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NGC 6445 (Little Gem)



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Magnetic interaction of jets and molecular clouds in NGC 4258
Context: NGC 4258 is a well known spiral galaxy witha peculiar large scale jet flow detected in the radio and in Hα.Due to the special geometry of the galaxy, the jets emerge from thenuclear region through the galactic disk - at least in the inner region. Aims: Also the distribution of molecular gas looks different from thatin other spiral galaxies: 12CO(1-0) emission has only beendetected in the center and along the jets and only up to distances ofabout 50'' (1.8 kpc) from the nucleus. This concentration of CO alongthe jets is similar to what is expected as fuel for jet-induced starformation in more distant objects. The reason for the CO concentrationalong the inner jets in NGC 4258 was not understood and is themotivation for the observations presented here. Methods: Using theIRAM interferometer at Plateau de Bure, we mapped the12CO(1-0) emission of the central part of NGC 4258 along thenuclear jet direction in the inner 3 kpc. In order to get a properlypositioned overlay with Hα we observed NGC 4258 in Hα at theHoher List Observatory of the University of Bonn. Results: We detectedtwo parallel CO ridges along a position angle of -25° with a totallength of about 80'' (2.8 kpc), separated by a CO depleted funnel with awidth of about 5'' (175 pc). The Hα emission is more extended andbroader than the CO emission with its maximum just in between the two COridges. It seems to be mixed in location and in velocity with the COemission. In CO we see a peculiar velocity distribution in theiso-velocity map and p-v diagrams. We discuss different scenarios for aninterpretation and present a model which can explain the observationalresults consistently. Conclusions: We propose here that theconcentration of CO along the ridges is due to interaction of therotating gas clouds with the jet's magnetic field by ambipolar diffusion(ion-neutral drift). This magnetic interaction is thought to increasethe time the molecular clouds reside near the jet thus leading to thequasi-static CO ridge.

Planetary nebulae abundances and stellar evolution
A summary is given of planetary nebulae abundances from ISOmeasurements. It is shown that these nebulae show abundance gradients(with galactocentric distance), which in the case of neon, argon, sulfurand oxygen (with four exceptions) are the same as HII regions and earlytype star abundance gradients. The abundance of these elements predictedfrom these gradients at the distance of the Sun from the center areexactly the solar abundance. Sulfur is the exception to this; the reasonfor this is discussed. The higher solar neon abundance is confirmed;this is discussed in terms of the results of helioseismology. Evidenceis presented for oxygen destruction via ON cycling having occurred inthe progenitors of four planetary nebulae with bilobal structure. Theseprogenitor stars had a high mass, probably greater than 5 Mȯ. Thisis deduced from the high values of He/H and N/H found in these nebulae.Formation of nitrogen, helium and carbon are discussed. The high massprogenitors which showed oxygen destruction are shown to have probablydestroyed carbon as well. This is probably the result of hot bottomburning.

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.

The importance of soft X-rays for the excitation of H2 emission in planetary nebulae
We note that H2 emitting planetary nebulae tend to haveZanstra temperatures TZ(HeII) > 90 kK. This is shown to beconsistent with a large evolutionary lifetime, and the kinematic ages ofthe envelopes. Non-local thermodynamic equilibrium stellar atmosphericmodelling also shows that levels of soft X-ray emission increase morerapidly than has previously been assumed, and are preferentially largein H2 emitting sources. It is suggested that this may holdthe key to explaining the strengths of the H2 transitions.

The Use of K_S Band Photometric Excesses to Investigate H(2) Emission in Planetary Nebulae
We have determined the distribution of H(2) emission in 14 planetarynebulae (PNe), using imaging and photometry published by the 2MASSinfrared survey. This technique is only applicable under certainstringent conditions, and requires precise broad band photometry, andaccurate spatial registration between the K_S and H band images. It is,in addition, only applicable to certain sources, and excludes outflowsin which central star and grain thermal excesses are appreciable. Ourresults for NGC 3132, NGC 6720, IC 4406 and M 2-9 are closely similar tothose of previous narrow band imaging, and confirm that H(2) emissionis confined to narrow, highly fragmented shells. Similar results areobtained for M 1-7, M 1-8, and M 3-5. Our spatial profiles also confirmthat the emission extends outside of the primary ionised shells. Whereenvelopes are large, and the PNe are more evolved, then the fractionalextensions Deltatheta/$theta appear to be at their smallest. They arealso similar to the radial widths predicted for H(2) abundanceprofiles, and to the values DeltaR/R determined throughmagnetohydrodynamic modelling of shocks. There appears, finally, to beevidence for an evolution in this parameter, such that Deltatheta/$thetavaries with increasing envelope size d(H) as Deltatheta/$theta ~d(H)(-2.2) .

The Fine-Structure Lines of Hydrogen in H II Regions
The 2s1/2 state of hydrogen is metastable and overpopulatedin H II regions. In addition, the 2p states may be pumped by ambientLyα radiation. Fine-structure transitions between these states maybe observable in H II regions at 1.1 GHz(2s1/2-2p1/2) and/or 9.9 GHz(2s1/2-2p3/2), although the details of absorptionversus emission are determined by the relative populations of the 2s and2p states. The n=2 level populations are solved with a parameterizationthat allows for Lyα pumping of the 2p states. The Lyαpumping rate has long been considered uncertain, as it involves solutionof the difficult Lyα transfer problem. The density of Lyαphotons is set by their creation rate, easily determined from therecombination rate, and their removal rate. Here we suggest that thedominant removal mechanism of Lyα radiation in H II regions isabsorption by dust. This circumvents the need to solve the Lyαtransfer problem and provides an upper limit to the rate at which the 2pstates are populated by Lyα photons. In virtually all cases ofinterest, the 2p states are predominantly populated by recombination,rather than Lyα pumping. We then solve the radiative transferproblem for the fine-structure lines in the presence of free-freeradiation. In the likely absence of Lyα pumping, the2s1/2-->2p1/2 lines will appear in stimulatedemission, and the 2s1/2-->2p3/2 lines inabsorption. Because the final 2p states are short lived, these lines aredominated by intrinsic line width (99.8 MHz). In addition, eachfine-structure line is a multiplet of three blended hyperfinetransitions. Searching for the 9.9 GHz lines in high emission measure HII regions offers the best prospects for detection. The lines arepredicted to be weak; in the best cases, line-to-continuum ratios ofseveral tenths of a percent might be expected with line strengths oftens to a hundred mK with the Green Bank Telescope. Predicted linestrengths, at both 1.1 and 9.9 GHz, are given for a number of H IIregions, high emission measure components, and planetary nebulae, basedon somewhat uncertain emission measures, sizes, and structures. Theextraordinary width of these lines and their blended structure willcomplicate detection.

Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio
Airborne and space-based low-resolution spectroscopy in the 1980sdiscovered tantalizing quantitative relationships between the gas phaseC/O abundance ratio in planetary nebulae (PNe) and the fractions oftotal far-infrared (FIR) luminosity radiated by the 7.7- and 11.3-μmbands (the C = C stretch and C-H bend, respectively), of polycyclicaromatic hydrocarbons (PAHs). Only a very small sample of nebulae wasstudied in this context, limited by airborne observations of the7.7-μm band, or the existence of adequate IRAS Low ResolutionSpectrometer data for the 11.3-μm band. To investigate these trendsfurther, we have expanded the sample of planetaries available for thisstudy using Infrared Space Observatory (ISO) low-resolution spectrasecured with the Short Wavelength Spectrometer and the Long WavelengthSpectrometer. The new sample of 43 PNe, of which 17 are detected in PAHemission, addresses the range from C/O = 0.2-13 with the objective oftrying to delineate the pathways by which carbon dust grains might haveformed in planetaries. For the 7.7-μm and 11.3-μm bands, weconfirm that the ratio of band strength to total infrared (IR)luminosity is correlated with the nebular C/O ratio. Expressed inequivalent width terms, the cut-on C/O ratio for the 7.7-μm band isfound to be 0.6+0.2-0.4, in good accord with thatfound from sensitive ground-based measurements of the 3.3-μ band.

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.

The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their Envelopes
The 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.

Evolution from AGB to planetary nebula in the MSX survey
We investigate the evolution of oxygen- and carbon-rich AGB stars,post-AGB objects, and planetary nebulae using data collected mainly fromthe MSX catalogue. Magnitudes and colour indices are compared with thosecalculated from a grid of synthetic spectra that describe the post-AGBevolution beginning at the onset of the superwind. We find that carbonstars and OH/IR objects form two distinct sequences in the (K-[8.3])×([8.3]-[14.7]) MSX colour diagram. OH/IR objects are distributedin two groups: the bluest ones are crowded near [14.7]-[21.3]≃ 1and [8.3]-[14.7]≃ 2, and a second, redder group is spread over alarge area in the diagram, where post-AGB objects and planetary nebulaeare also found. High mass-loss rate OH/IR objects, post-AGB stars, andplanetary nebulae share the same region in the (K-[8.3])×([8.3]-[14.7]) and [14.7]-[21.3]×([8.3]-[14.7]) colour-colourdiagrams. This region in the diagram is clearly separated from a bluerone where most OH/IR stars are found. We use a grid of models ofpost-AGB evolution, which are compared with the data. The gap in thecolour-colour diagrams is interpreted as the result of the rapidtrajectory in the diagram of the stars that have just left the AGB.Based on results obtained by the MSX survey.Tables 1 to 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/565

The structure of the planetary nebula NGC 6445
Narrow band CCD images and low and high dispersion longslit spectra fromthe planetary nebula NGC 6445 are presented. This is a non-symmetrictype I bipolar nebula with a very bright central ring, opened lobes, andremarkable features. Using high dispersion spectra we obtain thekinematical structure of NGC 6445 and some of its internal knots. Lowdispersion spectra have been used to get the chemical structure of thenebula and for mapping the physical conditions. Preliminary results showthe expansion of the lobes as well as an important variation of thekinematics and the physical and chemical parameters in different regionsof this object. Spectra were obtained with the 2.1m UNAM Telescope atSan Pedro Martir Observatory (Mexico) using the Boller & Chivens andthe Manchester Echelle spectrographs. CCD imagery was obtained with 1.5mTelescope at Observatorio de Sierra Nevada in Granada (Spain).This research was supported by grant IN111903-3 DGAPA-PAPIIT-UNAM(Mexico). GB is in grateful receipt of a graduate scholarship fromCONACYT (Mexico).

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.

Flux Ratio [Nev] 14.3/24.3 as a Test of Collision Strengths
From ISO [Nev] 14.3/24.3 μm line flux ratios, we find that 10 out of20 planetary nebulae (PNs) have measured ratios below the low-electrondensity (Ne) theoretical predicted limit. Such astronomicaldata serve to provide important tests of atomic data, collisionstrengths in this case. In principle, well-calibrated measurements ofthe [Nev] 14.3/24.3 flux ratio could improve upon the existing atomicdata.

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.

Probing AGB nucleosynthesis via accurate Planetary Nebula abundances
The elemental abundances of ten planetary nebulae, derived with highaccuracy including ISO and IUE spectra, are analysed with the aid ofsynthetic evolutionary models for the TP-AGB phase. The accuracy on theobserved abundances is essential in order to make a reliable comparisonwith the models. The advantages of the infrared spectra in achievingthis accuracy are discussed. Model prescriptions are varied until weachieve the simultaneous reproduction of all elemental features, whichallows placing important constraints on the characteristic masses andnucleosynthetic processes experienced by the stellar progenitors. Firstof all, it is possible to separate the sample into two groups of PNe,one indicating the occurrence of only the third dredge-up during theTP-AGB phase, and the other showing also the chemical signature ofhot-bottom burning. The former group is reproduced by stellar modelswith variable molecular opacities (see Marigo \cite{Marigo2002}),adopting initial solar metallicity, and typical efficiency of the thirddredge-up, lambda ~ 0.3-0.4. The latter group of PNe, with extremelyhigh He content (0.15 <=He/H <=0.20) and marked oxygen deficiency,is consistent with original sub-solar metallicity (i.e. LMCcomposition). Moreover, we are able to explain quantitatively both theN/H-He/H correlation and the N/H-C/H anti-correlation, thus solving thediscrepancy pointed out long ago by Becker & Iben(\cite{Becker1980}). This is obtained only under the hypothesis thatintermediate-mass TP-AGB progenitors (M >~ 4.5-5.0 Msun)with LMC composition have suffered a number of very efficient,carbon-poor, dredge-up events. Finally, the neon abundances of theHe-rich PNe can be recovered by invoking a significant production of22Ne during thermal pulses, which would imply a reduced roleof the 22Ne(alpha , n)25Mg reaction as neutronsource to the s-process nucleosynthesis in these stars.

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.

Near-infrared spectroscopy of (proto)-planetary nebulae: molecular hydrogen excitation as an evolutionary tracer
We present an in-depth analysis of molecular excitation in 11H2-bright planetary and protoplanetary nebulae (PN and PPN).From newly acquired K-band observations, we extract a number of spectraat positions across each source. H2 line intensities areplotted on `column density ratio' diagrams so that we may examine theexcitation in and across each region. To achieve this, we combine theshock models of Smith, Khanzadyan & Davis with the photodissociationregion (PDR) models of Black & van Dishoeck to yield ashock-plus-fluorescence fit to each data set.Although the combined shock + fluorescence model is needed to explainthe low- and high-energy H2 lines in most of the sourcesobserved (fluorescence accounts for much of the emission from thehigher-energy H2 lines), the relative importance of shocksover fluorescence does seem to change with evolutionary status. We findthat shock excitation may well be the dominant excitation mechanism inthe least evolved PPN (CRL 2688 - in both the bipolar lobes and in theequatorial plane) and in the most evolved PN considered (NGC 7048).Fluorescence, on the other hand, becomes more important at intermediateevolutionary stages (i.e. in `young' PN), particularly in the inner coreregions and along the inner edges of the expanding post-asymptotic giantbranch (AGB) envelope. Since H2 line emission seems to beproduced in almost all stages of post-AGB evolution, H2excitation may prove to be a useful probe of the evolutionary status ofPPN and PN alike. Moreover, shocks may play an important role in themolecular gas excitation in (P)PN, in addition to the low- and/orhigh-density fluorescence usually attributed to the excitation in thesesources.

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

Optical Slitless Spectroscopy of Large Magellanic Cloud Planetary Nebulae: A Study of the Emission Lines and Morphology
Hubble Space Telescope Space Telescope Imaging Spectrograph slitlessspectroscopy of LMC planetary nebulae (PNs) is the ideal tool to studytheir morphology and their ionization structures at once. We present theresults from a group of 29 PNs that have been spatially resolved, forthe first time, in all the major optical lines. Images in the light ofHα, [N II], and [O III] are presented, together with lineintensities, measured from the extracted one- and two-dimensionalspectra. A study on the surface brightness in the different opticallines, the electron densities, the ionized masses, the excitationclasses, and the extinction follows, illustrating an ideal consistencewith the previous results found by us on LMC PNs. In particular, we findthe surface brightness decline with the photometric radius to be thesame in most emission lines. We find that asymmetric PNs form awell-defined cooling sequence in the excitation-surface brightnessplane, confirming their different origin and larger progenitor mass.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.

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

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.

The Distances of Planetary Nebulae: A Scale Based upon Nearby Sources
The statistical distances of planetary nebulae (PNe) are known to benotoriously uncertain, differing from each by factors approaching ~2,and also from more reliable distances based upon trigonometric andspectroscopic parallaxes, expansion distances, and so forth. We pointout that many of these problems arise from the use of inappropriaterelations between surface brightness and nebular radius; of incompletesamples of standard sources; and the use of indirect (and/or inaccurate)methods for the evaluation of distances. Many of these problems can beside-stepped, however, where the standard sources are confined to nearbynebulae; planetaries whose distances are less than ~0.7 kpc or so. Wehave, using such sources, undertaken a detailed revision of the surfacebrightness-radius calibration, and thereby defined a new statisticalscale which is consistent with the most direct, recent, and reliabledistance measures. This scale is used to determine distances to 448Population I Galactic planetary nebulae, and the results are compared toalternative statistical scales. It is shown that much of the disparitybetween the present distance scale and that of Zhang is likely to arisethrough differences in calibration source distances. It does not (assupposed by previous authors) appear to be a product of selectioneffects within the nebular samples. A disparity with the van de Steene& Zijlstra distances may arise from differences between Galacticbulge and disk nebulae, while the scales of Daub and Cahn et al. appearto be based upon inappropriate trends between surface brightness andradius. Finally, we discuss the implications of these results for theformation rates of planetary nebulae and determine a galaxy-wide totalof PNe NT=3.0×104. The luminosity specificformation rate χL~3×10-11L-1solar,BOL appears ~2.5 times larger than determined inprevious analyses, while the rate per unit area of Galactic disk[χA(PNe)~1.6×10-3yr-1kpc-2] is comparable to the value deduced for white dwarfs.

The 3-D ionization structure of the planetary nebula NGC 6565
A detailed study of the planetary nebula NGC 6565 has been carried outon long-slit echellograms (lambda /Delta lambda =60 000, spectral range= lambda lambda 3900-7750 Å) at six, equally spaced positionangles. The expansion velocity field, the c(Hβ ) distribution andthe radial profile of the physical conditions (electron temperature anddensity) are obtained. The distance, radius, mass and filling factor ofthe nebula and the temperature and luminosity of the central star arederived. The radial ionization structure is analyzed using both theclassical method and the photo-ionization code CLOUDY. Moreover, wepresent the spatial structure in a series of images from differentdirections, allowing the reader to ``see'' the nebula in 3-D. NGC 6565results to be a young (2000-2500 years), patchy, optically thicktriaxial ellipsoid (a=10.1 arcsec, a/b=1.4, a/c=1.7) projected almostpole-on. The matter close to major axis was swept-up by someaccelerating agent (fast wind? ionization? magnetic fields?), formingtwo faint and asymmetric polar cups. A large cocoon of almost neutralgas completely embeds the ionized nebula. NGC 6565 is in a recombinationphase, because of the luminosity drop of the massive powering star,which is reaching the white dwarf domain (log T* =~ 5.08 K;log L*/Lsun =~ 2.0). The stellar decline startedabout 1000 years ago, but the main nebula remained optically thin forother 600 years before the recombination phase occurred. In the nearfuture the ionization front will re-grow, since the dilution factor dueto the expansion will prevail on the slower and slower stellar decline.NGC 6565 is at a distance of 2.0 (+/-0.5) kpc and can be divided intothree radial zones: the ``fully ionized'' one, extending up to0.029-0.035 pc at the equator (0.050 pc at the poles), the``transition'' one, up to 0.048-0.054 pc (0.080 pc), the ``halo'',detectable up to 0.110 pc. The ionized mass ( =~ 0.03 Msun)is only a fraction of the total mass (>= 0.15 Msun), whichhas been ejected by an equatorial enhanced superwind of 4 (+/-2) x10-5 Msun yr-1 lasted for 4 (+/-2) x103 years. Based on observations made with ESO Telescopes atthe La Silla Observatories, under programme ID 65.I-0524, and onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive at the Space Telescope Institute (observingprogram GO 7501; P.I. Arsen Hajian). STScI is operated by theassociation of Universities for Research in Astronomy, Inc. under theNASA contract NAS 5-26555. We have applied the photoionization codeCLOUDY, developed at the Institute of Astronomy of the CambridgeUniversity.

Chemical Composition of Planetary Nebulae: Including ISO Results
The method of determining abundances using Infrared Space Observatoryspectra is discussed. The results for seven planetary nebula are given.Using these data, a preliminary discussion of their evolution is given.

Study of electron density in planetary nebulae. A comparison of different density indicators
We present a comparison of electron density estimates for planetarynebulae based on different emission-line ratios. We have considered thedensity indicators [O Ii]lambda 3729/lambda 3726, [S Ii]lambda6716/lambda 6731, [Cl Iii]lambda 5517/lambda 5537, [Ar Iv]lambda4711/lambda 4740, C Iii]lambda 1906/lambda 1909 and [N I]lambda5202/lambda 5199. The observational data were extracted from theliterature. We have found systematic deviations from the densityhomogeneous models, in the sense that: Ne(ion {N}i) <~Ne(ion {O}{ii}) < Ne(ion {S}{ii}, ion {C}{iii},ion {Cl}{iii} or ion {Ar}{iv}) and Ne(ion {S}{ii}) ~Ne(ion {C}{iii}) ~ Ne(ion {Cl}{iii}) ~Ne(ion {Ar}{iv}). We argue that the lower [O Ii] densityestimates are likely due to errors in the atomic parameters used.

Abundances of planetary nebulae NGC 7662 and NGC 6741
The ISO and IUE spectra of the elliptical nebulae NGC7662 and NGC 6741 are presented. Thesespectra are combined with the spectra in the visual wavelength region toobtain a complete, extinction corrected, spectrum. The chemicalcomposition of the nebulae is then calculated and compared to previousdeterminations. The abundances found are compared to determinations madein other nebulae using ISO data. A discussion is given to see ifpossible evolutionary effects can be found from the abundancedifferences. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA.

The distance scale of planetary nebulae
By collecting distances from the literature, a set of 73 planetarynebulae with mean distances of high accuracy is derived. This sample isused for recalibration of the mass-radius relationship, used by manystatistical distance methods. An attempt to correct for a statisticalpeculiarity, where errors in the distances influences the mass-radiusrelationship by increasing its slope, has been made for the first time.Distances to PNe in the Galactic Bulge, derived by this new method aswell as other statistical methods from the last decade, are then usedfor the evaluation of these methods as distance indicators. In order ofachieving a Bulge sample that is free from outliers we derive newcriteria for Bulge membership. These criteria are much more stringentthan those used hitherto, in the sense that they also discriminateagainst background objects. By splitting our Bulge sample in two, onewith optically thick (small) PNe and one with optically thin (large)PNe, we find that our calibration is of higher accuracy than most othercalibrations. Differences between the two subsamples, we believe, aredue to the incompleteness of the Bulge sample, as well as the dominanceof optical diameters in the ``thin'' sample and radio diameters in the``thick'' sample. Our final conclusion is that statistical methods givedistances that are at least as accurate as the ones obtained from manyindividual methods. Also, the ``long'' distance scale of Galactic PNe isconfirmed.

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.

Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.
Not Available

Abundance in the planetary nebulae NGC 6537 and He2-111
The ISO and IUE spectra of the bipolar planetary nebulae NGC 6537 andHe2-111 are presented. These spectra are combined with the spectrum inthe visual wavelength region from the nebulae to obtain a completespectrum that is corrected for extinction. The chemical abundance of thenebulae is then determined and compared to previous determinations. Theabundance of the two nebulae is quite similar. A comparison is then madewith the abundance of two other bipolar planetary nebulae whoseabundance is also determined with the help of ISO observations. It isshown that not all bipolar nebulae have similar abundance. NGC 6445 hasa much lower nitrogen to oxygen ratio, similar to NGC 7027, but stillnot as low as the Orion nebula.

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Observation and Astrometry data

Right ascension:17h49m15.21s
Apparent magnitude:13

Catalogs and designations:
Proper NamesLittle Gem
NGC 2000.0NGC 6445

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