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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.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| The distances of less-evolved planetary nebulae: a further test of statistical distance scales
It has recently been pointed out that a number of the methods used todetermine planetary nebulae (PNe) distances may be appreciably in error.Whilst the scales of Zhang (1995), Bensby & Lundstrom (2001) andothers are appropriate for higher radio brightness temperaturesTB, those of Phillips and Daub are more relevant whereTB is small.We note, in the following, that the absolute bolometric magnitudes ofless-evolved PNe are likely to be similar. The mean value of can therefore be used to constrain PNe distancesD, and confirm the distance scales for higher TB outflows. Wehave used this procedure to evaluate distances to a further 47 PNe, andwe find that the mean values of are consistent with those ofCahn, Kaler & Stanghellini (1992), Zhang (1995), Phillips et al.(2004) and van de Steene & Zijlstra (1995). They are, as expected,inconsistent with the lower TB scale of Phillips (2002a).
| 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.
| Discovery of magnetic fields in central stars of planetary nebulae
For the first time we have directly detected magnetic fields in centralstars of planetary nebulae by means of spectro-polarimetry with FORS1 atthe VLT. In all four objects of our sample we found kilogauss magneticfields, in NGC 1360 and LSS 1362 with very highsignificance, while in EGB 5 and Abell 36 the existence ofa magnetic field is probable but with less certainty. This discoverysupports the hypothesis that the non-spherical symmetry of mostplanetary nebulae is caused by magnetic fields in AGB stars. Our highdiscovery rate demands mechanisms to prevent full conservation ofmagnetic flux during the transition to white dwarfs.Based on observations collected at the European Southern Observatory,Paranal, Chile, under programme ID 072.D-0089.
| The distances of highly evolved planetary nebulae
The central stars of highly evolved planetary nebulae (PNe) are expectedto have closely similar absolute visual magnitudes MV. Thisenables us to determine approximate distances to these sources where oneknows their central star visual magnitudes, and levels of extinction. Wefind that such an analysis implies values of D which are similar tothose determined by Phillips; Cahn, Kaler & Stanghellin; Acker, andDaub. However, our distances are very much smaller than those of Zhang;Bensby & Lundstrom, and van de Steene & Zijlstra. The reasonsfor these differences are discussed, and can be traced to errors in theassumed relation between brightness temperature and radius.Finally, we determine that the binary companions of such stars can be nobrighter than MV~ 6mag, implying a spectral type of K0 orlater in the case of main-sequence stars.
| 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.
| 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.
| The SAI Catalog of Supernovae and Radial Distributions of Supernovae of Various Types in Galaxies
We describe the Sternberg Astronomical Institute (SAI) catalog ofsupernovae. We show that the radial distributions of type-Ia, type-Ibc,and type-II supernovae differ in the central parts of spiral galaxiesand are similar in their outer regions, while the radial distribution oftype-Ia supernovae in elliptical galaxies differs from that in spiraland lenticular galaxies. We give a list of the supernovae that arefarthest from the galactic centers, estimate their relative explosionrate, and discuss their possible origins.
| Physical Structure of Planetary Nebulae. III. The Large and Evolved NGC 1360
NGC 1360 is a large planetary nebula (PN) without an obvious shellmorphology. We have analyzed Hα images and high-dispersion echellespectra of NGC 1360 in order to construct spatio-kinematic models and todetermine its density distribution. The best-fit model indicates thatNGC 1360 is a prolate ellipsoidal shell whose major axis is twice aslong as its minor axis and is tilted by 60° with respect to the lineof sight. The large kinematic age of the shell, ~10,000 yr, and the lowdensity of the nebula, <=130 H atom cm-3, imply that NGC1360 is an evolved PN and has begun to merge with the interstellarmedium. The observed morphology and surface brightness profiles of NGC1360 can be described well as a thick shell with a Gaussian radialdensity profile without a sharp inner edge, indicating a lack of ongoingcompression by a fast stellar wind. The fast, low-ionization emissionregions observed in NGC 1360 near the end of its major axis expandfaster than the shell and are younger than the nebular shell.
| A Far-Ultraviolet Spectroscopic Analysis of the Central Star of the Planetary Nebula Longmore 1
We have performed a non-LTE spectroscopic analysis using far-UV and UVdata of the central star of the planetary nebula K1-26 (Longmore 1) andfound Teff=120+/-10 kK,logg=6.7+0.3-0.7 cm s-2, and Y~=0.10.The temperature is significantly hotter than previous results based onoptical line analyses, highlighting the importance of analyzing thespectra of such hot objects at shorter wavelengths. The spectra showmetal lines (from, e.g., carbon, oxygen, sulfur, and iron). Thesignatures of most elements can be fit adequately using solarabundances, confirming the classification of Lo 1 as a high-gravity O(H)object. Adopting a distance of 800 pc, we derive R*~=0.04Rsolar, L~=250 Lsolar, and M~=0.6Msolar. This places the object on the white dwarf coolingsequence of the evolutionary tracks with an age ofτevol~=65 kyr.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer and data from the MAST archive. FUSE is operatedfor NASA by the Johns Hopkins University under NASA contract NAS5-32985.
| The relation between Zanstra temperature and morphology in planetary nebulae
We have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology.
| Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinates
We have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029}
| Astronomical seeing at the Magdalena Ridge Observatory
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| X-ray and UV Views of Hot Gas in Planetary Nebulae (invited review)
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| Temperature Scale and Iron Abundances of Very Hot Central Stars of Planetary Nebulae (invited review)
The determination of effective temperatures of very hot central stars(Teff>70000K) by model atmosphere analyses of optical H and He lineprofiles is afflicted with considerable uncertainty, primarily due tothe lack of neutral helium lines. Ionization balances of metals,accessible only with UV lines, allow more precise temperature estimates.The potential of iron lines is pointed out. At the same time iron andother metal abundances, hardly investigated until today, may be derivedfrom UV spectra. We describe recent HST spectroscopy performed for thispurpose. A search for iron lines in FUV spectra of the hottestH-deficient central stars (PG1159-type, Teff>100000K) taken with FUSEwas unsuccessful. The derived deficiency is interpreted in terms of irondepletion due to n-capture nucleosynthesis in intershell matter, whichis now exposed at the stellar surface as a consequence of a late Heshell flash.
| The Spectral Energy Distribution of the Seyfert Galaxy Ton S180
We present spectral results from a multisatellite, broadband campaign onthe narrow-line Seyfert 1 galaxy Ton S180 (PHL 912) performed at the endof 1999. We discuss the spectral energy distribution (SED) of thesource, combining simultaneous Chandra, ASCA, and Extreme UltravioletExplorer data with contemporaneous Far Ultraviolet SpectroscopicExplorer (FUSE), Hubble Space Telescope, and ground-based optical andinfrared data. The resulting SED shows that most of the energy isemitted in the 10-100 eV regime, which must be dominated by the primaryenergy source. No spectral turnover is evident in the UV regime. This,the strong soft X-ray emission, and the overall shape of the SEDindicate that emission from the accretion disk peaks between 15 and 100eV. High-resolution FUSE spectra showing UV absorption due to O VI andthe lack of detectable X-ray absorption in the Chandra spectrumdemonstrate the presence of a low column density of highly ionized gasalong our line of sight. The highly ionized state of the circumnucleargas is most likely linked to the high luminosity and steep spectrum ofthe active nucleus. Given the strong ionizing flux in Ton S180, it ispossible that the clouds within a few tens of light days of the centralsource are too highly ionized to produce much line emission. Thus, thenarrow width of the emission lines in Ton S180 is due to the emissionarising from large radii. Based (in part) on observations made with theDanish 1.5 m telescope at ESO, La Silla, Chile.
| Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.
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| Temperature of the Central Stars of Planetary Nebulae and the Effect of the Nebular Optical Depth
The effect of the nebula optical depth on the determination of thetemperature (T*) of the central stars in planetary nebulae isdiscussed. Based on photoionization models for planetary nebulae withdifferent optical depths, we show, quantitatively, that the details ofthe distribution of the H and He II Zanstra temperatures are mainlyexplained by an optical depth effect; in particular, that thediscrepancy is larger for low stellar temperatures. The results alsoshow that for high stellar temperatures the He II Zanstra temperatureunderestimates the stellar temperature, even for high optical depths.The stellar temperature, as well as the optical depth, can be obtainedfrom a Zanstra temperature ratio (ZR) plotZR=TZ(HeII)/TZ(H)versus TZ(He II). The effects of departures from a blackbodyspectrum, as well as of the He abundance in the nebulae, are alsodiscussed. For nebulae of very low optical depth and/or high stellartemperature the distribution ZR versus TZ(He II) onlyprovides lower limits for T*. In order to obtain bettervalues for the optical depth and T*, we propose the use ofthe line intensity ratio HeII/HeI versusTZ(He II) diagram.
| ROSAT Observations of X-Ray Emission from Planetary Nebulae
We have searched the entire ROSAT archive for useful observations tostudy X-ray emission from Galactic planetary nebulae (PNs). The searchyields a sample of 63 PNs, which we call the ROSAT PN sample. About20%-25% of this sample show X-ray emission; these include 13 definitedetections and three possible detections (at a 2 σ level). AllX-ray sources in these PNs are concentrated near the central stars. OnlyA30, BD +30°3639, and NGC 6543 are marginally resolved by the ROSATinstruments. Three types of X-ray spectra are seen in PNs. Type 1consists of only soft X-ray emission (<0.5 keV), peaks at 0.1-0.2keV, and can be fitted by blackbody models at temperatures1-2×105 K. Type 2 consists of harder X-ray emission,peaks above 0.5 keV, and can be fitted by thin plasma emission models attemperatures of a few times 106 K. Type 3 is a composite of abright type 1 component and a fainter type 2 component. Unresolved softsources with type 1 spectra or the soft component of type 3 spectra aremost likely photospheric emission from the hot central stars. Absorptioncross sections are large for these soft-energy photons; therefore, onlylarge, tenuous, evolved PNs with hot central stars and small absorptioncolumn densities have been detected. The origin of hard X-ray emissionfrom PNs is uncertain. PNs with type 2 spectra are small, dense, youngnebulae with relatively cool (<<105 K) central stars,while PNs with type 3 X-ray spectra are large, tenuous, evolved nebulaewith hot central stars. The hard X-ray luminosities are also differentbetween these two types of PNs, indicating perhaps different origins oftheir hard X-ray emission. Future Chandra and XMM observations with highspatial and spectral resolution will help to understand the origin ofhard X-ray emission from PNs.
| Extreme Ultraviolet Astronomy
Astronomical studies in the extreme ultraviolet (EUV) band of thespectrum were dismissed during the early years of space astronomy asimpossible, primarily because of the mistaken view that radiation inthis band would be absorbed by the interstellar medium. Observations inthe 1980s from sounding rockets and limited duration orbital spacecraftbegan to show the potential of this field and led to the deployment oftwo spacecraft devoted to EUV astronomy: the UK Wide Field Camera andthe Extreme Ultraviolet Explorer. The instrumentation in these missions,although quite limited in comparison with instrumentation in otherfields of space astronomy, provided unique and far-reaching results.These included new information on solar system topics, stellarchromospheres and corona, white dwarf astrophysics, cataclysmicvariables, the interstellar medium, galaxies, and clusters of galaxies.We summarize these findings herein.
| Spectrophotometry: Revised Standards and Techniques
The telluric features redward of 6700 Å have been removed from theaccurate spectrophotometric standards of Hamuy et al. to permit morereliable relative and absolute spectrophotometry to be obtained from CCDspectra. Smooth fluxes from 3300 to 10500 Å are best determined bydividing the raw spectra of all objects taken in a night by the rawspectrum of a ``smooth'' spectrum star before deriving the instrumentalresponse function using the revised standard star fluxes. In this waythe telluric features and any large instrumental variation withwavelength are removed from the raw data, leaving smooth spectra thatneed only small corrections to place them on an absolute flux scale.These small corrections with wavelength are well described by alow-order polynomial and result in very smooth flux-calibrated spectra.
| Spectroscopic investigation of old planetaries. IV. Model atmosphere analysis
The results of a NLTE model atmosphere analysis of 27 hydrogen-richcentral stars of old planetary nebulae (PN) are reported. These starswere selected from a previous paper in this series, where we gaveclassifications for a total of 38 central stars. Most of the analyzedcentral stars fill a previously reported gap in the hydrogen-richevolutionary sequence. Our observations imply the existence of twoseparated spectral evolutionary sequences for hydrogen-rich and -poorcentral stars/white dwarfs. This is in line with theoreticalevolutionary calculations, which predict that most post-AGB stars reachthe white dwarf domain with a thick hydrogen envelope of ~ 10(-4) M_sun.We determine stellar masses from the comparison with evolutionary tracksand derive a mass distribution for the hydrogen-rich central stars ofold PNe. The peak mass and the general shape of the distribution is inagreement with recent determinations of the white dwarf massdistribution. The properties of most analyzed stars are well explainedby standard post-AGB evolution. However, for eight stars of the sampleother scenarios have to be invoked. The properties of three of them areprobably best explained by born again post-AGB evolution. Two of theseare hybrid CSPN (hydrogen-rich PG 1159 stars), but surprisingly thethird star doesn't show any signs of chemical enrichment in itsatmosphere. The parameters of five stars are not in accordance withpost-AGB evolution. We discuss alternative scenarios such as thestripping of the hydrogen-rich envelope by a companion during the firstred giant phase or the formation of a common envelope with a possiblemerging of both components. Two stars (HDW 4 andHaWe 5) remain mysterious after all. They resembleordinary hot DA white dwarfs, but due to very large evolutionary agesthe presence of a PN cannot be explained. We speculate that the nebulaemay be shells produced by ancient nova outbursts. A wide spread ofhelium abundances is observed in the photospheres of central stars ofold PNe. It is shown that a good correlation between helium abundancesand luminosity is present. It is inferred that when the stars'luminosities fall below L~ 300 L_sun depletion starts and the heliumabundance steadily decreases with decreasing luminosity. The existenceof this correlation is in qualitative agreement with recent theoreticalcalculations of gravitational settling in the presence of a stellarwind.
| The iron abundance in hot central stars of planetary nebulae derived from IUE spectra
We present the first attempt to determine the iron abundance in hotcentral stars of planetary nebulae. We perform an analysis with fullymetal-line blanketed NLTE model atmospheres for a sample of ten stars(T_eff >~ 70 000 K) for which high-resolution UV spectra areavailable from the IUE archive. In all cases lines of Fe Vi or Fe Viican be identified. As a general trend, the iron abundance appears to besubsolar by 0.5-1 dex, however, the S/N of the IUE spectra is notsufficient to exclude a solar abundance in any specific case. Improvedspectroscopy by either FUSE or HST is necessary to verify thepossibility of a general iron deficiency in central stars. The suspecteddeficiency may be the result of gravitational settling in the case ofthree high-gravity objects. For the other stars with low gravity andhigh luminosity dust fractionation during the previous AGB phase is aconceivable origin. Based on observations with the InternationalUltraviolet Explorer (IUE) satellite.
| A HUBBLE SPACE TELESCOPE Survey for Resolved Companions of Planetary Nebula Nuclei
We report the results of a Hubble Space Telescope ``snapshot'' surveyaimed at finding resolved binary companions of the central stars ofGalactic planetary nebulae (PNe). Using the the Wide Field and PlanetaryCamera and Wide Field Planetary Camera 2, we searched the fields of 113PNe for stars whose close proximity to the central star suggests aphysical association. In all, we find 10 binary nuclei that are verylikely to be physically associated and another six that are possiblebinary associations. By correcting for interstellar extinction andplacing the central stars' companions on the main sequence (or, in onecase, on the white dwarf cooling curve), we derive distances to theobjects, and thereby significantly increase the number of PNe withreliable distances. Comparison of our derived distances with thoseobtained from various statistical methods shows that all of the latterhave systematically overestimated the distances, by factors ranging upto a factor of 2 or more. We show that this error is most likely due tothe fact that the properties of our PNe with binary nuclei aresystematically different from those of PNe used heretofore to calibratestatistical methods. Specifically, our PNe tend to have lower surfacebrightnesses at the same physical radius than the traditionalcalibration objects. This difference may arise from a selection effect:the PNe in our survey are typically nearby, old nebulae, whereas most ofthe objects that calibrate statistical techniques are low-latitude, highsurface brightness, and more distant nebulae. As a result, thestatistical methods that seem to work well with samples of distant PNe,for example, those in the Galactic bulge or external galaxies, may notbe applicable to the more diverse population of local PNe. Our distancedeterminations could be improved with better knowledge of themetallicities of the individual nebulae and central stars, measurementsof proper motions and radial velocities for additional candidatecompanions, and deeper HST images of several of our new binary nuclei.
| Final Astro-2 Calibration of the Hopkins Ultraviolet Telescope
We present the final calibration of the Hopkins Ultraviolet Telescope(HUT) for its flight during the Astro-2 space shuttle mission in 1995March. Aspects of mission operations and instrument performance thataffect data quality are described in detail, as are the data reductionprocedures applied to the archived data. The sensitivity calibration isdefined by a comparison between synthetic spectra and observationsobtained in flight of the hot DA white dwarfs HZ 43, GD 71, GD 153, andG191-B2B the synthetic spectra were calculated by D. Koester using modelparameters derived from fits to ground-based spectra. The resultingflux-calibrated spectra for these stars differ from their respectivemodel predictions by less than 3% at all wavelengths, except at thecores of the Lyman lines where the observed line profiles are shallowerthan the predicted profiles. As an additional consistency check, the HUTspectrum of BD +75 deg325 was found to differ from the Hubble SpaceTelescope Faint Object Spectrograph spectrum by at most 5% in the regionof overlap. The wavelength scale and spectral resolution were calibratedby observations of symbiotic stars and of the coronal star alpha Aur.The spectral resolution was found to vary from 1.8 to roughly 4.5Å, depending on wavelength and on the instrument configuration.The internal consistency of the HUT calibration provides a new andimportant test of white dwarf model atmospheres, as the predictedstellar spectra are more sensitive to changes in model parameters or toshortcomings in the models themselves at wavelengths shortward of 1100Å than at longer wavelengths. Combining this result with that ofthe Astro-1 HUT calibration (that a synthetic spectrum of G191-B2B andlaboratory flux standards gave results consistent within the laboratorymeasurement uncertainties) demonstrates that pure hydrogen white dwarfswith effective temperatures of 32,000-61,000 K may be used as primaryflux standards down to the Galactic Lyman edge.
| Narrow-band imaging and a search for planetary nebulae
We summarize the results of narrow-band (Hα and [\O 3] lambda 5007Angstroms) imaging of eight planetary nebulae taken during recentobserving campaigns: We present the probably largest known PN, MWP 1,with a linear diameter of about 6 pc. We show first narrow-band imagesof He 2-437, MeWe 1-5, MeWe 1-8, MeWe 1-9, NGC 1360, and of - like weterm them - the ``Galactic footballs'' A 43 and NGC 7094. We reportabout a search for planetary nebulae around eleven extremely hot (pre-)white dwarfs which had not been successful. Based on observationsobtained at the German-Spanish Astronomical Center, Calar Alto, operatedby the Max-Planck-Institut für Astronomie Heidelberg jointly withthe Spanish National Commission for Astronomy and at the EuropeanSouthern Observatory, La Silla, Chile.
| Possible trends in expansion velocities of planetary nebulae evolved from common envelope binary systems
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