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 Mapping the Instability Domains of GW Vir Stars in the Effective Temperature-Surface Gravity DiagramWe 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 starsAims.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 A search for photometric variability of hydrogen-deficient planetary-nebula nucleiAims.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. Discovery of Ne VII in the Winds of Hot Evolved StarsWe show that a strong P Cygni feature seen in the far-UV spectra of somevery hot (Teff>~85 kK) central stars of planetary nebulae(CSPN), which has been previously identified as C III λ977,actually originates from Ne VII λ973. Using stellar atmospheremodels, we reproduce this feature seen in the spectra of two [WR] PG1159-type CSPN, A78 and NGC 2371, and in one PG 1159 CSPN, K1-16 (the[WR]'' nomenclature denotes a CSPN with spectral features similar tothose of Wolf-Rayet stars). In the latter case, our analysis suggests anenhanced neon abundance. Strong neon features in CSPN spectra areimportant because an overabundance of this element is indicative ofprocessed material that has been dredged up to the surface from theintershell region in the born-again'' scenario, an explanation ofhydrogen-deficient CSPN. Our modeling indicates that the Ne VIIλ973 wind feature may be used to discern enhanced neon abundancesfor stars showing an unsaturated P Cygni profile, such as some PG 1159stars. We explore the potential of this strong feature as a winddiagnostic in stellar atmosphere analyses for evolved objects. For the[WR] PG 1159 objects, the line is present as a P Cygni line forTeff>~85 kK and becomes strong for100<~Teff<~155 kK when the neon abundance is solar, andcan be significantly strong beyond this range for higher neonabundances. When unsaturated, i.e., for very high Teff and/orvery low mass-loss rates, it is sensitive to M˙ and very sensitiveto the neon abundance. The Ne VII classification is consistent withrecent identification of this line, seen in absorption in many PG 1159spectra.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer and data from the Multimission Archive at theSpace Telescope Science Institute (MAST). FUSE is operated for NASA bythe Johns Hopkins University, under NASA contract NAS5-32985. The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their EnvelopesThe 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. Some implications of the introduction of scattered starlight in the spectrum of reddened starsThis 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 1429-1209We 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 scaleThe 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. Central Stars of Planetary Nebulae in the Large Magellanic Cloud: A Far-Ultraviolet Spectroscopic AnalysisWe observed seven central stars of planetary nebulae (PNe) in the LargeMagellanic Cloud (LMC) with the Far Ultraviolet Spectroscopic Explorerand performed a model-based analysis of these spectra in conjunctionwith Hubble Space Telescope spectra in the UV and optical range todetermine the stellar and nebular parameters. Most of the objects showwind features, and they have effective temperatures ranging from 38 to60 kK with mass-loss rates of ~=5×10-8Msolar yr-1. Five of the objects have typical LMCabundances. One object (SMP LMC 61) is a [WC4] star, and we fit itsspectra with He/C/O-rich abundances typical of the [WC] class and findits atmosphere to be Fe-deficient. Most objects have very hot(T>~2000 K) molecular hydrogen (H2) in their nebulae,which may indicate a shocked environment. One of these (SMP LMC 62) alsodisplays O VI λλ1032, 1038 nebular emission lines, rarelyobserved in PNe.Based on observations made with the NASA-CNES-CSA FUSE and archivaldata. FUSE is operated for NASA by the Johns Hopkins University underNASA contract NAS5-32985. Far-Ultraviolet Spectroscopic Analyses of Four Central Stars of Planetary NebulaeWe analyze the far-UV/UV spectra of four central stars of planetarynebulae (CSPNs) with strong wind features-NGC 2371, A78, IC 4776, andNGC 1535-and derive their photospheric and wind parameters by modelinghigh-resolution FUSE (Far Ultraviolet Spectroscopic Explorer) data inthe far-UV and HST (Hubble Space Telescope) STIS and IUE data in the UVwith spherical, non-LTE, line-blanketed model atmospheres. A78 is ahydrogen-deficient transitional [WR]-PG 1159 object, and we find NGC2371 to be in the same stage, both migrating from theconstant-luminosity phase to the white dwarf cooling sequence, withTeff~=120 kK, M~=5×10-8 Msolaryr-1. NGC 1535 is a hydrogen-rich'' O(H) CSPN, and theexact nature of IC 4776 is ambiguous, although it appears to behelium-burning. Both objects lie on the constant-luminosity branch ofpost-AGB evolution and have Teff~=65 kK,M~=1×10-8 Msolar yr-1. Thus, boththe H-rich and H-deficient channels of PN evolution are represented inour sample. We also investigate the effects of including higherionization stages of iron (up to Fe X) in the model atmospherecalculations of these hot objects (usually neglected in previousanalyses) and find iron to be a useful diagnostic of the stellarparameters in some cases. The far-UV spectra of all four objects showevidence of hot (T~300 K) molecular hydrogen in their circumstellarenvironments.ID="FN1"> 1Based on observations made with the NASA-CNES-CSA FarUltraviolet Spectroscopic Explorer (FUSE) and data from the MASTarchive. FUSE is operated for NASA by the Johns Hopkins University,under NASA contract NAS5-32985. 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. Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IV. Synthesis and the Sulfur AnomalyWe have compiled a large sample of O, Ne, S, Cl, and Ar abundances thathave been determined for 85 Galactic planetary nebulae in a consistentand homogeneous manner using spectra extending from 3600 to 9600Å. Sulfur abundances have been computed using the near-IR lines of[S III] λλ9069, 9532 along with [S III] temperatures. Wefind average values, expressed logarithmically with a standarddeviation, of log(S/O)=-1.91+/-0.24, log(Cl/O)=-3.52+/-0.16, andlog(Ar/O)=-2.29+/-0.18, numbers consistent with previous studies of bothplanetary nebulae and H II regions. We also find a strong correlationbetween [O III] and [S III] temperatures among planetary nebulae. Inanalyzing abundances of Ne, S, Cl, and Ar with respect to O, we find atight correlation for Ne-O, and loose correlations for Cl-O and Ar-O.All three trends appear to be colinear with observed correlations for HII regions. S and O also show a correlation, but there is a definiteoffset from the behavior exhibited by H II regions and stars. We suggestthat this S anomaly is most easily explained by the existence ofS+3, whose abundance must be inferred indirectly when onlyoptical spectra are available, in amounts in excess of what is predictedby model-derived ionization correction factors in PNe. Finally for thedisk PNe, abundances of O, Ne, S, Cl, and Ar all show gradients whenplotted against Galactocentric distance. The slopes are statisticallyindistinguishable from one another, a result which is consistent withthe notion that the cosmic abundances of these elements evolve inlockstep. 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 fourlarge-scale'', mean-to-high excitation sub-systems (the internalshell, the main shell, the outer shell and the halo), and as manysmall-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). UV Observations of Neutron Capture Elements in Planetary NebulaeWe 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. The relation between Zanstra temperature and morphology in planetary nebulaeWe 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 coordinatesWe 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 nebulaeWe 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 nebulaeWe analyse 42 emission-line nuclei of Planetary Nebulae (PNe), in theframework of a large spectrophotometric survey of [WC] nuclei of PNeconducted since 1994, using low/medium resolution spectra obtained atESO and at OHP. We construct a grid of selected line-intensities(normalized to C Iv-5806 Å= 100) ordered by decreasing ionisationpotential going from 871 to 24 eV. In this grid, the stars appear tobelong clearly to prominent O (hot [WO1-4] types) or C (cooler [WC4-11]types) line-sequences, in agreement with the classification of massiveWR stars applied to Central Stars of Planetary Nebulae (CSPNe) byCrowther et al. \cite{crowther98} (CMB98). We propose 20 selected lineratios and the FWHM of C Iv and C Iii lines as classificationdiagnostics, which agree well with the 7 line ratios and the FWHMproposed by CMB98. This classification based on ionisation is related tothe evolution of the temperature and of the stellar wind, reflecting themass-loss history. In particular, inside the hot [WO4]-class, wediscover four stars showing very broad lines over the whole spectralrange. These stars possibly mark the transition from the initialmomentum-driven phase to the later energy-driven phase of the CSPNealong their evolution from the post-Asymptotic Giant Branch (post-AGB)phase through [WC] late, [WC4] and [WO]-types. The HR diagram and thediagram linking the terminal velocity and the temperature indicatehighly dispersed values of the stellar mass for our sample, around amean mass higher than for normal CSPNe. The distribution of the 42 starsalong the ionisation sequence shows 24% of [WO1-3], 21% of [WO4], 17% of[WC4] hot stars, and 26% of [WC9-11] cool stars. The [WC5-8] classesremain poorly represented (12%). This distribution is confirmed on thebasis of a large compilation of the 127 known emission-lines CSPNe,which represent about 5% of the known PNe.Based on observations obtained at the European Southern Observatory(ESO), La Silla (Chile), and at the Observatoire de Haute-Provence (OHP,France).Table \ref{liste} is only available in electronic form athttp://www.edpsciences.org The Extraordinary X-ray Light Curve of the Classical Nova V1494 Aquilae (1999 No. 2) in Outburst: The Discovery of Pulsations and a Burst''V1494 Aql (Nova Aql 1999 No. 2) was discovered on 1999 December 2. Weobtained Chandra ACIS-I spectra on 2000 April 15 and June 7 which appearto show only emission lines. Our third observation, on August 6, showedthat its spectrum had evolved to that characteristic of a Super SoftX-ray Source. We then obtained Chandra LETG+HRC-S spectra on September28 (8 ks) and October 1 (17 ks). We analyzed the X-ray light curve ofour grating observations and found both a short timescale burst'' andoscillations. Neither of these phenomena has previously been seen in thelight curve of a nova in outburst. The burst'' was a factor of ~10rise in X-ray counts near the middle of the second observation, andwhich lasted about 1000 s; it exhibited at least two peaks, in additionto other structure. Our time series analysis of the combined 25 ksobservation shows a peak at ~2500 s which is present in independentanalyses of both the zeroth-order image and the dispersed spectrum andis not present in similar analyses of grating data for HZ 43 and SiriusB. Further analyses of the V1494 Aql data find other periods presentwhich implies that we are observing nonradial g+ modes fromthe pulsating, rekindled white dwarf. Properties of Galactic and MC CSPN disclosed by Far-Ultraviolet SpectroscopyWe are performing a comprehensive spectral analysis of FUSE, IUE, HSTand ground-based data of Galactic, and Large and Small Magellanic Cloud(LMC and SMC) central stars of planetary nebulae (CSPN), with the aim ofunderstanding the overall evolution of PN in environments of differingmetallicities. We determine the parameters of the central stars as wellas the characteristics of the circumstellar environment. Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. III. Observations and Results for a Final SampleThis paper is the fourth in a series whose purpose is to study theinterstellar abundances of sulfur, chlorine, and argon in the Galaxyusing a sample of 86 planetary nebulae. Here we present new high-qualityspectrophotometric observations of 20 Galactic planetary nebulae withspectral coverage from 3700 to 9600 Å. A major feature of ourobservations throughout the entire study has been the inclusion of thenear-infrared lines of [S III] λλ9069, 9532, which allowsus to calculate accurate S+2 abundances and to either improveupon or convincingly confirm results of earlier sulfur abundancestudies. For each of the 20 objects here, we calculate ratios of S/O,Cl/O, and Ar/O and find average values ofS/O=(1.1+/-1.1)×10-2,Cl/O=(4.2+/-5.3)×10-4, andAr/O=(5.7+/-4.3)×10-3. For six objects, we are able tocompare abundances of S+3 calculated directly from available[S IV] 10.5 μm measurements with those inferred indirectly from thevalues of the ionization correction factors for sulfur. In the finalpaper of the series, we will compile results from all 86 objects, searchfor and evaluate trends, and use chemical evolution models to interpretour results. X-ray and UV Views of Hot Gas in Planetary Nebulae (invited review)Not Available Gas temperature and excitation classes in planetary nebulaeEmpirical 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 Physical Conditions and Chemical Structure of the PNe NGC 2440 and NGC 2371-72We present preliminary results of the physical conditions and chemicalabundances determined through the planetary nebulae (PNe) NGC 2440 andNGC 2371-72. We have used low-dispersion long-slit spectra of regionsdefined by main axes and bright knots. Infrared properties of planetary nebulae with [WR] central starsWe 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 ratioIn 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. Galactic planetary nebulae with Wolf-Rayet nuclei. II. A consistent observational data setWe 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 Statistics of planetary nebulae with [WR] central starsThe 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 nebulaeeWe 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. Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.Not Available
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