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Optically Thick Radio Cores of Narrow-Waist Bipolar Nebulae
We report our search for optically thick radio cores in 16 narrow-waistbipolar nebulae. Optically thick cores are a characteristic signature ofcollimated ionized winds. Eleven northern nebulae were observed with theVery Large Array (VLA) at 1.3 and 0.7 cm, and five southern nebulae wereobserved with the Australia Telescope Compact Array (ATCA) at 6 and 3.6cm. Two northern objects, 19W32 and M1-91, and three southern objects,He 2-25, He 2-84, and Mz 3, were found to exhibit a compact radio corewith a rising spectrum consistent with an ionized jet. Such jets havebeen seen in M2-9 and may be responsible for shaping bipolar structurein planetary nebulae.
| High-Resolution Spectra of Bright Central Stars of Bipolar Planetary Nebulae and the Question of Magnetic Shaping
We present ESO New Technology Telescope high-resolution echellespectroscopy of the central stars (CSs) of eight southern bipolarplanetary nebulae (PNe) selected for their asymmetry. Our aim was todetermine or place limits on the magnetic fields of the CSs of thesenebulae, and hence to explore the role played by magnetic fields innebular morphology and PN shaping. If magnetic fields do play a role, weexpect these CSs to have fields in the range102-107 G from magnetic flux conservation on thereasonable assumption that they must evolve into the high-field magneticwhite dwarfs. We were able to place an upper limit of ~20,000 G on themagnetic fields of the central stars of He 2-64 and MyCn 18. Thespectrum of He 2-64 also shows a P Cygni profile in He I λ5876and λ6678, corresponding to an expanding photosphere withvelocity ~100 km s-1. The detection of helium absorptionlines in the spectrum of He 2-36 confirms the existence of a hot stellarcomponent. We did not reach the necessary line detection for magneticfield analysis in the remaining objects. Overall, our results indicatethat if magnetic fields are responsible for shaping bipolar planetarynebulae, these are not required to be greater than a few tens ofkilogauss.Based on observations made with ESO telescopes at the La SillaObservatory under program ID 70.D-0339.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| The structure of planetary nebulae: theory vs. practice
Context.This paper is the first in a short series dedicated to thelong-standing astronomical problem of de-projecting the bi-dimensional,apparent morphology of a three-dimensional mass of gas. Aims.Wefocus on the density distribution in real planetary nebulae (and alltypes of expanding nebulae). Methods. We introduce some basictheoretical notions, discuss the observational methodology, and developan accurate procedure for determining the matter radial profile withinthe sharp portion of nebula in the plane of the sky identified by thezero-velocity-pixel-column (zvpc) of high-resolution spectral images.Results. The general and specific applications of the method (andsome caveats) are discussed. Moreover, we present a series of evolutivesnapshots, combining illustrative examples of both model and trueplanetary nebulae. Conclusions. The zvpc radial-densityreconstruction - added to tomography and 3D recovery developed at theAstronomical Observatory of Padua (Italy) - constitutes a very usefultool for looking more closely at the spatio-kinematics, physicalconditions, ionic structure, and evolution of expanding nebulae.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edge
We infer the gas kinematics, diagnostics and ionic radial profiles,distance and central star parameters, nebular photo-ionization model,spatial structure and evolutionary phase of the Planetary Nebula NGC6741 by means of long-slit ESO NTT+EMMI high-resolution spectra at nineposition angles, reduced and analysed according to the tomographic and3-D methodologies developed at the Astronomical Observatory of Padua(Italy). NGC 6741 (distance≃2.0 kpc, age≃ 1400 yr, ionizedmass Mion≃ 0.06 Mȯ) is a dense(electron density up to 12 000 cm-3), high-excitation,almost-prolate ellipsoid (0.036 pc × 0.020 pc × 0.018 pc,major, intermediate and minor semi-axes, respectively), surrounded by asharp low-excitation skin (the ionization front), and embedded in aspherical (radius≃ 0.080 pc), almost-neutral, high-density (n(HI)≃ 7 ×103 atoms cm-3) halo containinga large fraction of the nebular mass (Mhalo≥ 0.20Mȯ). The kinematics, physical conditions and ionicstructure indicate that NGC 6741 is in a deep recombination phase,started about 200 years ago, and caused by the rapid luminosity drop ofthe massive (M*=0.66{-}0.68 Mȯ), hot (logT* ≃ 5.23) and faint (logL*/Lȯ ≃ 2.75) post-AGB star, which hasexhausted the hydrogen-shell nuclear burning and is moving along thewhite dwarf cooling sequence. The general expansion law of the ionizedgas in NGC 6741, Vexp(km s-1)=13 × R arcsec,fails in the innermost, highest-excitation layers, which move slowerthan expected. The observed deceleration is ascribable to the luminositydrop of the central star (the decreasing pressure of the hot-bubble nolonger balances the pressure of the ionized gas), and appears instriking contrast to recent reports inferring that acceleration is acommon property of the Planetary Nebulae innermost layers. A detailedcomparative analysis proves that the "U"-shaped expansion velocity fieldis a spurious, incorrect result due to a combination of: (a) simplisticassumptions (spherical shell hypothesis for the nebula); (b) unfitreduction method (emission profiles integrated along the slit); and (c)inappropriate diagnostic choice (λ4686 Å of He II, i.e. athirteen fine-structure components recombination line). Some generalimplications for the shaping mechanisms of Planetary Nebulae arediscussed.
| Planetary nebula distances re-examined: an improved statistical scale
The distances of planetary nebulae (PNe) are still quite uncertain.Although observational estimates are available for a small proportion ofPNe, based on statistical parallax and the like, such distances are verypoorly determined for the majority of galactic PNe. In particular,estimates of so-called `statistical' distance appear to differ byfactors of ~2.7.We point out that there is a well-defined correlation between the 5-GHzluminosity of the sources, L5, and their brightnesstemperatures, TB. This represents a different trend to thoseinvestigated in previous statistical analyses, and permits us todetermine independent distances to a further 449 outflows. Thesedistances are shown to be closely comparable to those determined using aTB-R correlation, providing that the latter trend is taken tobe non-linear.This non-linearity in the TB-R plane has not been noted inprevious analyses, and is likely responsible for the broad (andconflicting) ranges of distance that have previously been published.Finally, we point out that there is a close accord between observedtrends within the L5-TB and TB-Rplanes, and the variation predicted through nebular evolutionarymodelling. This is used to suggest that observational biases areprobably modest, and that our revised distance scale is reasonablytrustworthy.
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| The distances of Type I planetary nebulae
The distances D of planetary nebulae (PNe) are still extremelyuncertain. Although a variety of methods have been used to evaluate thisparameter, these are often in conflict, and subject to large random andsystematic errors. It is therefore important to evaluate D using as manyindependent procedures as possible. We outline here one further way inwhich this parameter may be assessed. It is noted that where the nebularmass range is narrow, then one might expect observed PNe radii to beroughly similar. This, where it occurs, would also result in acorrelation between their angular diameters Θ, and distances D.We find that just such a trend occurs for Type I nebulae, and we employthis to determine distances to a further 44 such outflows. Our meanvalues of D appear similar to those of Zhang [ApJS 98 (1995) 659],implying a relatively long PNe distance scale.
| A reanalysis of chemical abundances in galactic PNe and comparison with theoretical predictions
New determinations of chemical abundances for He, N, O, Ne, Ar and Sare derived for all galactic planetary nebulae (PNe) so far observedwith a relatively high accuracy, in an effort to overcome differences inthese quantities obtained over the years by different authors usingdifferent procedures. These include: ways to correct for interstellarextinction, the atomic data used to interpret the observed line fluxes,the model nebula adopted to represent real objects and the ionizationcorrections for unseen ions. A unique `good quality' classical-typeprocedure, i.e. making use of collisionally excited forbidden lines toderive ionic abundances of heavy ions, has been applied to allindividual sets of observed line fluxes in each specific position withineach PN. Only observational data obtained with linear detectors, andsatisfying some `quality' criteria, have been considered. Suchobservations go from the mid-1970s up to the end of 2001. Theobservational errors associated with individual line fluxes have beenpropagated through the whole procedure to obtain an estimate of theaccuracy of final abundances independent of an author's `prejudices'.Comparison of the final abundances with those obtained in relevantmulti-object studies on the one hand allowed us to assess the accuracyof the new abundances, and on the other hand proved the usefulness ofthe present work, the basic purpose of which was to take full advantageof the vast amount of observations done so far of galactic PNe, handlingthem in a proper homogeneous way. The number of resulting PNe that havedata of an adequate quality to pass the present selection amounts to131. We believe that the new derived abundances constitute a highlyhomogeneous chemical data set on galactic PNe, with realisticuncertainties, and form a good observational basis for comparison withthe growing number of predictions from stellar evolution theory. Owingto the known discrepancies between the ionic abundances of heavyelements derived from the strong collisonally excited forbidden linesand those derived from the weak, temperature-insensitive recombinationlines, it is recognized that only abundance ratios between heavyelements can be considered as satisfactorily accurate. A comparison withtheoretical predictions allowed us to assess the state of the art inthis topic in any case, providing some findings and suggestions forfurther theoretical and observational work to advance our understandingof the evolution of low- and intermediate-mass stars.
| Metallicity distribution on the galactic disk
Depending mainly on UBVCCD data, the metallicities of 91 open starclusters nearby the galactic disk have been estimated using Cameron's[A&A 147 (1985b) 39] method. The metallicity radial gradient alongthe galactic plane is found to be -0.09 dex/kpc; which is in a very goodagreement with Panagia and Tosi [A&A 96 (1981) 306] and Carraro etal. [MNRAS 296 (1998) 1045]. Vertically on the galactic disk, withinabout 800 pc, the metallicity gradient is found to be so trivial. Anaverage age-metallicity relation has been examined, which confirms theprevious suggestion that the metallicity of a cluster depending mainlyon its position on the galactic disk more than its age.
| 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}
| Angular dimensions of planetary nebulae
We 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 nebulae
An 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.
| Planetary nebula carbon yields and the chemical evolution of the Galactic disc
Two sets of observational carbon stellar yields for low-and-intermediate mass stars are computed based on planetary nebulaabundances derived from CIIλ4267 and CIIIλλ1906 +1909 lines, respectively. These observational yields are assumed inchemical evolution models for the solar vicinity and the Galactic disc.C/O values observed in stars in the solar vicinity and Galactic HIIregions are compared with those predicted by chemical evolution modelsfor the Galaxy. It is concluded that the C yields derived from permittedlines are in better agreement with the observational constraints thanare those derived from forbidden lines.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Infrared Imaging and Optical Imaging and Spectroscopy of (mostly) typePlanetary Nebulae.
Optical spectra, images in the main optical emission lines and IR imagesdirected towards the detection of molecular hydrogen, were obtained for14 planetary nebulae (PNe). Eleven of these comply with the two criteriadefining a typePNe (He-rich with a large N/O ratio). All are classifiedas bipolar, though two of them (A 24 and M 3-5) are either bipolars seenpole-on or elliptical. Shocked molecular hydrogen was detected in eightof these objects, but not in A 14, A 24, and M 1-57. KjPn 6 wastentatively classified as a typePN, though its helium abundance issmaller than the minimum prescribed value and the sum of its nitrogenand oxygen abundances is low. The object is about 800above the galacticplane, is bipolar and shows no trace of molecular hydrogen. The chemicalcomposition of A 24, A 79, M 1-57, and Sh 1-89 indicates that thirddredge-up episodes occured in their progenitor star. Shock waves arepropagating in the ionized component of A 79, M 1-28 and NGC 2818, andpossibly A 14, A 24, and K 3-46. An emission arc in the periphery ofWray 16-22, a typePN, may be interpreted as a bow shock produced by thesupersonic motion of this object in the interstellar medium.
| Imaging and Spectroscopy of Type I Planetary Nebulae
Not Available
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| Gas temperature and excitation classes in planetary nebulae
Empirical methods to estimate the elemental abundances in planetarynebulae usually use the temperatures derived from the [O III] and [N II]emission-line ratios, respectively, for the high- and low-ionizationzones. However, for a large number of objects these values may not beavailable. In order to overcome this difficulty and allow a betterdetermination of abundances, we discuss the relationship between thesetwo temperatures. Although a correlation is not easily seen when asample of different PNe types is used, the situation is improved whenthey are gathered into excitation classes. From [OII]/[OIII] andHeII/HeI line ratios, we define four excitation classes. Then, usingstandard photoionization models which fit most of the data, a linearrelation between the two temperatures is obtained for each of the fourexcitation classes. The method is applied to several objects for whichonly one temperature can be obtained from the observed emission linesand is tested by recalculation of the radial abundance gradient of theGalaxy using a larger number of PNe. We verified that our previousgradient results, obtained with a smaller sample of planetary nebulae,are not changed, indicating that the temperature relation obtained fromthe photoionization models are a good approximation, and thecorresponding statistical error decreases as expected. Tables 3-5, 7 and9 are only available in electronic form at http://www.edpsciences.org
| The Extreme Type I Planetary Nebula M2-52
High-resolution spectrophotometric data of the central zone of theBr-type planetary nebula M2-52 are presented. The nebula has a richspectrum, with high and low excitation lines. The chemical compositionderived from the spectra shows that He and N are very enhanced in M2-52.Thus, this object can be classified as an extreme Peimbert's Type I PN.The chemical composition of the ionized gas is: He/H = 0.165+/-0.010,O/H = (2.6+/-0.5)×10-4, N/O = 2.3+/-0.3, Ne/O =0.37+/-0.10, Ar/O = (9.2+/-2.0)×10-3 and S/O >2.0×10-3. The expansion velocity of the nebula is, onaverage, about 20+/-2 km s-1, but the low ionization species (N+ and S+)seem to systematically show slightly lower expansion velocities (18 kms-1) than O+ + and He+ which have v[exp] = 20 km s-1, whileH+ and He++ have v[exp] ~ 22 km s-1. This behavior couldindicate that the outer zones of the ionized gas are being deceleratedby the molecular ring located around the central star.
| A possible observational measure of evolution in bipolar nebulae
Bipolar planetary nebulae (BPNe) possess a broad range of shapes,ranging from narrow-waisted butterfly-like structures, through to thosewhich are more nearly cylindrical. We point out that these morphologiesappear to be correlated with radio surface brightness Tb, inthe sense that higher values of Tb are associated withenvelopes having narrower waists. If one interprets the variation inTb as arising from shell evolution, as is usually assumed forother planetaries, this would then imply that shell morphology varieswith time in a manner which appears not to be consistent withevolutionary models. It also remains possible, however, that differentBPNe morphologies arise as a result of differing mechanisms offormation, and that the pre-collimation of high velocity central windsgives rise to narrower waists, and higher surface brightness nuclei. Ourresults, if this is true, may then imply that central winds possess abroad range of collimations.
| Membership, Binarity and Stellar Evolution of Red Giants in the Open Clusters NGC 2324, NGC 2818, NGC 3960 and NGC 6259
Not Available
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| Red giants in open clusters. IX. NGC 2324, 2818, 3960 and 6259
We present accurate radial velocities and photoelectric UBV photometryfor 73 and 57 red-giant candidates, respectively, in theintermediate-age open clusters NGC 2324, 2818, 3960 and 6259. These dataconfirm the membership of 47 stars, 12 of which (26%) are spectroscopicbinaries; three preliminary orbits have been determined in NGC 3960.From Washington photometry of 8 red giant members, the metallicity ofNGC 6259 is found to be [Fe/H] = 0.06 +/- 0.08. At the age of theseclusters, most of the red giants are observed in the core-helium (clump)burning phase, the general morphology of which is well reproduced bytheoretical models with convective overshooting. However, a number ofbona fide cluster giant members are found significantly to the red ofthe isochrones fitting the rest of the CMD of these and a few otherclusters. Some of these stars are binaries, but others seem to besingle. In either case, their red colours and/or low luminosities remainunexplained by current stellar evolution theory. Based on observationscollected with the Danish 1.54-m telescope at the European SouthernObservatory, La Silla (Chile) and at Cerro Tololo Inter-AmericanObservatory, National Optical Astronomy Observatories, operated by theAssociation of the Universities for Research in Astronomy, Inc., undercontract with the National Science Foundation.
| Bipolar Nebulae: The Missing Population
We have undertaken an analysis of observed aspect ratios for thecollimation disks in bipolar nebulae (BPNe). We find, as a result, thatmost such structures are likely to have a ratio of thickness to diameterb/a~0.23. Similarly, the paucity of disks having aspect ratios >=0.7implies that many of the sources are either unobserved or, what is moreprobably the case, misidentified. It seems likely, as a result, that thetrue population of BPNe is ~1.7 times greater than previously supposed.Similarly, we determine that as many as of 43% of annular or ringlikeplanetaries may represent bipolar sources oriented along the line ofsight.
| The low-excitation structures of planetary nebulae
The low excitation properties of the planetary nebula (PN) NGC 6720 areknown to be unusual, and to imply large ring/core emission ratios. Wepoint out that such characteristics are by no means confined to thissource alone, and that high ratios may occur in a large fraction ofelliptical and circular PNe. Such trends may arise because of thepresence of thin low-excitation emission sheets `wrapped' within andaround the primary outflows. The widths of such shells are required tobe exceedingly small, and may (for certain cases) be of order<<10-2pc. Such a mechanism appears capable ofexplaining most of the observed emission properties, and may arisethrough shock interaction between differing envelopes. Alternativeexplanations in terms of bipolar or cylindrical outflows are shown to beimplausible.
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