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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.

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.

Trigonometric Parallaxes of Central Stars of Planetary Nebulae
Trigonometric parallaxes of 16 nearby planetary nebulae are presented,including reduced errors for seven objects with previous initial resultsand results for six new objects. The median error in the parallax is0.42 mas, and 12 nebulae have parallax errors of less than 20%. Theparallax for PHL 932 is found here to be smaller than was measured byHipparcos, and this peculiar object is discussed. Comparisons are madewith other distance estimates. The distances determined from theseparallaxes tend to be intermediate between some short distance estimatesand other long estimates; they are somewhat smaller than those estimatedfrom spectra of the central stars. Proper motions and tangentialvelocities are presented. No astrometric perturbations from unresolvedclose companions are detected.

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.

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.

Asteroseismic inferences on GW Virginis variable stars in the frame of new PG 1159 evolutionary models
An adiabatic, nonradial pulsation study of GW Vir stars is presented.The pulsation calculations were based on PG 1159 evolutionary sequenceswith different stellar masses artificially derived from a fullevolutionary sequence of 0.5895 Mȯ that has beencomputed by taking the evolutionary history of the progenitor star intoaccount. The artificial sequences were constructed by appropriatelyscaling the stellar mass of the 0.5895-Mȯ sequence wellbefore the models reached the low-luminosity, high-gravity stage of theGW Vir domain. We computed g-mode pulsation periods appropriate to GWVir variable stars. The implications for the mode-trapping properties ofour PG 1159 models are discussed at length. We found that themode-trapping features characterizing our PG 1159 models are mostlyfixed by the stepped shape of the core chemical profile left by priorconvective overshooting. This is particularly true at least for therange of periods observed in GW Vir stars. In addition, we makeasteroseismic inferences about the internal structure of the GW Virstars PG 1159-035, PG 2131+066, PG 1707+427, and PG 0122+200.

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.

New pulsating white dwarfs in cataclysmic variables
The number of discovered non-radially pulsating white dwarfs (WDs) incataclysmic variables (CVs) is increasing rapidly by the aid of theSloan Digital Sky Survey (SDSS). We performed photometric observationsof two additional objects, SDSS J133941.11+484727.5 (SDSS 1339),independently discovered as a pulsator by Gänsicke et al., and SDSSJ151413.72+454911.9, which we identified as a CV/ZZ Ceti hybrid. In thisLetter we present the results of the remote observations of thesetargets performed with the Nordic Optical Telescope (NOT) during theNordic-Baltic Research School at Molėtai Observatory, and follow-upobservations executed by NOT in service mode. We also present threecandidates we found to be non-pulsating.The results of our observations show that the main pulsation frequenciesagree with those found in previous CV/ZZ Ceti hybrids, but specificallyfor SDSS 1339 the principal period differs slightly between individualobservations and also from the recent independent observation byGänsicke et al. Analysis of SDSS colour data for the small sampleof pulsating and non-pulsating CV/ZZ Ceti hybrids found so far seems toindicate that the r - i colour could be a good marker for theinstability strip of this class of pulsating WDs.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden, in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.E-mail: ricky@astro.lu.se

SDSS J212531.92-010745.9 - the first definite PG 1159 close binary system
Aims.The archival spectrum of SDSS J212531.92-010745.9 shows not onlythe typical signature of a PG 1159 star, but also indicates the presenceof a companion. Our aim was the proof of the binary nature of thisobject and the determination of its orbital period.Methods.We performedtime-series photometry of SDSS J212531.92-010745.9. We observed theobject during 10 nights, spread over one month, with the Tübingen80 cm and the Göttingen 50 cm telescopes. We fitted the observedlight curve with a sine and simulated the light curve of this systemwith the nightfall program. Furthermore, we compared the spectrum ofSDSS J212531.92-010745.9 with NLTE models, the results of which alsoconstrain the light curve solution. Results.An orbital period of6.95616(33) h with an amplitude of 0.354(3) mag is derived from ourobservations. A pulsation period could not be detected. For the PG 1159star we found, as preliminary results from comparison with our NLTEmodels, Teff ~ 90 000 K, log g ~ 7.60, and the abundanceratio C/He ~ 0.05 by number fraction. For the companion we obtained witha mean radius of 0.4 ± 0.1~Rȯ, a mass of 0.4± 0.1~M&odot/, and a temperature of 8200 K on theirradiated side, good agreement between the observed light curve and thenightfall simulation, but we do not regard those values as final.

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.

Modeling the red edge of the GW Vir (PG 1159) instability strip : evolutionary calculations taking into account diffusion and mass loss .
Computations of PG 1159 models show that a red edge for theirpulsational instability is naturally reached around T_{eff } ˜73,000 K when we make use of a simple mass loss law in conjunction withdiffusion theory.

The mean properties of planetary nebulae as a function of Peimbert class
Planetary nebulae are known to possess a broad range of abundances, andthese (with other characteristics) have been used to define five classesof outflow. Peimbert Type I sources, for instance, possess high N and Heabundances, filamentary structures, and low mean scaleheights above theGalactic plane, whilst those of Type III have much lower abundances,high peculiar velocities, and belong to the Galactic thick disc. Apartfrom some rather ill-defined indications, however, very little is knownconcerning their mean physical, spatial, structural, kinematic andthermal characteristics.We have performed a comprehensive study of all of these properties, andfind evidence for strong variations between the various Peimbertclasses. Certain of these differences are consistent with Type I sourceshaving the highest progenitor masses, although it seems that thesenebulae also possess the lowest rms densities and 5-GHz brightnesstemperatures. The latter results are in conflict with a range of recentmodelling.

Imaging Fabry-Perot spectrometric studies of velocity fields in gaseous nebulae
In order to study the spatio-kinematics of extended gaseous nebulae suchas the HII regions associated with giant molecular clouds and planetarynebulae, we had designed and built an Imaging Fabry-Perot Spectrometerfor the 1.2m Mt Abu Telescope. We describe here some of the significantscientific results that came out of these studies in the past one decadeat Mt Abu.

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.

Unresolved Hα Enhancements at High Galactic Latitude in the WHAM Sky Survey Maps
We have identified 85 regions of enhanced Hα emission at|b|>10deg subtending approximately 1° or less on theWisconsin Hα Mapper (WHAM) sky survey. These high-latitude ``WHAMpoint sources'' have Hα fluxes of 10-11-10-9ergs cm-2 s-1, radial velocities within about 70km s-1 of the LSR, and line widths that range from less than20 to about 80 km s-1 (FWHM). Twenty-nine of theseenhancements are not identified with either cataloged nebulae or hotstars and appear to have kinematic properties that differ from thoseobserved for planetary nebulae. Another 14 enhancements are near hotevolved low-mass stars that had no previously reported detections ofassociated nebulosity. The remainder of the enhancements are catalogedplanetary nebulae and small, high-latitude H II regions surroundingmassive O and early B stars.

Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.

Radial-Velocity Survey of Central Stars of Southern Planetary Nebulae .
We have monitored selected southern-hemisphere planetary-nebula nuclei(PNNi) in order to search for radial-velocity (RV) variations. Theobservations have been carried out regularly since early 2003 with theSMARTS Consortium 1.5-m telescope and Cassegrain spectrograph at CerroTololo Inter-American Observatory, Chile. This study is a followup to anearlier survey of northern PNNi made by \citet{demarco04}, whichsuggested that there is a high incidence of RV variability among PNNi.If the variations are due to motion in binary orbits, the fraction ofclose binaries among PNNi must be very high, suggesting that mostplanetary nebulae are ejected through binary-star processes, such ascommon-envelope interactions. We presente here the results of thesouthern portion of our RV survey. Preliminary results indicate that thefraction of variable RVs is also very high among southern PNNi.

X-ray Observations of Hot Gas in Planetary Nebulae
The formation and shaping of planetary nebulae (PNe) is a complexprocess that involves the action of multiple agents, including faststellar winds and collimated outflows. Both fast stellar winds andcollimated outflows can produce shock-heated gas that emits diffuseX-rays. Hot gas in PN interiors was hinted by ROSAT observations,but unambiguous detections of diffuse X-ray emission were not made untilChandra and XMM-Newton became available. The unprecedentedangular resolution and sensitivity of these new X-ray observations allowus to investigate in detail the physical properties and origin of thehot gas content of PNe and to assess its dynamical effects on theshaping and expansion of PNe. This paper reviews the results from recentX-ray observations of PNe and discusses their implications to ourunderstanding of the formation and evolution of PNe.

Winds from Low Mass Stars: impact on the ISM
A large fraction of all stars, after experiencing heavy winds at the endof the Asymptotic Giant Branch (AGB) phase, leave behind a core that isbelow the Chandrasekhar mass limit. This core eventually photoionizesthe stellar ejecta giving birth to a planetary nebula (PN). PN and AGBstars are therefore the remnants of stars that through their windscontribute to the chemical enrichment of the ISM with elements He, C, Nand O. We have explored the impact of such winds in the ISM and in theformation and evolution of the circumstellar envelopes around AGB starsand PN shells. In our simulations, we find that huge shells (up to 2.5pc in radius) are formed as a consequence of the mass-loss during theearly AGB phase. When the star is at rest with respect to the ISM, theseshells contain a large fraction of ISM material that has been swept upby the stellar wind. We find that when the star is moving and the rampressure is high enough, significant dispersal between the stellarejecta and the ISM material still takes place due to instabilities,however, the mass and the size of the envelope are highly reduced due toram pressure stripping.

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).

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.

PFP 1: A Large Planetary Nebula Caught in the First Stages of ISM Interaction
This paper presents (Hα+[NII]) imaging and spectroscopy of apreviously unknown, highly evolved planetary nebula of low excitationwhich is in the first stages of an interaction with the interstellarmedium (ISM). It was discovered serendipitously from AAO/UKST HαSurvey images as part of a project to exploit the survey data and hasevaded detection by previous surveys due to its very low surfacebrightness. It is a remarkable hollow-sphere planetary nebula, some 19'across, making it one of the largest examples of its type. We estimate aradius of 1.5pc and a distance of 550pc as derived from a new Hαsurface brightness-radius relation. PFP 1 has near-perfect circularsymmetry, broken only at the north-western edge which is coupled withsignificantly increased (Hα+[NII]) intensity, both of whichprovide evidence for an interaction with the ISM. We find a near solarcomposition for this object with possibly enhanced He and N abundances.A blue central star candidate has been identified from the SuperCosmosSky Survey data.

Infrared Array Camera (IRAC) Observations of Planetary Nebulae
We present the initial results from the Infrared Array Camera (IRAC)imaging survey of planetary nebulae (PNs). The IRAC colors of PNs arered, especially in the 8.0 μm band. Emission in this band is likelydue to contributions from two strong H2 lines and a [Ar III]line in that bandpass. IRAC is sensitive to the emission in the halos aswell as in the ionized regions that are optically bright. In NGC 246, wehave observed an unexpected ring of emission in the 5.8 and 8.0 μmIRAC bands not seen previously at other wavelengths. In NGC 650 and NGC3132, the 8.0 μm emission is at larger distances from the centralstar compared to the optical and other IRAC bands, possibly related tothe H2 emission in that band and the tendency for themolecular material to exist outside of the ionized zones. In theflocculi of the outer halo of NGC 6543, however, this trend is reversed,with the 8.0 μm emission bright on the inner edges of the structures.This may be related to the emission mechanism, where the H2is possibly excited in shocks in the NGC 6543 halo, whereasH2 emission is likely fluorescently excited in the UV fieldsnear the central star.

Identification of a DO white dwarf and a PG 1159 star in the ESO SN Ia progenitor survey (SPY)
We present high-resolution VLT spectra of a new helium-rich DO whitedwarf (HE1314+0018) and a new PG 1159 star(HE1429-1209), which we identified in the ESO SPYsurvey. We performed NLTE model atmosphere analyses and found that thePG 1159 star is a low-gravity, extremely hot (Teff,= 160 000K, log g,= 6) star, having a carbon-helium dominated atmosphere withconsiderable amounts of oxygen and neon (He = 38%, C = 54%, O = 6%, Ne =2% by mass). It is located within the planetary nebula nucleiinstability strip, hence, future searches for an associated PN as wellas for stellar pulsations might be successful. The DO white dwarf is aunique object. From the relative strength of neutral and ionized heliumlines we found Teff,≈ 60 000 K, however, the He II linesare extraordinarily strong and cannot be fitted by any model. Similarproblems were encountered with hot subdwarfs and white dwarfs showingsignatures of a super-hot wind. The reason is unknown.Based on observations at the Paranal Observatory of the EuropeanSouthern Observatory for programs No. 165.H-0588(A) and 167.D-0407(A).

On the Driving Mechanism and the Coexistence of Variable and Nonvariable Stars in the Domain of the Pulsating PG 1159 Stars
We revisit the controversial question of the excitation of pulsationmodes in models of PG 1159 stars that feature homogeneous envelopes withcompositions comparable to those observed at the surface. We find, inagreement with some authors but contrary to others, that g-modepulsations are naturally excited in such models and that there is noneed to invoke composition gradients between the photosphere and thedriving region in PG 1159 pulsators. We further find an excellentqualitative agreement between the range of predicted periods and therange of observed periods for all objects in our sample of pulsatingstars, except for two models corresponding to the stars with the lowestsurface gravities (central stars of planetary nebulae). We also addressthe problem of the coexistence of both variable and nonvariable PG 1159stars in the same region of the logg-Teff diagram. We find anatural explanation for this cohabitation in terms of a dispersion inatmospheric parameters and in terms of a variation in surfacecomposition from star to star. In particular, the most He-rich starstend to be stable. We finally address the puzzling question of theexistence of a correlation between pulsations and the presence of tracesof nitrogen in the atmospheres of pulsating PG 1159 stars, a challengethat has remained unanswered so far.

Hard X-Ray Emission Associated with White Dwarfs. II.
We have previously conducted a search for X-ray sources coincident withwhite dwarfs using the white dwarf catalog compiled by McCook & Sionand the ROSAT sources in the WGA Catalog (Paper I). To include the whitedwarfs discovered since 1999 and to include the X-ray sources detectedin ROSAT Position Sensitive Proportional Counter (PSPC) observationsmade with a boron filter, we have carried out another search using anupdated list of white dwarfs and the final catalogs of the ROSAT PSPCobservations with and without a boron filter. Forty-seven new X-raysources convincingly coincident with white dwarfs are found and reportedin this paper. Among these, only five show hard X-ray emission: threepossess confirmed or suggested late-type companions, one is apparentlysingle, and the other is likely a misclassified BL Lac object. Theapparently single white dwarf with hard X-ray emission, KPD 0005+5106,was discussed extensively in Paper I. Photospheric origin for the hardX-ray emission from hot DO and DQZO white dwarfs remains a tantalizingpossibility, but high-quality near-IR spectroscopic observations andmonitoring of the Hα emission line are needed to rule out theexistence of a faint dMe companion.

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.

The High-Excitation Planetary Nebula NGC 246: Optical and Near-Ultraviolet Observations and Two-dimensional Numerical Models
We have imaged the planetary nebula (PN) NGC 246 in the near-ultravioletwavelengths [Ne V] 342.6 nm, the Bowen fluorescence line of O III at344.4 nm, and a nearby line-free region centered on 338.6 nm, as well asHα, [O III] 500.7 nm, and [S II] 673.0 and 671.5 nm. Imaging inthe 344.4 nm line is necessary to deconvolve contamination of the [Ne V]images by O III 342.9 nm. The emission from the shell and inner parts ofthe nebula is detected in [Ne V]. The radial profiles of the [Ne V]brightness decrease with radius from the exciting star, indicating thatthe bulk of the emission from this ion is due to the hard UV stellarradiation field, with a (probably) marginal contribution fromcollisional ionization in a shock between the PN shell and theinterstellar medium (ISM). In contrast, the radial profiles of theemission in Hα, [O III] 500.7 nm, and [S II] are flatter and peakat the location of the shell. The emission of [S II] probably traces theinteraction of the PN with the ambient ISM. We also presenttwo-dimensional numerical simulations for this PN-ISM interaction. Thesimulations consider the stellar motion with respect to the ambient ISM,with a velocity of 85 km s-1, and include the time evolutionof the wind parameters and UV radiation field from the progenitor star.

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Observation and Astrometry data

Right ascension:00h47m03.34s
Apparent magnitude:8

Catalogs and designations:
Proper NamesSkull Nebula
NGC 2000.0NGC 246

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