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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.

White dwarf masses derived from planetary nebula modelling
Aims.We compare the mass distribution of central stars of planetarynebulae (CSPNe) with those of their progeny, white dwarfs (WD). Methods: We use a dynamical method to measure masses with an uncertaintyof 0.02 M_ȯ. Results: The CSPN mass distribution is sharplypeaked at 0.61~M_ȯ. The WD distribution peaks at lower masses(0.58~M_ȯ) and shows a much broader range of masses. Some of thedifference can be explained if the early post-AGB evolution is fasterthan predicted by the Blöcker tracks. Between 30 and 50 per cent ofWD may avoid the PN phase because they have too low a mass. However, thediscrepancy cannot be fully resolved and WD mass distributions may havebeen broadened by observational or model uncertainties.Data is only available in electronic form at http://www.aanda.org

Asteroseismology of the PG 1159 star PG 0122+200
Context: The variable pre-white dwarf PG 1159 stars (GW Vir) are g-modenon-radial pulsators. Asteroseismology puts strong constraints on theirglobal parameters and internal structure. PG 0122+200 defines the rededge of the instability strip and its evolutionary timescale ispredicted to be dominated by neutrino emission. Its study offers theopportunity to better understand the instability mechanism and tovalidate the physics of the neutrino production in dense plasma. Aims:To achieve such a goal requires determining precisely its fundamentalparameters. This is the goal of this paper. Methods: We present newmulti-site photometric observations obtained in 2001 and 2002. Togetherwith previous data, they allow us to detect 23 frequencies, composed of7 triplets and 2 single frequencies, which are used to constrain itsinternal structure and derive its fundamental parameters. Results: Allthe observed frequencies correspond to ℓ=1 g-modes. The perioddistribution shows a signature of mode trapping from which we constrainthe He-rich envelope mass fraction to be -6.0≤ log(qy)≤ -5.3. The comparison of the mode trapping amplitudes among GW Virstars suggests that the mass-loss efficiency must decrease significantlybelow T_eff≤ 140 kK. We measure an average period spacing of 22.9 sfrom which we derive a mass of 0.59±0.02 Mȯ. Fromthe triplets we measure a mean rotational splitting of 3.74 μHz and arotational period of 1.55 days. We derive an upper limit to the magneticfield of B≤4×103 G. The luminosity (logL/Lȯ = 1.3±0.5) and the distance (D =0.7^+1.0_-0.4 kpc) are only weakly constrained due to the largeuncertainty on the spectroscopically derived surface gravity and theabsence of a measured parallax. Conclusions: From the asteroseismicmass, the ratio of the neutrino luminosity on the photon luminosity is1.6±0.2 confirming that the PG 0122+200 evolutionary time scaleshould be dominated by neutrino cooling. A measurement of dot{P} for thelargest amplitude untrapped modes should verify this prediction.Based on data obtained at the Haute-Provence Observatory, INSU/CNRS,France; the Nordic Optical Telescope, La Palma, Spain; the Xinglongstation of National Astronomical Observatories, China; the BohyunsanObservatory, South-Korea; the Gunma Astrophysical Observatory, Japan;the Teide Observatory, Tenerife, Spain; the Calar Alto Observatory,Spain; the Kitt Peak National Observatory, Arizona, USA; the McDonaldObservatory, Texas, USA; the South Africa Astronomical Observatory,South Africa; the Wise Observatory, Israel and thePiszkéstető Observatory, Hungary.

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( 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.

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.

Gravitational radiation from differentially rotating and oscillating white dwarfs
We examine the possible emission of gravitational waves from whitedwarfs undergoing self-similar oscillations driven by the energyreleased during relaxation of their differential rotation. Twodistributions of the initial angular momentum are considered. It isassumed that 1% of the energy dissipated by a rotating white dwarf isconverted into the energy of self-similar oscillations and, therefore,into gravitational radiation. The relative amplitude of thegravitational radiation from an isolated white dwarf at a distance of 50pc is found to be less than 10‑27. The emission fromthe galactic population of white dwarfs may create a background whichoverlaps the random cosmological background of gravitational radiationfor the improved decihertz detectors currently being proposed.

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.

On the nitrogen abundance of fast, low-ionization emission regions: the outer knots of the planetary nebula NGC 7009
We have constructed a 3D photoionization model of a planetary nebula(PN) similar in structure to NGC 7009 with its outer pair of knots (alsoknown as FLIERs - fast, low-ionization emission regions). The work ismotivated by the fact that the strong [NII]λ6583 line emissionfrom FLIERs in many PNe has been attributed to a significant localoverabundance of nitrogen. We explore the possibility that the apparentenhanced nitrogen abundance previously reported in the FLIERs may be dueto ionization effects. The model is constrained by the results obtainedby Gonçalves et al. from the analysis of both Hubble SpaceTelescope (HST) [OIII] and [NII] images, and long-slit spectra of NGC7009. Our model is indeed able to reproduce the main spectroscopic andimaging characteristics of the bright inner rim of NGC 7009 and itsouter pairs of knots, assuming homogeneous elemental abundancesthroughout the nebula, for nitrogen as well as all the other elementsincluded in the model.We also study the effects of a narrow slit on our non-sphericallysymmetric density distribution, via the convolution of the model resultswith the profile of the long slit used to obtain the spectroscopicobservations that constrained our model. This effect significantlyenhances the [NII]/Hβ emission, more in the FLIERs than in theinner rim.Because of the fact that the (N+/N)/(O+/O) ratiopredicted by our models is 0.60 for the rim and is 0.72 for the knots,so clearly in disagreement with the N+/N = O+/Oassumption of the ionization correction factor (icf) method, the icfswill be underestimated by the empirical scheme, in both components, rimand knots, but more so in the knots. This effect is partly responsiblefor the apparent inhomogeneous N abundance empirically derived. Thedifferences in the above ratio in these two components of the nebula maybe due to a number of effects including charge exchange - as pointed outpreviously by other authors - and the difference in the ionizationpotentials of the relevant species - which makes this ratio extremelysensitive to the shape of the local radiation field. Because of thelatter, a realistic density distribution is essential to the modellingof a non-spherical object, if useful information is to be extracted fromspatially resolved observations, as in the case of NGC 7009.

The abundance discrepancy - recombination line versus forbidden line abundances for a northern sample of galactic planetary nebulae
We present deep optical spectra of 23 galactic planetary nebulae, whichare analysed in conjunction with archival infrared and ultravioletspectra. We derive nebular electron temperatures based on standardcollisionally excited line (CEL) diagnostics as well as the hydrogenBalmer jump and find that, as expected, the Balmer jump almost alwaysyields a lower temperature than the [OIII] nebular-to-auroral lineratio. We also make use of the weak temperature dependence of helium andOII recombination line ratios to further investigate the temperaturestructure of the sample nebulae. We find that, in almost every case, thederived temperatures follow the relation , which is the relationpredicted by two-component nebular models in which one component is coldand hydrogen-deficient. Te(OII) may be as low as a fewhundred Kelvin, in line with the low temperatures found for thehydrogen-deficient knots of Abell 30 by Wesson, Liu and Barlow.Elemental abundances are derived for the sample nebulae from both CELsand optical recombination lines (ORLs). ORL abundances are higher thanCEL abundances in every case, by factors ranging from 1.5 to 12. Fiveobjects with O2+ abundance discrepancy factors greater than 5are found. DdDm 1 and Vy 2-2 are both found to have a very largeabundance discrepancy factor of 11.8.We consider the possible explanations for the observed discrepancies.From the observed differences between Te(OIII) andTe(BJ), we find that temperature fluctuations cannot resolvethe abundance discrepancies in 22 of the 23 sample nebulae, implyingsome additional mechanism for enhancing ORL emission. In the oneambiguous case, the good agreement between abundances derived fromtemperature-insensitive infrared lines and temperature-sensitive opticallines also points away from temperature fluctuations being present. Theobserved recombination line temperatures, the large abundancediscrepancies and the generally good agreement between infrared andoptical CEL abundances all suggest instead the existence of a coldhydrogen-deficient component within the `normal' nebular gas. The originof this component is as yet unknown.

The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edge
We infer the gas kinematics, diagnostics and ionic radial profiles,distance and central star parameters, nebular photo-ionization model,spatial structure and evolutionary phase of the Planetary Nebula NGC6741 by means of long-slit ESO NTT+EMMI high-resolution spectra at nineposition angles, reduced and analysed according to the tomographic and3-D methodologies developed at the Astronomical Observatory of Padua(Italy). NGC 6741 (distance≃2.0 kpc, age≃ 1400 yr, ionizedmass Mion≃ 0.06 Mȯ) is a dense(electron density up to 12 000 cm-3), high-excitation,almost-prolate ellipsoid (0.036 pc × 0.020 pc × 0.018 pc,major, intermediate and minor semi-axes, respectively), surrounded by asharp low-excitation skin (the ionization front), and embedded in aspherical (radius≃ 0.080 pc), almost-neutral, high-density (n(HI)≃ 7 ×103 atoms cm-3) halo containinga large fraction of the nebular mass (Mhalo≥ 0.20Mȯ). The kinematics, physical conditions and ionicstructure indicate that NGC 6741 is in a deep recombination phase,started about 200 years ago, and caused by the rapid luminosity drop ofthe massive (M*=0.66{-}0.68 Mȯ), hot (logT* ≃ 5.23) and faint (logL*/Lȯ ≃ 2.75) post-AGB star, which hasexhausted the hydrogen-shell nuclear burning and is moving along thewhite dwarf cooling sequence. The general expansion law of the ionizedgas in NGC 6741, Vexp(km s-1)=13 × R arcsec,fails in the innermost, highest-excitation layers, which move slowerthan expected. The observed deceleration is ascribable to the luminositydrop of the central star (the decreasing pressure of the hot-bubble nolonger balances the pressure of the ionized gas), and appears instriking contrast to recent reports inferring that acceleration is acommon property of the Planetary Nebulae innermost layers. A detailedcomparative analysis proves that the "U"-shaped expansion velocity fieldis a spurious, incorrect result due to a combination of: (a) simplisticassumptions (spherical shell hypothesis for the nebula); (b) unfitreduction method (emission profiles integrated along the slit); and (c)inappropriate diagnostic choice (λ4686 Å of He II, i.e. athirteen fine-structure components recombination line). Some generalimplications for the shaping mechanisms of Planetary Nebulae arediscussed.

Whole Earth Telescope observations of the pulsating hot white dwarf PG 1707+427
We report on the analysis of multisite time-series photometry of thepulsating pre-white dwarf (GW Vir star) PG 1707+427, obtained by theWhole Earth Telescope collaboration. This is the last of the known GWVir stars without surrounding nebulae to be resolved by multisite data.Successful resolution of the pulsation spectrum resulted from thecombination of high signal-to-noise observations with a large telescopeand wide coverage in longitude with smaller telescopes. We find a seriesof 8 pulsation frequencies (along with two nonlinear combinationfrequencies), and identify 7 of them as part of a sequence of ℓ=1modes, with a common period spacing of 23.0 s. This spacing implies thatthe mass of PG 1707+427 is 0.57 Mȯ. Preliminary modelfits suggest that the mass determined via asteroseismology is consistentwith the mass determined from spectroscopy combined with evolutionarytracks.

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.

Observations and three-dimensional photoionization modelling of the Wolf-Rayet planetary nebula NGC 1501
Deep optical spectra of the high-excitation planetary nebula NGC 1501and its W04 central star are presented. A recombination line abundanceanalysis of the emission-line spectrum of the central star yields He:C:Omass fractions of 0.36:0.48:0.16, similar to those of PG 1159 stars. Adetailed empirical analysis of the nebular collisionally excited line(CEL) and optical recombination line (ORL) spectrums are presented,together with fully three-dimensional photoionization modelling of thenebula. We found very large ORL-CEL abundance discrepancy factors (ADFs)for O2+ (32) and Ne2+ (33). The mean value of~5100 K for the Te derived from HeI recombination linesratios is 6000 K, lower than the value of 11100 K implied by the [OIII]line ratio. This result indicates the existence of a second,low-temperature nebular component, which could account for the observedORL emission. Electron temperature fluctuations (t2) cannotaccount for the high ADFs found from our optical spectra of this nebula.A three-dimensional photoionization model of NGC 1501 was constructedusing the photoionization code MOCASSIN, based on our new spectroscopicdata and using the three-dimensional electron density distributiondetermined from long-slit echellograms of the nebula by Ragazzoni et al.The central star ionizing radiation field is approximated by a modelatmosphere, calculated using the Tübingen non-local thermodynamicequilibrium model atmosphere package, for abundances typical of the W04nucleus of NGC 1501 and PG 1159 stars. The nebular emission-linespectrum was best reproduced using a central star model with aneffective temperature of Teff= 110 kK and a luminosity ofL*= 5000Lsolar. The initial models showed higherdegrees of ionization of heavy elements than indicated by observations.We investigated the importance of the missing low-temperaturedielectronic recombination rates for third-row elements and haveestimated upper limits to their rate coefficients.Our single-phase, three-dimensional photoionization model heavilyunderpredicts the optical recombination line emission. We conclude thatthe presence of a hydrogen-deficient, metal-rich component is necessaryto explain the observed ORL spectrum of this object. The existence ofsuch knots could also provide a softening of the radiation field, viathe removal of ionizing photons by absorption in the knots, therebyhelping to alleviate the overionization of the heavy elements in ourmodels.

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.

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 ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}

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

Amplitude and frequency variability of the pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 observed with the Whole Earth Telescope
We have acquired new time series photometry of the two pulsating DBwhite dwarf stars KUV 05134+2605 and PG 1654+160 with the Whole EarthTelescope. Additional single-site photometry is also presented. We useall these data plus all available archival measurements to study thetemporal behaviour of the pulsational amplitudes and frequencies ofthese stars for the first time.We demonstrate that both KUV 05134+2605 and PG 1654+160 pulsate in manymodes, the amplitudes of which are variable in time; some frequencyvariability of PG 1654+160 is also indicated. Beating of multiplepulsation modes cannot explain our observations; the amplitudevariability must therefore be intrinsic. We cannot find stable modes tobe used for determinations of the evolutionary period changes of thestars. Some of the modes of PG 1654+160 appear at the same periodswhenever detected. The mean spacing of these periods (~40 s) suggeststhat they are probably caused by non-radial gravity-mode pulsations ofspherical degree l= 1. If so, PG 1654+160 has a mass around 0.6Msolar.The time-scales of the amplitude variability of both stars (down to twoweeks) are consistent with theoretical predictions of resonant modecoupling, a conclusion which might however be affected by the temporaldistribution of our data.

The 3-D ionization structure of NGC 6818: A Planetary Nebula threatened by recombination
Long-slit NTT+EMMI echellograms of NGC 6818 (the Little Gem) at nineequally spaced position angles, reduced according to the 3-D methodologyintroduced by Sabbadin et al. (\cite{Sabbadin00}a,b), allowed us toderive: the expansion law, the diagnostics and ionic radial profiles,the distance and the central star parameters, the nebularphoto-ionization model, the 3-D reconstruction in He II, [O III] and [NII], the multicolor projection and a series of movies. The Little Gemresults to be a young (3500 years), optically thin (quasi-thin in somedirections) double shell (Mion =~ 0.13 Msun) ata distance of 1.7 kpc, seen almost equatorial on: a tenuous and patchyspherical envelope (r =~ 0.090 pc) encircles a dense and inhomogeneoustri-axial ellipsoid (a/2 =~ 0.077 pc, a/b =~ 1.25, b/c =~ 1.15)characterized by a hole along the major axis and a pair of equatorial,thick moustaches. NGC 6818 is at the start of the recombination phasefollowing the luminosity decline of the 0.625 Msun centralstar, which has recently exhausted the hydrogen shell nuclear burningand is rapidly moving toward the white dwarf domain (log T*=~ 5.22 K; log L*/Lsun =~ 3.1). The nebula isdestined to become thicker and thicker, with an increasing fraction ofneutral, dusty gas in the outermost layers. Only over some hundreds ofyears the plasma rarefaction due to the expansion will prevail againstthe slower and slower stellar decline, leading to a gradual re-growingof the ionization front. The exciting star of NGC 6818 (mV =~17.06) is a visual binary: a faint, red companion (mV =~17.73) appears at 0.09 arcsec in PA =190degr , corresponding to aseparation ge 150 AU and to an orbital period ge 1500 years.Based on observations made with ESO Telescopes at the La SillaObservatories, under programme ID 65.I-0524, and on observations madewith the NASA/ESA Hubble Space Telescope, obtained from the data archiveat the Space Telescope Institute (observing programs GO 7501 and GO8773; P.I. Arsen Hajian). STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under the NASA contract NAS5-26555. We have applied the photo-ionization code CLOUDY, developed atthe Institute of Astronomy of the Cambridge University.

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.

Wind inhomogeneities in [WC] central stars of planetary nebulae
Not Available

The Correlation of PN Morphology and Parameters (invited review)
Not Available

Abundances of [WC] Central Stars of PN and the Double Dust Chemistry Problem (invited review)
Not Available

WR Central Stars (invited review)
Not Available

PNN NGC 246: A Complex Photometric Behaviour that Requires WET
We present a study over three single-site campaigns to investigate thephotometric behaviour of the PNN NGC 246. We observed this object in2000 and 2001. The analysis of the light curves indicates complex andvariable temporal spectra. Using wavelet analysis we have foundevidences for changes on time scales of hours in the 2000 dataset. Thetemporal spectra obtained during 2001 are quite different from theresults of the previous year. The modulations in the light curve aremore noticeable and the temporal spectra present a higher number ofmodulation frequencies. One peculiar characteristic is the presence ofa variable harmonic structure related to one of these modulationfrequencies. This complex photometric behaviour may be explained by amore complicated unresolved combination of modulation frequencies, butmore likely due to a combination of pulsations of the star plusmodulations related to interaction with a close companion, maybeindicating a disc. However, these characteristics cannot be confirmedfrom single site observations. The complex and variable behaviour ofNGC 246 needs the WET co-operation in order to completely resolve itslight curve.

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.

The 3-D ionization structure of the planetary nebula NGC 6565
A detailed study of the planetary nebula NGC 6565 has been carried outon long-slit echellograms (lambda /Delta lambda =60 000, spectral range= lambda lambda 3900-7750 Å) at six, equally spaced positionangles. The expansion velocity field, the c(Hβ ) distribution andthe radial profile of the physical conditions (electron temperature anddensity) are obtained. The distance, radius, mass and filling factor ofthe nebula and the temperature and luminosity of the central star arederived. The radial ionization structure is analyzed using both theclassical method and the photo-ionization code CLOUDY. Moreover, wepresent the spatial structure in a series of images from differentdirections, allowing the reader to ``see'' the nebula in 3-D. NGC 6565results to be a young (2000-2500 years), patchy, optically thicktriaxial ellipsoid (a=10.1 arcsec, a/b=1.4, a/c=1.7) projected almostpole-on. The matter close to major axis was swept-up by someaccelerating agent (fast wind? ionization? magnetic fields?), formingtwo faint and asymmetric polar cups. A large cocoon of almost neutralgas completely embeds the ionized nebula. NGC 6565 is in a recombinationphase, because of the luminosity drop of the massive powering star,which is reaching the white dwarf domain (log T* =~ 5.08 K;log L*/Lsun =~ 2.0). The stellar decline startedabout 1000 years ago, but the main nebula remained optically thin forother 600 years before the recombination phase occurred. In the nearfuture the ionization front will re-grow, since the dilution factor dueto the expansion will prevail on the slower and slower stellar decline.NGC 6565 is at a distance of 2.0 (+/-0.5) kpc and can be divided intothree radial zones: the ``fully ionized'' one, extending up to0.029-0.035 pc at the equator (0.050 pc at the poles), the``transition'' one, up to 0.048-0.054 pc (0.080 pc), the ``halo'',detectable up to 0.110 pc. The ionized mass ( =~ 0.03 Msun)is only a fraction of the total mass (>= 0.15 Msun), whichhas been ejected by an equatorial enhanced superwind of 4 (+/-2) x10-5 Msun yr-1 lasted for 4 (+/-2) x103 years. Based on observations made with ESO Telescopes atthe La Silla Observatories, under programme ID 65.I-0524, and onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive at the Space Telescope Institute (observingprogram GO 7501; P.I. Arsen Hajian). STScI is operated by theassociation of Universities for Research in Astronomy, Inc. under theNASA contract NAS 5-26555. We have applied the photoionization codeCLOUDY, developed at the Institute of Astronomy of the CambridgeUniversity.

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Observation and Astrometry data

Right ascension:04h06m59.19s
Apparent magnitude:13

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NGC 2000.0NGC 1501

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