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# M 97 (Owl Nebulae)

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 Bubbles in planetary nebulae and clusters of galaxies: jet bendingWe study the bending of jets in binary stellar systems. A compactcompanion accretes mass from the slow wind of the mass-losing primarystar, forms an accretion disc and blows two opposite jets. These fastjets are bent by the slow wind. Disregarding the orbital motion, we findthe dependence of the bending angle on the properties of the slow windand the jets. Bending of jets is observed in planetary nebulae which arethought to be the descendants of interacting binary stars. For example,in some of these planetary nebulae, the two bubbles (lobes) which areinflated by the two opposite jets are displaced to the same side of thesymmetry axis of the nebula. Similar displacements are observed inbubble pairs in the centre of some clusters and groups of galaxies. Wecompare the bending of jets in binary stellar systems with that inclusters of galaxies. 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 distances of highly evolved planetary nebulaeThe 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. 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. Antoine Darquier et la decouverte de la nebuleuse de la Lyre.Not Available 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. The Binarity of η Carinae and Its Similarity to Related Astrophysical ObjectsI examine some aspects of the interaction between the massive star ηCarinae and its companion, in particular during the eclipse-like event,known as the spectroscopic event or the shell event. The spectroscopicevent is thought to occur when near periastron passages the stellarcompanion induces much higher mass-loss rate from the primary starand/or enters into a much denser environment around the primary star. Ifind that enhanced mass-loss rate during periastron passages, if itoccurs, might explain the high eccentricity of the system. However,there is not yet a good model to explain the presumed enhanced mass-lossrate during periastron passages. In the region where the winds from thetwo stars collide, a dense slow flow is formed, such that large dustgrains may be formed. Unlike the case during the 19th century GreatEruption, the companion does not accrete mass during most of its orbitalmotion. However, near periastron passages short accretion episodes mayoccur, which may lead to pulsed ejection of two jets by the companion.The companion may ionize a nonnegligible region in its surrounding,resembling the situation in symbiotic systems. I discuss the relation ofsome of these processes to other astrophysical objects, by incorporatingη Car into a large class of astrophysical bipolar nebulae. Smart focal plane masks: rewritable photochromic films for astronomical multi-object spectroscopyModern astronomical spectroscopy makes use of multi-aperture slitsplaced in the focal plane of telescopes before light enters thespectrograph. Multiple object spectroscopy (MOS) allows several spectrato be obtained simultaneously with a multiplexing gain from the order ofdozens of objects in 4m class telescopes to few hundreds in larger 8mtelescopes. Many of these devices make use of metal plates which arepunched, milled or laser cut and can be used only for observation of agiven astronomical target. A typical observing night requires from 4 to20 MOS masks, which have to be prepared during an off-line procedure,usually days before. Here we report an innovative technique to carry outmulti object spectroscopy based on changes of properties of photochromicmaterials. Photochromic MOS masks consist of polymer thin films whichcan be made opaque or transparent in a restricted wavelength range usingalternatively UV and visible light. Slit patterns can thus be easilywritten by means of a red diode laser on a UV preflashed plate. Writingtime for a 10x10 arcmin plate is a few minutes and the whole procedurecan be performed promptly after the acquisition of the field image andwithout mechanical debris as in milling or laser cutting. A computercontrolled writing device suited for the AFOSC camera of the Asiago 1.8mtelescope was built. The same focal plane mask can be UV erased and usedmore than 450 times. High contrasts have been reached by means of anappropriate passband filter in the light beam of the spectrograph. Ourfirst successful observation run took place in January 2003. Spectra ofselected stars in the crowded M67 cluster field and emission lines fromthe gaseous planetary nebula M97 were obtained. Unresolved Hα Enhancements at High Galactic Latitude in the WHAM Sky Survey MapsWe 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. Bubbles in planetary nebulae and clusters of galaxies: instabilities at bubble' frontsI study the stability of off-center low-density more or less spherical(fat) bubbles in clusters of galaxies and in planetary nebulae (PNs) toRayleigh-Taylor (RT) instability. As the bubble expands and decelerates,the interface between the low-density bubble's interior and the denseshell formed from the accreted ambient medium is RT-stable. If, however,in a specific direction the density decreases such that this segment isaccelerated by the pressure inside the bubble, then this acceleratedregion is RT-unstable. The outermost region, relative to the center ofthe system, is the most likely to become unstable because there thedensity gradient is the steepest. Using simple analytical analysis, Ifind that off-center fat bubbles in PNs are much less stable than inclusters. In PNs bubbles become unstable when they are very smallrelative to their distance from the center; they can be stabilizedsomewhat if the mass loss rate from the stellar progenitor decreases fora time, such that the negative density gradient is much shallower. Inclusters fat bubbles become unstable when their size is comparable totheir distance from the center. I discuss some implications of thisinstability in clusters and in PNs. Why a Single-Star Model Cannot Explain the Bipolar Nebula of η CarinaeI examine the angular momentum evolution during the 1837-1856 GreatEruption of the massive star η Carinae. I find that the new estimateof the mass blown during that eruption implies that the envelope ofη Car substantially spun down during the 20 yr eruption. Single-starmodels, most of which require the envelope to rotate at close to thebreak-up velocity, cannot account for the bipolar nebula-theHomunculus-formed from matter expelled in that eruption. The kineticenergy and momentum of the Homunculus further constrain single-starmodels. I discuss how η Car can fit into a unified model for theformation of bipolar lobes in which two oppositely ejected jets inflatetwo lobes (or bubbles). These jets are blown by an accretion disk, whichrequires stellar companions in the case of bipolar nebulae aroundstellar objects. Physical Structure of Planetary Nebulae. III. The Large and Evolved NGC 1360NGC 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. 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. Bubbles in planetary nebulae and clusters of galaxies: Jet propertiesI derive constraints on jet properties for inflating pairs of bubbles inplanetary nebulae and clusters of galaxies. This work is motivated bythe similarity in morphology and some non-dimensional quantities betweenX-ray-deficient bubbles in clusters of galaxies and theoptical-deficient bubbles in planetary nebulae, which were pointed outin an earlier work. In the present Paper I find that for inflating fatbubbles, the opening angle of the jets must be large, i.e., the halfopening angle measured from the symmetry axis of the jets shouldtypically be α  40 o. For such wide-opening anglejets, a collimated fast wind (CFW) is a more appropriate term. Narrowjets will form elongated lobes rather than fat bubbles. I emphasize theneed to include jets with large opening angle, i.e., α ≃30-70o, in simulating bubble inflation in both planetarynebulae and (cooling flow) clusters of galaxies.Research supported by the Israel Science Foundation. Pairs of Bubbles in Planetary Nebulae and Clusters of GalaxiesI point to an interesting similarity in the morphology and somenondimensional quantities between pairs of X-ray-deficient bubbles inclusters of galaxies and pairs of optical-deficient bubbles in planetarynebulae (PNs). This similarity leads me to postulate a similar formationmechanism. This postulate is used to strengthen models for PN shaping byjets (or collimated fast winds [CFWs]). The presence of dense materialin the equatorial plane observed in the two classes of bubblesconstrains the jets and CFW activity in PNs to occur while theasymptotic giant branch star still blows its dense wind, or very shortlyafter. I argue that only a stellar companion can account for such jetsand CFWs. 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. 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. X-ray and UV Views of Hot Gas in Planetary Nebulae (invited review)Not Available Physical Structure of Planetary Nebulae. I. The Owl NebulaThe Owl Nebula is a triple-shell planetary nebula with the outermostshell being a faint bow-shaped halo. We have obtained deep narrowbandimages and high-dispersion echelle spectra in the Hα, [O III], and[N II] emission lines to determine the physical structure of each shellin the nebula. These spatiokinematic data allow us to rule outhydrodynamic models that can reproduce only the nebular morphology. Ouranalysis shows that the inner shell of the main nebula is slightlyelongated with a bipolar cavity along its major axis, the outer nebulais a filled envelope coexpanding with the inner shell at 40 kms-1, and the halo has been braked by the interstellar mediumas the Owl Nebula moves through it. To explain the morphology andkinematics of the Owl Nebula, we suggest the following scenario for itsformation and evolution. The early mass loss at the TP-AGB phase formsthe halo, and the superwind at the end of the AGB phase forms the mainnebula. The subsequent fast stellar wind compressed the superwind toform the inner shell and excavated an elongated cavity at the center,but this has ceased in the past. At the current old age the inner shellis backfilling the central cavity.Based on observations made with the William Herschel Telescope, operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de Los Muchachos of the Instituto deAstrofísica de Canarias, and with the Burrell Schmidt telescopeof the Warner and Swasey Observatory, Case Western Reserve University. 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} Urban Astronomy: Observing the Messier Objects from the CityNot Available The Dynamical Evolution of the Circumstellar Gas around Low- and Intermediate-Mass Stars. II. The Planetary Nebula FormationWe have studied the effect of the mass of the central star (CS) on thegas evolution during the planetary nebula (PN) phase. We have performednumerical simulations of PN formation using CS tracks for six stellarcore masses corresponding to initial masses from 1 to 5Msolar. The gas structure resulting from the previousasymptotic giant branch (AGB) evolution is used as the startingconfiguration. The formation of multiple shells is discussed in thelight of our models, and the density, velocity, and Hα emissionbrightness profiles are shown for each stellar mass considered. We havecomputed the evolution of the different shells in terms of radius,expansion velocity, and Hα peak emissivity. We find that theevolution of the main shell is controlled by the ionization front ratherthan by the thermal pressure provided by the hot bubble during the earlyPN stages. This effect explains why the kinematical ages overestimatethe age in young CSs. At later stages in the evolution and for low-massprogenitors the kinematical ages severely underestimate the CS age.Large (up to 2.3 pc), low surface brightness shells (less than 2000times the brightness of the main shell) are formed in all of our models(with the exception of the 5 Msolar model). These PN haloscontain most of the ionized mass in PNe, which we find is greatlyunderestimated by the observations because of the low surface brightnessof the halos. The Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic SampleNorthern Galactic planetary nebulae (PNs) are studied to disclosepossible correlations between the morphology of the nebulae and theevolution of the central stars (CSs). To this end, we have built thebest database available to date, accounting for homogeneity andcompleteness. We use updated statistical distances and an updatedmorphological classification scheme, and we calculate Zanstratemperatures for a large sample of PNs. With our study we confirm thatround, elliptical, and bipolar PNs have different spatial distributionswithin the Galaxy, with average absolute distances to the Galactic planeof 0.73, 0.38, and 0.21 kpc, respectively. We also find evidence thatthe distributions of the CS masses are different across thesemorphological groups, although we do not find that CSs hosted by bipolarPNs are hotter, on average, than CSs within round and elliptical PNs.Our results are in broad agreement with previous analyses, indicatingthat round, elliptical, and bipolar PNs evolve from progenitors indifferent mass ranges and might belong to different stellar populations,as also indicated by the helium and nitrogen abundances of PNs ofdifferent morphology. Les nebuleuses planetaires et leurs etoiles centrales.Not Available The rise and rise of the deep sky imagePresidential Address to the British Astronomical Association, 2000October 25 Sulfur, Chlorine, and Argon in Planetary Nebulae. I. Observations and Abundances in a Northern SampleThis paper is the first of a series specifically studying the abundancesof sulfur, chlorine, and argon in type II planetary nebulae (PNe) in theGalactic disk. Ratios of S/O, Cl/O, and Ar/O constitute important testsof differential nucleosynthesis of these elements and serve as strictconstraints on massive star yield predictions. We present newground-based optical spectra extending from 3600-9600 Å for asample of 19 type II northern PNe. This range includes the strongnear-infrared lines of [S III] λλ9069,9532, which allowsus to test extensively their effectiveness as sulfur abundanceindicators. We also introduce a new, model-tested ionization correctionfactor for sulfur. For the present sample, we find average values ofS/O=1.2×10-2+/-0.71×10-2,Cl/O=3.3×10-4+/-1.6×10-4, andAr/O=5.0×10-3+/-1.9×10-3. 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 Spectroscopic investigation of old planetaries. V. Distance scalesWe use the results of our recent NLTE model atmosphere analysis ofcentral stars of old planetary nebulae (PN) to calculate distances. Weperform a comparison with three other methods (trigonometric parallaxes,interstellar Na D lines, and Shklovsky distances) and discuss theproblem of the PNe distance scale. The result of the comparison of ourspectroscopic distances with the trigonometric distances is that thespectroscopic distances are 55% larger. Since using trigonometricparallaxes with large relative measurement errors can introducesystematic errors, we carried out a Monte Carlo simulation of the biasesintroduced by selection effects and measurement errors. It turns outthat a difference between both distance scales of the observed size isexpected for the present day data if the underlying distance scales areidentical. Thus our finding is essentially a confirmation of thespectroscopic distance scale! Good agreement is found between thespectroscopic distances and distances derived from the interstellar NaDlines. All three independent methods of distance measurement indicatethat the widely used statistical'' distance scales of the Shklovskytype are too short for old PNe. A correlation with nebular radii exists.The most likely explanation is an underestimate of the nebula masses forlarge PN. Implications for the nebula masses are discussed. Estimates ofthe PNe space density and birthrate, which are based on Shklovsky typedistances, therefore give too large values.
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