of a Keplerian disk. In addition, the historical lightcurves for the three Galactic hot RCB stars show evidence for asignificant fading in their maximum-light brightnesses of ~1 mag overthe last 70 yr. From this we deduce that their effective temperaturesincreased by a few thousand degrees. If V348 Sgr is a born-again star,as we presume, this means that the star is returning from the born-againAGB phase to the phase of a central star of PN. Spectroscopically, nodramatic change is observed over the last 50 years for V348 Sgr and MVSgr. However, there is some evidence that the winds of V348 Sgr and DYCen have increased in strength in the last decade. HV 2671, located inthe LMC, has not been analyzed in detail but at 5 Å resolution isalmost identical to V348 Sgr. Through the bolometric correction derivedfor V348 Sgr and the known distance, we can estimate the absolute Vmagnitude of HV 2671 (MV=-3.0 mag) and its bolometricluminosity (~6000 Lsolar).
| 3-D ionization structure (in stereoscopic view) of planetary nebulae: the case of NGC 1501.
Long-slit echellograms of the high excitation planetary nebula NGC 1501,reduced according to the methodology developed by Sabbadin et al.(2000a,b), allowed us to obtain the "true" distribution of the ionizedgas in the eight nebular slices covered by the spectroscopic slit. A 3-Drendering procedure is described and applied, which assembles thetomographic maps and rebuilds the spatial structure. The images of NGC1501, as seen in 12 directions separated by 15o, form a series ofstereoscopic pairs giving surprising 3-D views in as many directions.The main nebula consists of an almost oblate ellipsoid of moderateellipticity (a ~= 44 arcsec, a/b ~= 1.02, a/c ~= 1.11), brighter in theequatorial belt, deformed by several bumps, and embedded in a quitehomogeneous, inwards extended cocoon. Some reliability tests are appliedto the rebuilt nebula; the radial matter profile, the small scaledensity fluctuations and the 2-D (morphology) -3-D (structure)correlation are presented and analysed. The wide applications of the 3-Dreconstruction to the morphology, physical conditions, ionizationparameters and evolutionary status of expanding nebulae in general(planetary nebulae, nova and supernova remnants, shells aroundPopulation I Wolf-Rayet stars, nebulae ejected by symbiotic stars,bubbles surrounding early spectral type main sequence stars etc.) areintroduced.
| Spectroscopic investigation of old planetaries. V. Distance scales
We use the results of our recent NLTE model atmosphere analysis ofcentral stars of old planetary nebulae (PN) to calculate distances. Weperform a comparison with three other methods (trigonometric parallaxes,interstellar Na D lines, and Shklovsky distances) and discuss theproblem of the PNe distance scale. The result of the comparison of ourspectroscopic distances with the trigonometric distances is that thespectroscopic distances are 55% larger. Since using trigonometricparallaxes with large relative measurement errors can introducesystematic errors, we carried out a Monte Carlo simulation of the biasesintroduced by selection effects and measurement errors. It turns outthat a difference between both distance scales of the observed size isexpected for the present day data if the underlying distance scales areidentical. Thus our finding is essentially a confirmation of thespectroscopic distance scale! Good agreement is found between thespectroscopic distances and distances derived from the interstellar NaDlines. All three independent methods of distance measurement indicatethat the widely used ``statistical'' distance scales of the Shklovskytype are too short for old PNe. A correlation with nebular radii exists.The most likely explanation is an underestimate of the nebula masses forlarge PN. Implications for the nebula masses are discussed. Estimates ofthe PNe space density and birthrate, which are based on Shklovsky typedistances, therefore give too large values.
| 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.
| The IUE and Hubble Space Telescope GHRS Spectrum of the ``Hybrid''/PG 1159-Type Central Star of the Planetary Nebula NGC 7094
Absorption features in the IUE spectrum of the ``hybrid'' PG 1159-typecentral star of NGC 7094 have been identified and measured. We reportequivalent widths for photospheric Fe VI and Fe VII features,interstellar and circumstellar absorptions, and variability of O Vλ1371 and C IV λ1549 P Cygni profiles. The terminal windvelocity of -3680+/-300 km s-1 was determined from thehigh-dispersion C IV P Cyg feature. Some features from IUE and HubbleSpace Telescope GHRS data are compared.
| The tetra-lobed planetary nebula NGC 1501
Direct imagery and long-slit, spatially resolved echellograms of thehigh excitation planetary nebula NGC 1501 allowed us to study in detailthe expansion velocity field, the physical conditions (electrontemperature, electron density, ionization) and the spatial distributionof the nebular gas. An electron temperature of 11500 K and a turbulenceof 18 km s-1 are derived by comparing the Hα and [OIII]emission line profiles, but large, small scale fluctuations of boththese quantities are present in the ionized gas. The radial densitydistribution shows external peaks up to 1400 cm-3; they havesteep outwards profiles and extended inwards tails probably originatedby Rayleigh-Taylor instability and winds interaction. The complexity ofthe expanding motions indicates that the main part of NGC 1501 is a thinellipsoid of moderate ellipticity, but the presence of a pair of largelobes along both the major and the intermediate axes and of a multitudeof smaller bumps spread on the whole nebular surface, makes the general3-D structure of NGC 1501 like a boiling, tetra-lobed shell. Thispeculiar morphology can be qualitatively explained in terms ofinteraction of the slow nebular material with the intense and fast windfrom the WC4/OVI central star.
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