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 Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfsContext: 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. Chemical abundances in seven galactic planetary nebulaeAn observational study of chemical abundances in the galactic planetarynebulae NGC 1535, NGC 2438,NGC 2440, NGC 3132, NGC3242, NGC 6302, and NGC7009 based on long-slit spectra of high signal-to-noise ratioin the 3100 to 6900 Å range is presented. We determined the N, O,Ne, S, and Cl abundances from collisionally excited lines and the He andO++ abundances from recombination lines. TheO++/H+ estimates derived from recombination linesare about a factor of four and two higher than those derived fromforbidden lines for NGC 7009 and NGC3242, respectively. Spatial profiles ofO++/H+ abundance from O II permitted lines andfrom [O III] forbidden lines were obtained for the planetary nebulaNGC 7009. The differences betweenO++/H+ derived from recombination and fromforbidden lines present smooth variations along the nebular surface ofNGC 7009, with the differences decreasing from thecenter to the edges of the nebula. If these abundance differences areexplained by the presence of electron temperature fluctuations,quantified by the parameter t2, a value of aboutt2=0.09 is required for NGC 3242 andNGC 7009. Sailing along River Eridani.Not Available Electron temperature fluctuations in planetary nebulaeAn observational study of the spatial variation of the electrontemperature and density in 10 galactic planetary nebulae is presented.The data consist of long-slit spectra of high signal-to-noise ratio inthe 3100 to 6900 Å range. Electron temperatures were determinedfrom the [O III](λ 4959 + λ 5007)/λ 4363 and [NII](λ 6548 + λ 6583)/λ 5755 ratios and from theBalmer discontinuity. Electron densities were estimated from the [SII]λ 6716/λ 6731, [Cl III]λ 5517/λ 5537, and[Ar IV]λ 4711/λ 4740 ratios. Electron temperaturevariations of low amplitude were found across the nebular surface in theplanetary nebulae studied. The temperature distribution across eachnebula presents a variance relative to the mean corresponding to 0.0003≤ t2s(Bal) ≤ 0.0078, 0.0003 ≤t2s(N II) ≤ 0.0097, and 0.0011 ≤t2s(O III) ≤ 0.0050. A systematic spatialvariation of electron density has been detected in most of objects(NGC 1535, NGC 2438, NGC2440, NGC 3132, NGC3242, NGC 6302, NGC6563, and NGC 7009). The remaining objects(NGC 6781 and NGC 6853) have notshown any significant electron density dependence on position.NGC 2438, NGC 6563, NGC6781, and NGC 6853 are in general the mostdiffuse and probably evolved objects studied here, with low meandensities in the range Ne(S II) ≈ 95-158~cm-3.An anti-correlation between temperature and density was found forNGC 2438 and NGC 3132, with theelectron temperature increasing with the decrease of electron densityand a correlation between temperature and density was found forNGC 2440, NGC 3242, NGC6302, and NGC 7009, with the electrontemperature increasing with the increase of electron density. Theserelationships seem to be associated with the structure of the nebula.The nebulae in which the correlation between temperature and density ispresent are ring shaped. The anti-correlation between temperature anddensity is found in bipolar planetary nebulae that are denser in thecentre of the nebula. The mean properties of planetary nebulae as a function of Peimbert classPlanetary 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. The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their EnvelopesThe results of a study of the chemical compositions of Galacticplanetary nebulae taking into account two types of inhomogeneity in thenebular gas density in their envelopes are reported. New analyticalexpressions for the ionization correction factors have been derived andare used to determine the chemical compositions of the nebular gas inGalactic planetary nebulae. The abundances of He, N, O, Ne, S, and Arhave been found for 193 objects. The Y Z diagrams for various Heabundances are analyzed for type II planetary nebulae separately andjointly with HII regions. The primordial helium abundance Y p andenrichment ratio dY/dZ are determined, and the resulting values arecompared with the data of other authors. Radial abundance gradients inthe Galactic disk are studied using type II planetary nebulae. Fluorine in extremely hot post-AGB stars: Evidence for nucleosynthesisWe have discovered lines of highly ionized fluorine (ion{F}{v} andion{F}{vi}) in the far-UV spectra of extremely hot (T_eff = 85 000-150000 K) post-AGB stars. Our sample comprises H-rich central stars ofplanetary nebulae as well as H-deficient PG1159 stars. We performednon-LTE calculations and find strong F overabundances (up to10-4 by mass, i.e., 250 times solar) in a number of PG1159stars, while F is essentially solar in the H-rich stars. Since PG1159stars are believed to exhibit intershell matter of the preceding AGBphase on their surface, their chemical analyses allow for a directinsight into nucleosynthesis processes during the AGB phase. The high Fabundances in PG1159 stars confirm the conclusion from abundancedeterminations in giants, that F is synthesized in AGB stars and thatthe F enrichment in the intershell must be very high.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985. The evolution of planetary nebulae. II. Circumstellar environment and expansion propertiesWe investigate and discuss the expansion properties of planetary nebulaeby means of 1D radiation-hydrodynamics models computed for differentinitial envelope configurations and central star evolutionary tracks. Inparticular, we study how the expansion depends on the initial densitygradient of the circumstellar envelope and show that it is possible toderive information on the very last mass-loss episodes during the star'sfinal evolution along and off the asymptotic giant branch. To facilitatethe comparison of the models with real objects, we have also computedobservable quantities like surface brightness and emission-lineprofiles. With the help of newly acquired high-resolution emission-lineprofiles for a sample of planetary nebulae we show that models withinitial envelopes based on the assumption of a stationary wind outflowfail to explain the observed expansion speeds of virtually all of theobserved planetary nebulae. Instead it must be assumed that during thevery last phase of evolution along the final asymptotic giant branchevolution the mass-loss rate increases in strength, resulting in a muchsteeper slope of the circumstellar radial density distribution. Underthese conditions, the expansion properties of the nebular gas differconsiderably from the self-similar solutions found for isothermalconditions. Furthermore, the mass loss must remain at a rather highlevel until the stellar remnant begins to evolve quickly towards thecentral star regime. Current theoretical computations of dust-drivenmass-loss which are restricted to rather low temperatures cannot beapplied during the star's departure from the asymptotic giant branch.Based on observations obtained at the 3.5 μm NTT and the 1.2 μmCAT telescope of the European Southern Observatory, La Silla, and at the2.6 μm NOT telescope operated on the island of La Palma by NOTSA, inthe Spanish Observatorio del Roque de Los Muchachos of the InstitutodeAstrofísica de Canarias.Dedicated to Prof. V. Weidemann on the occasion of his 80th birthday,October 3, 2004. 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 NebulaeThe 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. Planetary nebula distances re-examined: an improved statistical scaleThe distances of planetary nebulae (PNe) are still quite uncertain.Although observational estimates are available for a small proportion ofPNe, based on statistical parallax and the like, such distances are verypoorly determined for the majority of galactic PNe. In particular,estimates of so-called statistical' distance appear to differ byfactors of ~2.7.We point out that there is a well-defined correlation between the 5-GHzluminosity of the sources, L5, and their brightnesstemperatures, TB. This represents a different trend to thoseinvestigated in previous statistical analyses, and permits us todetermine independent distances to a further 449 outflows. Thesedistances are shown to be closely comparable to those determined using aTB-R correlation, providing that the latter trend is taken tobe non-linear.This non-linearity in the TB-R plane has not been noted inprevious analyses, and is likely responsible for the broad (andconflicting) ranges of distance that have previously been published.Finally, we point out that there is a close accord between observedtrends within the L5-TB and TB-Rplanes, and the variation predicted through nebular evolutionarymodelling. This is used to suggest that observational biases areprobably modest, and that our revised distance scale is reasonablytrustworthy. Far-Ultraviolet Spectroscopic Analyses of Four Central Stars of Planetary NebulaeWe analyze the far-UV/UV spectra of four central stars of planetarynebulae (CSPNs) with strong wind features-NGC 2371, A78, IC 4776, andNGC 1535-and derive their photospheric and wind parameters by modelinghigh-resolution FUSE (Far Ultraviolet Spectroscopic Explorer) data inthe far-UV and HST (Hubble Space Telescope) STIS and IUE data in the UVwith spherical, non-LTE, line-blanketed model atmospheres. A78 is ahydrogen-deficient transitional [WR]-PG 1159 object, and we find NGC2371 to be in the same stage, both migrating from theconstant-luminosity phase to the white dwarf cooling sequence, withTeff~=120 kK, M~=5×10-8 Msolaryr-1. NGC 1535 is a hydrogen-rich'' O(H) CSPN, and theexact nature of IC 4776 is ambiguous, although it appears to behelium-burning. Both objects lie on the constant-luminosity branch ofpost-AGB evolution and have Teff~=65 kK,M~=1×10-8 Msolar yr-1. Thus, boththe H-rich and H-deficient channels of PN evolution are represented inour sample. We also investigate the effects of including higherionization stages of iron (up to Fe X) in the model atmospherecalculations of these hot objects (usually neglected in previousanalyses) and find iron to be a useful diagnostic of the stellarparameters in some cases. The far-UV spectra of all four objects showevidence of hot (T~300 K) molecular hydrogen in their circumstellarenvironments.ID="FN1"> 1Based on observations made with the NASA-CNES-CSA FarUltraviolet Spectroscopic Explorer (FUSE) and data from the MASTarchive. FUSE is operated for NASA by the Johns Hopkins University,under NASA contract NAS5-32985. 12C/13C Ratio in Planetary Nebulae from the IUE ArchivesWe investigated the abundance ratio of 12C/13C inplanetary nebulae by examining emission lines arising from C III2s2p3Po2,1,0-->2s21S0.Spectra were retrieved from the International Ultraviolet Explorerarchives, and multiple spectra of the same object were co-added toachieve improved signal-to-noise ratio. The 13C hyperfinestructure line at 1909.6 Å was detected in NGC 2440. The12C/13C ratio was found to be ~4.4+/-1.2. In allother objects, we provide an upper limit for the flux of the 1910Å line. For 23 of these sources, a lower limit for the12C/13C ratio was established. The impact on ourcurrent understanding of stellar evolution is discussed. The resultinghigh-signal-to-noise ratio C III spectrum helps constrain the atomicphysics of the line formation process. Some objects have the measured1907/1909 Å flux ratio outside the low-electron densitytheoretical limit for 12C. A mixture of 13C with12C helps to close the gap somewhat. Nevertheless, someobserved 1907/1909 Å flux ratios still appear too high to conformto the currently predicted limits. It is shown that this limit, as wellas the 1910/1909 Å flux ratio, are predominantly influenced byusing the standard partitioning among the collision strengths for themultiplet1S0-3PoJaccording to the statistical weights. A detailed calculation for thefine-structure collision strengths between these individual levels wouldbe valuable. Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IV. Synthesis and the Sulfur AnomalyWe have compiled a large sample of O, Ne, S, Cl, and Ar abundances thathave been determined for 85 Galactic planetary nebulae in a consistentand homogeneous manner using spectra extending from 3600 to 9600Å. Sulfur abundances have been computed using the near-IR lines of[S III] λλ9069, 9532 along with [S III] temperatures. Wefind average values, expressed logarithmically with a standarddeviation, of log(S/O)=-1.91+/-0.24, log(Cl/O)=-3.52+/-0.16, andlog(Ar/O)=-2.29+/-0.18, numbers consistent with previous studies of bothplanetary nebulae and H II regions. We also find a strong correlationbetween [O III] and [S III] temperatures among planetary nebulae. Inanalyzing abundances of Ne, S, Cl, and Ar with respect to O, we find atight correlation for Ne-O, and loose correlations for Cl-O and Ar-O.All three trends appear to be colinear with observed correlations for HII regions. S and O also show a correlation, but there is a definiteoffset from the behavior exhibited by H II regions and stars. We suggestthat this S anomaly is most easily explained by the existence ofS+3, whose abundance must be inferred indirectly when onlyoptical spectra are available, in amounts in excess of what is predictedby model-derived ionization correction factors in PNe. Finally for thedisk PNe, abundances of O, Ne, S, Cl, and Ar all show gradients whenplotted against Galactocentric distance. The slopes are statisticallyindistinguishable from one another, a result which is consistent withthe notion that the cosmic abundances of these elements evolve inlockstep. Rings in the haloes of planetary nebulaeWe present a search for rings or arcs in the haloes of planetary nebulae(PNe). We discovered such structures in eight PNe, tripling the sampleof PNe with known rings. This shows that, contrary to what was believedto date, the occurrence of mass loss fluctuations with timescales of102-103 yrs at the end of the asymptotic giantbranch phase (AGB) is common. We estimate a lower limit of theoccurrence rate of rings in PN haloes to be ˜35%. Using these newdetections and the cases previously known, we discuss the statisticalproperties of ring systems in PNe haloes. We estimate that the massmodulation producing the rings takes place during the last 10 000 or 20000 yrs of AGB evolution. In PNe, the spacing between rings ranges from<0.01 pc to 0.06 pc, significantly larger than those seen inproto-PNe. This, together with the finding of a possible positivecorrelation of spacing with the post-AGB age of the nebulae, suggeststhat the spacing of the rings increases with time. These properties, aswell as the modest surface brightness amplitudes of rings, areconsistent with the predictions of the dust-driven wind instabilitymodel explored by Meijerink et al. (\cite{Me03}), but do not immediatelyexclude other proposed models.Based on observations obtained at: the 2.5 INT telescope of the IsaacNewton Group and the 2.6 m NOT telescope operated by NOTSA in theSpanish Observatorio del Roque de Los Muchachos of the Instituto deAstrofísica de Canarias; the 3.5 m NTT and the 2.2 MPG/ESO at theEuropean Southern Observatory in Chile; and the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by AURA for NASA under contract NAS5-26555. The relation between Zanstra temperature and morphology in planetary nebulaeWe have created a master list of Zanstra temperatures for 373 galacticplanetary nebulae based upon a compilation of 1575 values taken from thepublished literature. These are used to evaluate mean trends intemperature for differing nebular morphologies. Among the most prominentresults of this analysis is the tendency forη=TZ(HeII)/TZ(HeI) to increase with nebularradius, a trend which is taken to arise from the evolution of shelloptical depths. We find that as many as 87 per cent of nebulae may beoptically thin to H ionizing radiation where radii exceed ~0.16 pc. Wealso note that the distributions of values η and TZ(HeII)are quite different for circular, elliptical and bipolar nebulae. Acomparison of observed temperatures with theoretical H-burning trackssuggests that elliptical and circular sources arise from progenitorswith mean mass ≅ 1 Msolar(although the elliptical progenitors are probably more massive).Higher-temperature elliptical sources are likely to derive fromprogenitors with mass ≅2 Msolar, however, implying thatthese nebulae (at least) are associated with a broad swathe ofprogenitor masses. Such a conclusion is also supported by trends in meangalactic latitude. It is found that higher-temperature ellipticalsources have much lower mean latitudes than those with smallerTZ(HeII), a trend which is explicable where there is anincrease in with increasing TZ(HeII).This latitude-temperature variation also applies for most other sources.Bipolar nebulae appear to have mean progenitor masses ≅2.5Msolar, whilst jets, Brets and other highly collimatedoutflows are associated with progenitors at the other end of the massrange (~ 1 Msolar). Indeed it ispossible, given their large mean latitudes and low peak temperatures,that the latter nebulae are associated with the lowest-mass progenitorsof all.The present results appear fully consistent with earlier analyses basedupon nebular scale heights, shell abundances and the relativeproportions of differing morphologies, and offer further evidence for alink between progenitor mass and morphology. Galactic Planetary Nebulae and their central stars. I. An accurate and homogeneous set of coordinatesWe have used the 2nd generation of the Guide Star Catalogue (GSC-II) asa reference astrometric catalogue to compile the positions of 1086Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue(SEC), its supplement and the version 2000 of the Catalogue of PlanetaryNebulae. This constitutes about 75% of all known PNe. For these PNe, theones with a known central star (CS) or with a small diameter, we havederived coordinates with an absolute accuracy of ~0\farcs35 in eachcoordinate, which is the intrinsic astrometric precision of the GSC-II.For another 226, mostly extended, objects without a GSC-II counterpartwe give coordinates based on the second epoch Digital Sky Survey(DSS-II). While these coordinates may have systematic offsets relativeto the GSC-II of up to 5 arcsecs, our new coordinates usually representa significant improvement over the previous catalogue values for theselarge objects. This is the first truly homogeneous compilation of PNepositions over the whole sky and the most accurate one available so far.The complete Table \ref{tab2} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/1029} Angular dimensions of planetary nebulaeWe have measured angular dimensions of 312 planetary nebulae from theirimages obtained in Hα (or Hα + [NII]). We have appliedthree methods of measurements: direct measurements at the 10% level ofthe peak surface brightness, Gaussian deconvolution and second-momentdeconvolution. The results from the three methods are compared andanalysed. We propose a simple deconvolution of the 10% levelmeasurements which significantly improves the reliability of thesemeasurements for compact and partially resolved nebulae. Gaussiandeconvolution gives consistent but somewhat underestimated diameterscompared to the 10% measurements. Second-moment deconvolution givesresults in poor agreement with those from the other two methods,especially for poorly resolved nebulae. From the results of measurementsand using the conclusions of our analysis we derive the final nebulardiameters which should be free from systematic differences between small(partially resolved) and extended (well resolved) objects in our sample.Table 1 is only available in electronic form athttp://www.edpsciences.org The relation between elemental abundances and morphology in planetary nebulaeAn investigation of the variation of elemental abundances with planetarynebula morphology is of considerable interest, since it has a bearingupon how such sources are formed, and from which progenitors they areejected. Recent advances in morphological classification now enable usto assess such trends for a statistically significant number of sources.We find, as a result, that the distribution N[log(X/H)] of sources withrespect to elemental abundance (X/H) varies between the differingmorphologies. Circular sources tend to peak towards low abundancevalues, whilst bipolar nebulae (BPNe) peak towards somewhat highervalues. This applies for most elemental species, although it is perhapsleast apparent for oxygen. In contrast, elliptical sources appear todisplay much broader functions N[log(X/H)], which trespass upon thedomains of both circular and elliptical planetary nebulae (PNe).We take these trends to imply that circular sources derive fromlower-mass progenitors, bipolar sources from higher-mass stars, and thatelliptical nebulae derive from all masses of progenitor, high and low.Whilst such trends are also evident in values of mean abundance, they are much less clear. Only in the cases of He/H, N/H,Ne/H and perhaps Ar/H is there evidence for significant abundancedifferences.Certain BPNe appear to possess low abundance ratios He/H and Ar/H, andthis confirms that a few such outflows may arise from lower-massprogenitors. Similarly, we note that ratios are quite modestin elliptical planetary nebulae, and not much different from those forcircular and bipolar PNe; a result that conflicts with the expectationsof at least one model of shell formation. Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical propertiesWe present a comprehensive observational study of haloes aroundplanetary nebulae (PNe). Deep Hα+[NII] and/or [OIII] narrow-bandimages have been obtained for 35 PNe, and faint extended haloes havebeen newly discovered in the following 10 objects: Cn 1-5, IC 2165, IC2553, NGC 2792, NGC 2867, NGC 3918, NGC 5979, NGC 6578, PB 4, andpossibly IC 1747. New deep images have also been obtained of other knownor suspected haloes, including the huge extended emission around NGC3242 and Sh 2-200. In addition, the literature was searched, andtogether with the new observations an improved data base containing some50 PN haloes has been compiled.The halo sample is illustrated in an image atlas contained in thispaper, and the original images are made available for use by thescientific community at http://www.ing.iac.es/~rcorradi/HALOES/.The haloes have been classified following the predictions of modernradiation-hydrodynamical simulations that describe the formation andevolution of ionized multiple shells and haloes around PNe. According tothe models, the observed haloes have been divided into the followinggroups: (i) circular or slightly elliptical asymptotic giant branch(AGB) haloes, which contain the signature of the last thermal pulse onthe AGB; (ii) highly asymmetrical AGB haloes; (iii) candidaterecombination haloes, i.e. limb-brightened extended shells that areexpected to be produced by recombination during the late post-AGBevolution, when the luminosity of the central star drops rapidly by asignificant factor; (iv) uncertain cases which deserve further study fora reliable classification; (v) non-detections, i.e. PNe in which no halois found to a level of <~10-3 the peak surface brightnessof the inner nebulae.We discuss the properties of the haloes: detection rate, morphology,location of the central stars in the Hertzsprung-Russell diagram, sizes,surface brightness profiles, and kinematical ages. Among the mostnotable results, we find that, as predicted by models, ionized AGBhaloes are a quite common phenomenon in PNe, having been found in 60 percent of elliptical PNe for which adequately deep images exist. Another10 per cent show possible recombination haloes. In addition, using thekinematical ages of the haloes and inner nebulae, we conclude that mostof the PNe with observed AGB haloes have left the AGB far from a thermalpulse, at a phase when hydrogen burning is the dominant energy source.We find no significant differences between the AGB haloes ofhydrogen-poor and hydrogen-rich central stars. Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. III. Observations and Results for a Final SampleThis paper is the fourth in a series whose purpose is to study theinterstellar abundances of sulfur, chlorine, and argon in the Galaxyusing a sample of 86 planetary nebulae. Here we present new high-qualityspectrophotometric observations of 20 Galactic planetary nebulae withspectral coverage from 3700 to 9600 Å. A major feature of ourobservations throughout the entire study has been the inclusion of thenear-infrared lines of [S III] λλ9069, 9532, which allowsus to calculate accurate S+2 abundances and to either improveupon or convincingly confirm results of earlier sulfur abundancestudies. For each of the 20 objects here, we calculate ratios of S/O,Cl/O, and Ar/O and find average values ofS/O=(1.1+/-1.1)×10-2,Cl/O=(4.2+/-5.3)×10-4, andAr/O=(5.7+/-4.3)×10-3. For six objects, we are able tocompare abundances of S+3 calculated directly from available[S IV] 10.5 μm measurements with those inferred indirectly from thevalues of the ionization correction factors for sulfur. In the finalpaper of the series, we will compile results from all 86 objects, searchfor and evaluate trends, and use chemical evolution models to interpretour results. X-ray and UV Views of Hot Gas in Planetary Nebulae (invited review)Not Available 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 WARPS Survey. VI. Galaxy Cluster and Source Identifications from Phase IWe present in catalog form the optical identifications for objects fromthe first phase of the Wide Angle ROSAT Pointed Survey (WARPS). WARPS isa serendipitous survey of relatively deep, pointed ROSAT observationsfor clusters of galaxies. The X-ray source detection algorithm used byWARPS is Voronoi Tessellation and Percolation (VTP), a technique whichis equally sensitive to point sources and extended sources of lowsurface brightness. WARPS-I is based on the central regions of 86 ROSATPSPC fields, covering an area of 16.2 square degrees. We describe herethe X-ray source screening and optical identification process forWARPS-I, which yielded 34 clusters at 0.06 Mass-loss rates of H-rich central stars of planetary nebulae as distance indicators?If the mass loss rate, dot {M}, or the modified wind momentum, Pi , ofcentral stars of planetary nebulae (CSPN) is strictly related to theluminosity, the study of their winds can be used to derive theirdistance as suggested in the literature. However, the mass loss ratesand modified wind momenta of a sample of 13 CSPN published in theliterature show a separation into two groups, which differ by a factor10 to 102 in Pi . This is partly, but not completely, due todifferences in the adopted methods for mass loss determinations, andpartly due to differences in the adopted stellar parameters, mainly theeffective temperature. We have adopted a homogeneous set of stellarparameters, based on the Zanstra method, the dynamical ages of thenebulae and on evolutionary tracks, and scaled the mass loss ratesaccordingly. The revised data show that there is a large jump in dot {M}and Pi near Teff =~ 60 000 K, with dot {M} and Pi beinglarger by a factor 10 to 102 for the cooler group of CSPN ofspectral type Of, than for the hotter group of type O. Thisdiscontinuity is most likely due to a bi-stability jump. The reviseddata do not show a clear relation between Pi and the luminosity. Theconsequences are discussed in terms of the post-AGB evolution theory andthe radiation driven wind models. A Catalogue of IJK Photometry of PNe with DENISNear-infrared photometry of planetary nebulae (PNe) allows theclassification of those objects (Whitelock 1985; Peña &Torres-Peimbert 1987). We present the largest homogeneous sample. Helium contamination from the progenitor stars of planetary nebulae: The He/H radial gradient and the ΔY / ΔZ enrichment ratioIn this work, two aspects of the chemical evolution of 4He inthe Galaxy are considered on the basis of a sample of disk planetarynebulae (PN). First, an application of corrections owing to thecontamination of 4He from the evolution of the progenitorstars shows that the He/H abundance by number of atoms is reduced by0.012 to 0.015 in average, leading to an essentially flat He/H radialdistribution. Second, a determination of the helium to heavy elementenrichment ratio using the same corrections leads to values in the range2.8 < ΔY / ΔZ < 3.6 for Y p = 0.23 and 2.0< ΔY / ΔZ < 2.8 for Y p = 0.24, in goodagreement with recent independent determinations and theoretical models. Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.Not Available The IUE and Hubble Space Telescope GHRS Spectrum of the Hybrid''/PG 1159-Type Central Star of the Planetary Nebula NGC 7094Absorption features in the IUE spectrum of the `hybrid'' PG 1159-typecentral star of NGC 7094 have been identified and measured. We reportequivalent widths for photospheric Fe VI and Fe VII features,interstellar and circumstellar absorptions, and variability of O Vλ1371 and C IV λ1549 P Cygni profiles. The terminal windvelocity of -3680+/-300 km s-1 was determined from thehigh-dispersion C IV P Cyg feature. Some features from IUE and HubbleSpace Telescope GHRS data are compared. Emission from an Inhomogeneous Plasma: Line Intensities and Determination of Elemental Abundances in Gaseous Nebulae with Fluctuations of Te and neA method is proposed for determining the abundances of chemical elementsin planetary nebulae based on allowance for the actual distributionfunctions of errors in measuring line intensities. Fluctuations both intemperature and in mass density of a nebula are taken into account inthe proposed method. The results of a determination of C and Oabundances and of the amplitudes of temperature and density fluctuationsare given.
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#### Observation and Astrometry data

 Constellation: Eridanus Right ascension: 04h14m15.76s Declination: -12Â°44'22.0" Apparent magnitude: 10

Catalogs and designations:
 Proper Names (Edit) NGC 2000.0 NGC 1535 → Request more catalogs and designations from VizieR