Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  

NGC 6072



Upload your image

DSS Images   Other Images

Related articles

Physical conditions in Photo-Dissociation Regions around Planetary Nebulae
We present observations of the infrared fine-structure lines of [Si II](34.8 μm), [O I] (63.2 and 145.5 μm) and [C II] (157.7 μm)obtained with the ISO SWS and LWS spectrographs of nine PlanetaryNebulae (PNe). These lines originate in the Photo-Dissociation Regions(PDRs) associated with the nebulae and provide useful information on theevolution and excitation conditions of the ejected material in theseregions. In order to interpret the observations, the measured lineintensities have been compared with those predicted byphoto-dissociation models. This comparison has been done taking intoaccount the C/O content in the nebulae. The densities derived with thiscomparison show a large scatter for some nebulae, probably because thedensity is higher than the critical density. Therefore, they are nolonger sensitive to this parameter implying that transitions from otherspecies with higher critical density should be used. The possiblecontribution of shocks to the observed emission characteristics of thesePNe is briefly discussed and it is shown that the radiation field is themain driving force responsible for the atomic lines in the PNe that havebeen studied. In addition, data on the pure rotational lines ofH2 in three nebulae (NGC 7027, NGC 6302 and Hb 5) are alsopresented. Assuming local thermal equilibrium the rotational temperatureand densities have been derived. We have derived the mass of atomic gasin the PDR associated with these PNe and compared those to ionic massesderived from Hβ and molecular masses derived from low J COobservations. This comparison shows that for these nebulae, the PDR isthe main reservoir of gas surrounding these objects. A comparison of theresults of these evolved PNe with very young PNe from the literaturesuggests that as the nebula ages the relative amount of ionic gasincreases at the expense of the atomic and molecular mass.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr ( or via http: / /cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/523

A reexamination of electron density diagnostics for ionized gaseous nebulae
We present a comparison of electron densities derived from opticalforbidden line diagnostic ratios for a sample of over a hundred nebulae.We consider four density indicators, the [O II]λ3729/λ3726, [S II] λ6716/λ6731, [Cl III]λ5517/λ5537 and [Ar IV] λ4711/λ4740 doubletratios. Except for a few H II regions for which data from the literaturewere used, diagnostic line ratios were derived from our own high qualityspectra. For the [O II] λ3729/λ3726 doublet ratio, we findthat our default atomic data set, consisting of transition probabilitiesfrom Zeippen (\cite{zeippen1982}) and collision strengths from Pradhan(\cite{pradhan}), fit the observations well, although at high electrondensities, the [O II] doublet ratio yields densities systematicallylower than those given by the [S II] λ6716/λ6731 doubletratio, suggesting that the ratio of transition probabilities of the [OII] doublet, A(λ3729)/A(λ3726), given by Zeippen(\cite{zeippen1982}) may need to be revised upwards by approximately 6per cent. Our analysis also shows that the more recent calculations of[O II] transition probabilities by Zeippen (\cite{zeippen1987a}) andcollision strengths by McLaughlin & Bell (\cite{mclaughlin}) areinconsistent with the observations at the high and low density limits,respectively, and can therefore be ruled out. We confirm the earlierresult of Copetti & Writzl (\cite{copetti2002}) that the [O II]transition probabilities calculated by Wiese et al. (\cite{wiese}) yieldelectron densities systematically lower than those deduced from the [SII] λ6716/λ6731 doublet ratio and that the discrepancy ismost likely caused by errors in the transition probabilities calculatedby Wiese et al. (\cite{wiese}). Using our default atomic data set for [OII], we find that Ne([O II])  Ne([S II]) ≈Ne([Cl III])< Ne([Ar IV]).

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.

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 ( 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 chemistry of compact planetary nebulae
We report high-sensitivity millimetre observations of several molecularspecies (13CO, HCN, HNC, CN, HCO+ andN2H+) in a sample of compact planetary nebulae.Some species such as HCO+ and CN are particularly abundantcompared to envelopes around AGB stars or even interstellar clouds. Wehave estimated the following average values for the column densitiesratios: CN/HCN ~ 2.6, HCO+/HCN ~ 0.5, and HNC/HCN ~ 0.4.Thus, the chemical composition of the molecular envelopes in thesecompact PNe appears somewhat intermediate between the composition ofproto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe (such asthe Ring, M4-9, or the Helix). From observations of the CO isotopomers,we have estimated that the 12C/13C ratio is in therange 10 la 12C/13C <~ 40. These values arebelow those expected from standard asymptotic giant branch models andsuggest non-standard mixing processes. The observed molecular abundancesare compared to very recent modelling work, and we conclude that theobservations are well explained, in general terms, by time-dependentgas-phase chemical models in which the ionization rate is enhanced byseveral orders of magnitude with respect to the average interstellarvalue. Thus, our observations confirm that the chemistry in the neutralshells of PNe is essentially governed by the high energy radiation fromthe hot central stars. The complexity of the chemical processes isincreased by numerous factors linked to the properties of the centralstar and the geometry and degree of clumpiness of the envelope. Severalaspects of the PN chemistry that remains to be understood are discussedwithin the frame of the available chemical models. Based on observationscarried out with the IRAM 30m telescope. IRAM is supported by INSU/CNRS(France), MPG (Germany) and IGN (Spain).

Photochemistry and Molecular Formation in Planetary Nebulae (invited review)
Not Available

CO Isotopes in Planetary Nebulae
Standard stellar evolution theory is inconsistent with the observedisotopic carbon ratio, 12C/13C, in evolved stars.This theory is also inconsistent with the 3He/H abundanceratios observed in Galactic H II regions, when combined with chemicalevolution theory. These discrepancies have been attributed to an extra,nonstandard mixing, which further processes material during the redgiant branch and should lower both the 12C/13C and3He/H abundance ratios for stars with masses <=2Msolar. Measurements of isotopic ratios in planetary nebulaeprobe material that escapes the star to be further processed by futuregenerations of stars. We have measured the carbon isotopic abundanceratio, 12C/13C, in 11 planetary nebulae (PNe) byobserving the J=2-->1 and J=3-->2 millimeter transitions of12CO and 13CO in molecular clouds associated withthe PNe. A large velocity gradient (LVG) model has been used todetermine the physical conditions for each PN for which both transitionshave been detected. We detect both 12CO and 13COin nine PNe. If12CO/13CO=12C/13C, the rangeof 12C/13C is 2.2-31. Our results support theoriesthat include some form of extra mixing.

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.

First Detections of Molecular Gas Associated with the Wolf-Rayet Ring Nebula NGC 3199
This paper presents the first observations of molecular gas associatedwith the Wolf-Rayet ring nebula NGC 3199 around the WR star WR 18. Thisincludes first observations of the molecules HCN, HCO+, CN,and HNC seen in any Wolf-Rayet ring nebula. Our observations immediatelysuggest the presence of high-density molecular gas (>104cm-3) in the nebula with significant amounts of associatedmolecular gas, which is in the form of clumpy ejecta and/or interstellarmaterial. Molecular CO gas was mapped across the optically brightportion of the nebula and out into the diffuse ionized component usingthe 12CO J=1-->0 line. CO gas is not seen within theoptically bright rim of NGC 3199 but adjacent to it. The opticalemission rim therefore appears to mark regions of photodissociation.Velocity components in the CO data are consistent with those seen inhigh-resolution optical spectra of the Hα line but extend beyondthe visible emission. A prior suggestion of the formation of the nebulavia a bow shock appears unlikely since Hipparcos measurements show theproper motion of WR 18 is almost at right angles to the directionrequired for the bow shock model. Instead, line splitting toward thenorth of the nebula suggests that a possible blowout of the Wolf-Rayetwind through surrounding ejecta may be responsible for some of thevelocity features observed. Preliminary estimates of molecularabundances in the nebula seen toward the central star are significantlyhigher than for the interstellar medium and are similar to those inplanetary nebulae, although CN is distinctly underabundant in comparisonto the very high values found in many planetary nebulae. The abundancesfound are consistent with the idea that at least a portion of themolecular material is associated with ejecta from the central star.Based on observations collected at the Swedish-ESO SubmillimetreTelescope (SEST) at the European Southern Observatory, La Silla, Chile.The Swedish-ESO Submillimetre Telescope is operated jointly by theEuropean Southern Observatory (ESO) and the Swedish National Facilityfor Radio Astronomy, Onsala Space Observatory, at Chalmers University ofTechnology.

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.

ISO LWS observations of planetary nebula fine-structure lines
We have obtained 43-198μm far-infrared (IR) spectra for a sample of51 Galactic planetary nebulae (PN) and protoplanetary nebulae (PPN),using the Long Wavelength Spectrometer (LWS) on board the Infrared SpaceObservatory (ISO). Spectra were also obtained of the former PN candidateLo 14. The spectra yield fluxes for the fine-structure lines [Nii]122μm, [Niii] 57μm and [Oiii] 52 and 88μm emitted in theionized regions and the [Oi] 63- and 146-μm and [Cii] 158-μm linesfrom the photodissociation regions (PDRs), which have been used todetermine electron densities and ionic abundances for the ionizedregions and densities, temperatures and gas masses for the PDRs. Thestrong [Niii] and [Oiii] emission lines detected in the LWS spectrumtaken centred on Lo 14 could be associated with the nearby strong radioand infrared source G 331.5-0.1. We find that the electron densitiesyielded by the [Oiii] 88μm/52μm doublet ratio are systematicallylower than those derived from the optical [Ariv]λ4740/λ4711 and [Cliii] λ5537/λ5517 doubletratios, which have much higher critical densities than the 52- and88-μm lines, suggesting the presence of density inhomogeneities inthe nebulae. Ionic abundances, N+/H+,N2+/H+ and O2+/H+, as wellas the N2+/O2+ abundance ratio, which provides agood approximation to the N/O elemental abundance ratio, are derived.Although ionic abundances relative to H+ deduced from thefar-IR fine-structure lines are sensitive to the adopted electrondensity and the presence of density inhomogeneities, the strongdependence on the nebular physical conditions is largely cancelled outwhen N2+/O2+ is calculated from the57μm/(52μm+88μm) flux ratio, owing to the similarity of thecritical densities of the lines involved. The temperatures and densitiesof the PDRs around 24 PN have been determined from the observed [Oi] and[Cii] line intensity ratios. Except for a few objects, the deducedtemperatures fall between 200 and 500K, peaking around 250K. Thedensities of the PDRs vary from104-105cm-3, reaching3×105cm-3 in some young compact PN. With aderived temperature of 1600K and a density of105cm-3, the PDR of NGC 7027 is one of the warmestand at the same time one of the densest amongst the nebulae studied. Formost of the PN studied, the [Cii]-emitting regions contain only modestamounts of material, with gas masses <~0.1Msolar.Exceptional large PDR masses are found for a few nebulae, including NGC7027, the bipolar nebulae M2-9 and NGC 6302, the young dense planetarynebulae BD+30°3639, IC 418 and NGC 5315, and the old, probablyrecombining, nebulae IC 4406 and NGC 6072.

The Formation of Very Narrow Waist Bipolar Planetary Nebulae
We discuss the interaction of the slow wind blown by an asymptotic giantbranch (AGB) star with a collimated fast wind (CFW) blown by itsmain-sequence or white dwarf companion, at orbital separations in therange of several AU<~a<~200 AU. The CFW results from accretion ofthe AGB wind into an accretion disk around the companion. The fast windis collimated by the accretion disk. We argue that such systems are theprogenitors of bipolar planetary nebulae and bipolar symbiotic nebulaewith a very narrow equatorial waist between the two polar lobes. The CFWwind will form two lobes along the symmetry axis and will furthercompress the slow wind near the equatorial plane, leading to theformation of a dense slowly expanding ring. Therefore, contrary to thecommon claim that a dense equatorial ring collimates the bipolar flow,we argue that in the progenitors of very narrow waist bipolar planetarynebulae, the CFW, through its interaction with the slow wind, forms thedense equatorial ring. Only later in the evolution, and after the CFWand slow wind cease, does the mass-losing star leave the AGB and blow asecond, more spherical, fast wind. At this stage the flow structurebecomes the one that is commonly assumed for bipolar planetary nebulae,i.e., collimation of the fast wind by the dense equatorial material.However, this results in the broadening of the waist in the equatorialplane and cannot by itself account for the presence of very narrowwaists or jets. We conduct a population synthesis study of the formationof planetary nebulae in wide binary systems which quantitativelysupports the proposed model. The population synthesis code follows theevolution of both stars and their arbitrarily eccentric orbit, includingmass loss via stellar winds, for 5×104 primordialbinaries. We show the number of expected systems that blow a CFW is inaccord with the number found from observations, to within the manyuncertainties involved. Overall, we find that ~5% of all planetarynebulae are bipolars with very narrow waists. Our population synthesisnot only supports the CFW model but more generally supports the binarymodel for the formation of bipolar planetary nebulae.

Photometric Constraints upon Binaries in Bipolar Nebulae
We summarize current information concerning the photometry, distances,and extinctions for the central stars of bipolar nebulae (BPNs). Theseare used to place tight constraints upon the properties of binarycompanions and to derive upper limit mass and spectral functions forthese stars. A comparison of these results with model binary systemssuggests that many BPN central stars may have binary companions,although these are either unrecognized or undetected. However, thephotometric limits also imply that such binaries are unlikely to undergostable mass transfer during AGB Roche lobe overflow. Constraints uponthe masses of the secondary stars also enable limits to be placed uponorbital velocities, whence it is apparent that the motion of very few ofthese binaries is likely to be detected at current levels ofobservational precision.

The dust content of planetary nebulae: a reappraisal
We have performed a statistical analysis using broad band IRAS data onabout 500 planetary nebulae with the aim of characterizing their dustcontent. Our approach is different from previous studies in that it usesan extensive grid of photoionization models to test the methods forderiving the dust temperature, the dust-to-gas mass ratio and theaverage grain size. In addition, we use only distance independentdiagrams. With our models, we show the effect of contamination by atomiclines in the broad band IRAS fluxes during planetary nebula evolution.We find that planetary nebulae with very different dust-to-gas massratios exist, so that the dust content is a primordial parameter for theinterpretation of far infrared data of planetary nebulae. In contrastwith previous studies, we find no evidence for a decrease in thedust-to-gas mass ratio as the planetary nebulae evolve. We also showthat the decrease in grain size advocated by Natta & Panagia(\cite{NattaPanagia}) and Lenzuni et al. (\cite{Lenzuni}) is an artefactof their method of analysis. Our results suggest that the timescale fordestruction of dust grains in planetary nebulae is larger than theirlifetime. Table~1 is only accessible in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

A Survey of Planetary Nebulae in the Southern Galactic Bulge
We present the results of a deep and uniform narrowband Hα imagingsurvey for planetary nebulae (PNs) in the southern Galactic bulge. Inour survey, we have found 56 new PNs and have rediscovered 45 known PNs.We have measured the radial velocities of this uniformly selected sampleand have also remeasured radial velocities for a subset of 317 PNs fromthe Acker catalog. Using the COBE/DIRBE 1.25, 2.2, and 3.5 μm images,we show that there is a similar longitude distribution of the PNs andthe COBE light in the zone of our deep survey. Also, we find that theextinction in our surveyed fields is not severe and that itsdistribution is fairly uniform. Finally, we present Hα fluxes for47 of our 56 newly discovered PNs and estimate the survey detectionlimit.

Electron densities in planetary nebulae, and the unusual characteristics of the [S BT II] emission zone} ] densities in planetary nebulae
We investigate the radial variation of electron densities in planetarynebulae, using values of ne deriving from the [S ii]<~mbda6717/<~mbda6730 line ratio. As a result, we are able to showthat there is a sharp discontinuity in densities of order 1.4 dex closeto nebular radii R=0.1 pc. It is proposed, as a consequence, that mostnebulae contain two primary [S ii] emission zones, with densitiesdiffering by a factor ~ 10(2) . The intensity of emission from thedenser component increases by an order of magnitude where nebulae passfrom radiation to density-bound expansion regimes, resulting in acorresponding discontinuous jump in [S ii]/Hβ line ratios. Theorigins of these changes are not entirely clear, although one mechanismis investigated whereby the superwind outflows shock interact withexterior AGB envelopes. Finally, the derived trends in ne(R)are used to determine distances for a further 262 nebulae. The resultingdistance scale appears to be comparable to that of Daub (1982) and Cahnet al. (1992).

The kinematics of 867 galactic planetary nebulae
We present a compilation of radial velocities of 867 galactic planetarynebulae. Almost 900 new measurements are included. Previously publishedkinematical data are compared with the new high-resolution data toassess their accuracies. One of the largest samples in the literatureshows evidence for a systematic velocity offset. We calculate weightedaverages between all available data. Of the final values in thecatalogue, 90% have accuracies better than 20 km s(-1) . We use thiscompilation to derive kinematical parameters of the galacticdifferential rotation obtained from least-square fitting and toestablish the Disk rotation curve; we find no significal trend for thepresence of an increasing external rotation curve. We examine also therotation of the bulge; the derived curve is consistent with a linearlyincreasing rotation velocity with l: we find V_b,r=(9.9+/-1.3)l -(6.7+/-8.5) km s(-1) . A possible steeper gradient in the innermostregion is indicated. Table 2 is available in electronic form only, viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

The Smith cloud: HI associated with the SGR dwarf?
The Smith high-velocity cloud (V_LSR=98 km s^-1) has been observed attwo locations in the emission lines [OIII]lambda5007, [NII]lambda6548and Hα. Both the [NII] and Hα profiles show bright cores due tothe Reynolds layer, and red wings with emission extending to V_LSR~130km s^-1. This is the first simultaneous detection of two emission linestowards a high-velocity cloud, allowing us to form the ratio of theseline profiles as a function of local standard of rest (LSR) velocity. Atboth cloud positions, we see a clear distinction between emission at thecloud velocity, and the Reynolds layer emission (V_LSR~0). The[NII]/Hα ratio (~0.25) for the Reynolds layer is typical of the warmionized medium. At the cloud velocity, this ratio is enhanced by afactor of 3-4 compared to emission at rest with respect to the LSR. Amoderately deep upper limit at [OIII] (0.12R at 3sigma) was derived fromour data. If the emission arises from dilute photoionization from hotyoung stars, the highly enhanced [NII]/Hα ratio, the [OIII]non-detection and weak Hα emission (0.24-0.30R) suggest that theSmith cloud is 26+/-4 kpc from the Sun, at a Galactocentric radius of20+/-4 kpc. This value assumes that the emission arises from anoptically thick slab, with a covering fraction of unity as seen by theionizing photons, the orientation of which is either (a) parallel to theGalactic disc, or (b) such as to maximize the received flux from thedisc. The estimated mass and size of the cloud are 4x10^6 Msolar and 6kpc. We discuss a possible association with the much larger Sgr dwarf,at a Galactocentric radius of 16+/-2 kpc, which lies within 35 deg (~12kpc) of the Smith cloud.

Planetary Nebulae in the NRAO VLA Sky Survey
The 1.4 GHz NRAO VLA Sky Survey (NVSS) images and source catalog wereused to detect radio emission from the 885 planetary nebulae north ofJ2000 declination delta = -40 deg in the Strasbourg-ESO Catalogue ofGalactic Planetary Nebulae. We identified 680 radio sources brighterthan about S = 2.5 mJy beam-1 (equivalent to T ~ 0.8 K in the 45" FWHMNVSS beam) with planetary nebulae by coincidence with accurate opticalpositions measured from Digitized Sky Survey (DSS) images. Totalextinction coefficients c at lambda = 4861 Angstroms were calculated forthe 429 planetary nebulae with available H beta fluxes and low free-freeoptical depths at 1.4 GHz. The variation of c with Galactic latitude andlongitude is consistent with the extinction being primarily interstellarand not intrinsic.

A Morphological Study of Planetary Nebulae
We have produced simulated images of 110 planetary nebulae using theellipsoidal shell model. This process has allowed us to remove theprojection effects from the morphological classification of planetarynebulae and has provided quantitative measures of the intrinsicasymmetries of the nebulae. It is shown that the morphology of mostplanetary nebulae can be reproduced with pole-to-equator density ratiosof 0.1-1. Many planetary nebulae also show a modest departure from axialsymmetry. Contrary to previous findings by Khromov & Kohoutek, thesky orientation of planetary nebulae in this sample is consistent with apurely random distribution. Extremely bipolar nebulae (e.g., those ofbutterfly shape) point to a steep density profile in the AGB envelopeand are more likely to be type I (high helium and/or nitrogen abundance)nebulae. We found evidence that these nebulae are likely to have moremassive progenitors and are at a more advanced stage of dynamicalevolution.

Orientation of planetary nebulae within the Galaxy
Narrow-band CCD images of 209 axially symmetrical planetary nebulae(PNe) have been examined in order to determine the orientation of theiraxes within the disc of the Galaxy. The nebulae have been divided intothe bipolar (B) and elliptical (E) PNe morphological types, according tothe scheme of Corradi & Schwarz. In both classes, contrary to theresults of Melnick & Harwit and Phillips we do not find any strongevidence for non-random orientations of the nebulae in the Galaxy.Compared with previous work in this field, the present study takesadvantage of the use of larger and morphologically more homogeneoussamples and offers a more rigorous statistical analysis.

The chemical structure of bipolar planetary nebulae. II. 13 objects
Long slit spectrophotometry of 13 bipolar planetary nebulae has beenused to study their physical and chemical properties. In each nebula,one to seven different spatial regions have been considered in order tosearch for possible abundance variations through the nebulae. Weobtained the following main results: - within the errors, the He, O, andN abundances are constant through all the nebulae. - the Ne, Ar, and Sabundances are also constant, within the errors, but their face valueshave systematic increases toward the outer regions of the nebulae. Thesetrends may be attributed to inaccuracies in the ionization correctionfactors, as predicted by Alexander & Balick (1997) for long-slitobservations of extended PNe. The corresponding increase of the Nabundance predicted by those authors is, however, generally not observed(with one exception). - The present sample contains some of the GalacticPNe with the highest He and N/O abundances known to date (M 3-2, He2-111, NGC 6537). The highest He overabundances cannot by reproduced byany current model of AGB evolution. - Oxygen depletion is suggested forthe nebulae with the highest N/O abundances, indicating that efficientON cycle process has occurred in their progenitors. Tables 4 to 29 areonly available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr ( or via/ttp://cdsweb.u-strasbg.fr/Abstract.html}. Based on observations made atthe European Southern Observatory

Where Do the Disks of Spiral Galaxies End?
In spiral galaxies, the H I surface density declines with increasingradius to a point at which it is seen to truncate dramatically in thebest observed cases. If the ambient radiation field is sufficientlystrong, there exists a maximum radius beyond which the cold gas isunable to support itself against ionization. We have now succeeded indetecting ionized gas beyond the observed H I disk in spirals. Here wereport on our findings for the Sculptor galaxy NGC 253. The H I disks inSculptor galaxies extend to only about 1.2R25, although we have detectedionized gas to the limits of our survey, out to 1.4R25. This hasimportant ramifications for spiral galaxies in that it now becomespossible to trace the gravitational potential beyond where the H I diskends. The detections confirm that the rotation curve continues to risein NGC 253, as it appears to do for other Sculptor galaxies, from the HI measurements, but there is a hint that the rotation curve may fallabruptly not far beyond the edge of the H I disk. If this is correct, itsuggests that the dark halo of NGC 253 may be truncated near the H Iedge and provides further support for the link between dark matter and HI. The line ratios are anomalous with [N II] lambda 6548 to H alpharatios close to unity. While metallicities at these large radii areuncertain, such enhanced ratios, compared to solar-abundance H IIregions ([N II] lambda 6548/H alpha = 0.05-0.2), are likely to requireselective heating of the electron population without further ionizationof N+. We discuss the most likely sources of ionization and heating, andthe possible role of refractory element depletion (e.g., Ca, Si, and Fe)onto dust grains.

Properties That Cannot Be Explained by the Progenitors of Planetary Nebulae
I classify a large number of planetary nebulae (458) according to theprocess that caused their progenitors to blow axisymmetrical winds. Theclassification is based primarily on the morphologies of the differentplanetary nebulae, assuming that binary companions, stellar orsubstellar, are necessary in order to have axisymmetrical mass loss onthe asymptotic giant branch. I propose four evolutionary classes,according to the binary-model hypothesis: (1) Progenitors of planetarynebula that did not interact with any companion. These amount to ~10% ofall planetary nebulae. (2) Progenitors that interact with stellarcompanions that avoided a common envelope, 11^{+2}_{-3}% of all nebulae.(3) Progenitors that interact with stellar companions via a commonenvelope phase, 23^{+11}_{-5}% of all nebulae. (4) Progenitors thatinteract with substellar (i.e., planets and brown dwarfs) companions viaa common envelope phase, 56^{+5}_{-8}% of all nebulae. In order todefine and build the different classes, I start with clarifying somerelevant terms and processes related to binary evolution. I then discusskinematical and morphological properties of planetary nebulae thatappear to require the interaction of the planetary nebula progenitorsand/or their winds with companions, stellar or substellar.

The spatial orientations of bipolar nebulae.
We investigate the spatial orientation of mass outflow axes inpost-main-sequence bipolar nebulae. As a consequence, some 71% ofnebulae are found to have their outflow axes oriented within +/-45^o^ ofthe Galactic plane; a distribution which has a probability level of5.8x10^-4^. There appear to be no clear mechanisms whereby such anon-random distribution may be explained.

The chemical evolution of planetary nebulae.
We report millimeter line observations of CO, ^13^CO, SiO, SiC_2_, CN,HCN, HNC, HCO^+^, CS, and HC_3_N to study the chemistry in planetarynebulae (PNe) with massive envelopes of molecular gas. The sampleobserved consists of representative objects at different stages ofdevelopment in order to investigate evolutionary effects: the proto-PNeCRL 2688 and CRL 618, the young PN NGC 7027, and the evolved PNe NGC6720 (the Ring), M4-9, NGC 6781, and NGC 7293 (the Helix). Theobservations confirm that the chemical composition of the molecular gasin PNe is radically different from that in interstellar clouds and thecircumstellar envelopes of Asymptotic Giant Branch (AGB) stars. Thereare also clear trends in the chemical evolution of the envelopes. As astar evolves beyond the AGB, through the proto-PN and PN phases, theabundances of SiO, SiC_2_, CS, and HC_3_N decrease, and they are notdetected in the PNe, while the abundances of CN, HNC, and HCO^+^increase dramatically. Once a PN has formed, the observed abundances inthe molecular clumps of the envelope remain relatively constant,although HNC is anomalously underabundant in NGC 7027. In the evolvedPNe, CN is about an order of magnitude more abundant than HCN, HNC, andHCO^+^, and the average abundance ratios are CN/HCN=9, HNC/HCN=0.5, andHCO^+^/HCN=0.5. These ratios are, respectively, one, two, and threeorders of magnitude higher than in the prototypical AGB envelopeIRC+10216. The ^12^C/^13^C ratios are =~10-25, within the large rangefound in AGB envelopes. The chemical evolution of the envelopes likelyoccurs through the development of photon-dominated regions produced bythe ultraviolet radiation field of the central star. The observationsalso provide important information on the physical conditions in themolecular gas. Multi-line observations of CN, CO, and HCO^+^ show thatthe clumps which form the envelopes of the evolved PNe maintainremarkably high gas densities (~ few x 10^5^cm^-3^) and low temperatures(~25K). These values are consistent with the idea that the clumps are inrough pressure equilibrium with the more diffuse, ionized gas and canlast for a significant part of the nebular lifetime, providing theenvironment needed for the survival of the molecules. Thus the clumpingof the gas in these PNe is an essential aspect of both their physicaland chemical evolution.

Chemical Evolution from the AGB to the Planetary Nebula Phase
An overview is given on the wealth of data recently provided by largemm-wave radiotelescopes on AGB stars, planetary nebulae (PNe), andtransition objects. The observations reveal that there is an observablechemical evolution in the neutral gas as a star evolves beyond the AGB,through the proto-PN and PN phases. Significant changes in theabundances of some key molecules (such as CS, CN, HCO+, HCN, and HC3N)take place during the fast evolution of the envelopes. Chemistry canthus be used as a rough clock to date the evolutionary stage of post-AGBenvelopes and proto-PN objects. However, once the PN is formed, theobserved abundances in the molecular clumps of the envelope remainrelatively constant. The chemical evolution of the molecular envelopeslikely occurs through the development of photon-dominated regionsproduced by the ultraviolet field of the central star. The main chemicalprocesses which likely control the evolution are also reviewed.

Planetary nebulae morphologies, central star masses and nebular properties.
We have constituted a sample of about 80 PN with defined morphologiesand well observed basic parameters (fluxes, angular radii, expansionvelocities and magnitudes of the central stars). For these PN, we havederived the central star masses by comparing the observed set ofparameters with those predicted by a simple evolutionary model of a PN,expanding at the same velocity as the observed one. We have thenexamined the relations between the PN morphological types and otherproperties, linked to the central star mass. Bipolar PN are shown tohave a wider distribution of central star masses than the rest of PN,and shifted towards higher values. They lie closer to the Galactic planeand tend to have larger N/O ratios. Point symmetric PN, which have notbeen much studied so far, are found to constitute an outstanding class.They show an almost perfect M_*_-v_exp_ correlation. They correspond toa rather short evolutionary stage of PN. They lie, on average, furtherfrom the Galactic plane than bipolar PN and tend to have lower N/O.Globally, PN with higher central star masses are found closer to theGalactic plane, and the observed relation between N/O and M_*_ isroughly consistent with the predictions from evolutionary models for AGBstars.

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Right ascension:16h12m58.08s
Apparent magnitude:14

Catalogs and designations:
Proper Names   (Edit)
NGC 2000.0NGC 6072

→ Request more catalogs and designations from VizieR