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The distances of less-evolved planetary nebulae: a further test of statistical distance scales
It has recently been pointed out that a number of the methods used todetermine planetary nebulae (PNe) distances may be appreciably in error.Whilst the scales of Zhang (1995), Bensby & Lundstrom (2001) andothers are appropriate for higher radio brightness temperaturesTB, those of Phillips and Daub are more relevant whereTB is small.We note, in the following, that the absolute bolometric magnitudes ofless-evolved PNe are likely to be similar. The mean value of can therefore be used to constrain PNe distancesD, and confirm the distance scales for higher TB outflows. Wehave used this procedure to evaluate distances to a further 47 PNe, andwe find that the mean values of are consistent with those ofCahn, Kaler & Stanghellini (1992), Zhang (1995), Phillips et al.(2004) and van de Steene & Zijlstra (1995). They are, as expected,inconsistent with the lower TB scale of Phillips (2002a).
| 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 (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.
| The distances of planetary nebulae: A scaling factor based upon radial velocities
We have used the observed radial velocities of planetary nebulae, andthe galactic rotation curve to define a new distance scale for planetarynebulae. Care has been taken to restrict the analysis to sources inwhich distance non-linearities are minimised, and for which distortionof the distance scale is likely to be small. We conclude, as a result,that the so-called ``long'' distance scales are the most valid, and thatvarious ``shorter'' scales are unlikely to be correct. The primarysystematic errors in this procedure derive from uncertainties in thegalactic rotation profile, and in Θ0 and R0.Such errors are less than those normally associated with PN distancedeterminations.
| Low-Ionization Structures in Planetary Nebulae: Confronting Models with Observations
Around 50 planetary nebulae (PNs) are presently known to possess``small-scale'' low-ionization structures (LISs) located inside oroutside their main nebular bodies. We consider here the different kindsof LISs (jets, jetlike systems, symmetrical and nonsymmetrical knots)and present a detailed comparison of the existing model predictions withthe observational morphological and kinematical properties. We find thatnebulae with LISs appear indistinctly spread among all morphologicalclasses of PNs, indicating that the processes leading to the formationof LISs are not necessarily related to those responsible for theasphericity of the large-scale morphological components of PNs. We showthat both the observed velocities and locations of most nonsymmetricalsystems of LISs can be reasonably well reproduced assuming either fossilcondensations originated in the asymptotic giant branch (AGB) wind or insitu instabilities. The jet models proposed to date (hydrodynamical andmagnetohydrodynamical interacting winds or accretion disk collimatedwinds) appear unable to account simultaneously for several keycharacteristics of the observed high-velocity jets, such as theirkinematical ages and the angle between the jet and the symmetry axes ofthe nebulae. The linear increase in velocity observed in several jetsfavors magnetohydrodynamical confinement compared to pure hydrodynamicalinteracting wind models. On the other hand, we find that the formationof jetlike systems characterized by relatively low expansion velocities(similar to those of the main shells of PNs) cannot be explained by anyof the existing models. Finally, the knots that appear in symmetricaland opposite pairs of low velocity could be understood as the survivalof fossil (symmetrical) condensations formed during the AGB phase or asstructures that have experienced substantial slowing down by the ambientmedium.
| Knots in the Outer Shells of the Planetary Nebulae IC 2553 and NGC 5882
We present images and high-resolution spectra of the planetary nebulaeIC 2553 and NGC 5882. Spatiokinematic modeling of the nebulae shows thatthey are composed of a markedly elongated inner shell and of a lessaspherical outer shell expanding at a considerably higher velocity thanthe inner one. Embedded in the outer shells of both nebulae are foundseveral low-ionization knots. In IC 2553, the knots show apoint-symmetric distribution with respect to the central star: onepossible explanation for their formation is that they are the survivorsof preexisting point-symmetric condensations in the asymptotic giantbranch wind, a fact that would imply a quite peculiar mass-loss geometryfrom the giant progenitor. In the case of NGC 5882, the lack of symmetryin the distribution of the observed low-ionization structures makes itpossible that they are the result of in situ instabilities. Based onobservations obtained at the 3.5 m New Technology Telescope (NTT) of theEuropean Southern Observatory and with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by AURA for NASA under contract NAS5-26555.
| High-Velocity Collimated Outflows in Planetary Nebulae: NGC 6337, HE 2-186, and K4-47
We have obtained narrowband images and high-resolution spectra of theplanetary nebulae NGC 6337, He 2-186, and K4-47 with the aim ofinvestigating the relation between their main morphological componentsand several low-ionization features present in these nebulae. The datasuggest that NGC 6337 is a bipolar PN seen almost pole-on, with polarvelocities of >=200 km s-1. The bright inner ring of thenebula is interpreted to be the ``equatorial'' density enhancement. Itcontains a number of low-ionization knots and outward tails that weascribe to dynamical instabilities leading to fragmentation of the ringor transient density enhancements due to the interaction of theionization front with previous density fluctuations in the interstellarmedium. The lobes show a pronounced point-symmetric morphology and twopeculiar low-ionization filaments, the nature of which remains unclear.The most notable characteristic of He 2-186 is the presence of twohigh-velocity (>=135 km s-1) knots from which an S-shapedlane of emission departs toward the central star. K4-47 is composed of acompact core and two high-velocity, low-ionization blobs. We interpretthe substantial broadening of line emission from the blobs as asignature of bow shocks, and using the modeling of Hartigan, Raymond,& Hartman, we derive a shock velocity of ~150 km s-1 anda mild inclination of the outflow on the plane of the sky. We discusspossible scenarios for the formation of these nebulae and theirlow-ionization features. In particular, the morphology of K4-47 hardlyfits into any of the usually adopted mass-loss geometries for singleasymptotic giant branch stars. Finally, we discuss the possibility thatpoint-symmetric morphologies in the lobes of NGC 6337 and the knots ofHe 2-186 are the result of precessing outflows from the central stars.Based on observations obtained at the 3.5 m New Technology Telescope(NTT) of the European Southern Observatory, and at the 2.6 m NordicOptical Telescope (NOT) operated on the island of La Palma by NOTSA, inthe Spanish Observatorio del Roque de Los Muchachos of the Instituto deAstrofísica de Canarias, and with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by AURA for NASA under contract NAS5-26555.
| Jets and Jet-Like Structures of Low-Ionization in Planetary Nebulae
In this contribution we discuss new results of a program aimed atstudying the physical properties, origin and evolution of``low-ionization, small-scale structures in PNe''. Within this projectwe classify and discuss the properties of a sample of more than 50 PNe,collected from the literature, in the light of their possible formationmechanisms. In addition, we obtained images and spectra for 10 of thesePNe, finding low-ionization structures with very different propertiesrelative to each other, in terms of expansion velocities, shapes, sizesand locations relative to the main nebular components. Several physicalprocesses have to be invoked in order to account for the formation andevolution of jets and jet-like structures observed. We present here someresults which are illustrative of our work--on IC 4593, NGC 3918, NGC6337--comparing the different low-ionization structures in these PNe andillustrating some of the questions that we try to address. The mostinteresting result we obtained is that the collimated LIS present alinear increase in the expansion velocity from inner to outer regions.This is an important constraint on the models and is, in fact, one ofthe predictions of the interacting-stellar-wind models for the formationof jets, if stellar magnetic fields are considered.
| Gas Dynamics in Planetary Nebulae: From Macro-structures to FLIERs
Purpose of this paper is to clarify how Planetary Nebulae (PNe) are veryinteresting laboratories to study cosmic gas dynamics. I first recallthe history of PNe which are generated from low and intermediate massstars through successive mass loss processes starting in the Reg Giantphase of evolution and continuing also after the termination of thepulsed AGB phase, where most of the nebular mass is believed to beejected. The correponding stellar winds are the ingredients of thenebula. Their initial properties and subsequent mutual interactions,under the action of the evolving stellar radiation field, areresponsible for the properties of the nebula. The observed structures ofPNe are considered in detail. Larger scale macroscopic structures (MACS)are examined separately from quite smaller scale microscopic structures(MICS). The formation of MACS, at least in cases of round to moderatelyelliptical PNe, is shown to be reasonably well understood in terms ofexisting hydrodynamical models. Considering the kinematical behaviour,MICS can be separated into FLIERs (Fast Low Ionization Emitting Regions)and SLOWERs (slowly moving). Attention is focussed on FLIERs and on theproposed mechanisms to interpret them. Recent observations with theHubble Space Telescope have provided us with a wealth of detailed(subarcsec) information on the nebular structures. The inner structureof FLIERs is here illustrated to consist of substructures of variousshapes with an high degree of individually from object to object, alsowithin the same PN. These new data call for deeper thoretical efforts tosolve the problems of cosmic gas dynamics, posed by their observedproperties. An ample account is given of the most relevant originalworks, in an effort to allow the non specialist reader to quickly becomeacquainted with the status of art in the various aspects of the subject.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Infrared Planetary Nebulae in the NRAO VLA Sky Survey
In order to construct a sample of planetary nebulae (PNe) unbiased bydust extinction, we first selected the 1358 sources in the IRAS PointSource Catalog north of J2000 declination delta=-40^deg having measuredS(25 μm)>=1 Jy and colors characteristic of PNe: detections orupper limits consistent with both S(12 μm)<=0.35S(25 μm) andS(25 μm)>=0.35S(60 μm). The majority are radio-quietcontaminating sources such as asymptotic giant branch stars. Free-freeemission from genuine PNe should make them radio sources. The 1.4 GHzNRAO VLA Sky Survey (NVSS) images and source catalog were used to rejectradio-quiet mid-infrared sources. We identified 454 IRAS sources withradio sources brighter than S~2.5 mJy beam^-1 (equivalent to T~0.8 K inthe 45" FHWM NVSS beam) by positional coincidence. They comprise 332known PNe in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulaeand 122 candidate PNe, most of which lie at very low Galactic latitudes.Exploratory optical spectroscopic observations suggest that most ofthese candidates are indeed PNe optically dimmed by dust extinction,although some contamination remains from H II regions, Seyfert galaxies,etc. Furthermore, the NVSS failed to detect only 4% of the known PNe inour infrared sample. Thus it appears that radio selection can greatlyimprove the reliability of PN candidate samples withoutsacrificingcompleteness.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| 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.
| 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.
| Jets and Brets in Planetary Nebulae (Invited Review)
Not Available
| A search for 5_1-6_0 A^+-methanol masers towards faint IRAS sources
We present the results of a search for 6.7-GHz methanol maser emissiontowards 241 colour-selected IRAS sources for which the flux density at60 μm is less than 100 Jy. All the candidate sources meet the Wood& Churchwell criteria for ultracompact HII regions. Five newmethanol maser sources are detected. We also present averagedistributions for the distances of the maser sources perpendicular tothe Galactic plane, the maser luminosities, and the masses of theexciting stars obtained by using a statistical procedure to handle theambiguity of kinematic distances. The mass distributions are used toobtain lower limits for the duration of the masing phase for masersassociated with embedded stars with mass greater than 35 M_solar.
| Unveiling low-ionization microstructures in planetary nebulae.
(Hα+[NII])/[OIII] ratio images have been used to search forlow-ionization small-scale structures in a sample of 258 planetarynebulae. Radially symmetrical knots or jet-like structures, which areenhanced in the ratio images compared to the neighbouring regions of thenebulae, have been identified in 23 objects. Some of these features arebarely detected or invisible in the Hα+[NII] and [OIII] images.Empirical evidence, as well as some simple modelling, demonstrate thatthe selected features are characterized by having a lower ionizationthan the surrounding gas. For the above properties, most of themresemble the low-ionization microstructures discussed recently by Balicket al. (1993, 1994), and named FLIERs (Fast Low-Ionization EmissionRegions). The present work shows that the image-division technique is avery useful tool to look for ionization structures in planetary nebulaeand other HII regions. Its main advantage is that the division removesthe morphological components which are pure density enhancements, sothat the output image remains mostly sensitive to excitation andabundance variations through the nebulae.
| A CS(2-1) survey of IRAS point sources with color characteristics of ultra-compact HII regions.
We have made a complete survey of the CS(2-1) emission toward IRAS pointsources in the galactic plane. The sources observed were selectedaccording to their far infrared (FIR) colors, which are characteristicof UC HII regions. They have 25μm/12μm flux ratios larger than 3.7and 60μm/12μm flux ratios larger than 19.3. The survey covered aregion from b=-2deg to b=2deg from l=300deg to l=0deg and from l=0deg tol=60deg, and from b=-4deg to b=4deg elsewhere. We detected 843 sourcesout of 1427 sources observed. The distributions of detected andundetected sources in a FIR color-color diagram show some differences,suggesting that more than one type of object is present in our observedsample.
| The UNAM Scanning Fabry-Perot Interferometer (puma) for the Study of the Interstellar Medium
Not Available
| Evolutionary Paths in Multiple-Shell Planetary Nebulae
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...452..286S
| A statistical distance scale for Galactic planetary nebulae
A statistical distance scale is proposed. It is based on the correlationbetween the ionized mass and the radius and the correlation between theradio continuum surface brightness temperature and the nebular radius.The proposed statistical distance scale is an average of the twodistances obtained while using the correlation. These correlations,calibrated based on the 1`32 planetary nebulae with well-determinedindividual distances by Zhang, can reproduce not only the averagedistance of a sample of Galactic Bulge planetary nebulae exactly at thedistance to the Galactic center, but also the expected Gaussiandistribution of their distances around the Galactic center. This newdistance scale is applied to 647 Galactic planetary nebulae. It isestimated that this distance scale can be accurate on average to35%-50%. Our statistical distance scale is in good agreement with theone recently proposed by Van de Steene and Zijlstra. The correlationsfound in this study can be attributed to the fact that the core mass ofthe central stars has a very sharp distribution, strongly peaked atapprox. 0.6 solar mass. We stress that the scatter seen in thestatistical distance scale is likely to be real. The scatter is causedby the fact that the core mass distribution, although narrow andstrongly peaked, has a finite width.
| On an alternative statistical distance scale for planetary nebulae. Catalog with statistical distances to planetary nebulae.
We have proposed a statistical method to determine distances toplanetary nebulae. The method is based on an empirical correlationbetween the radio-continuum brightness temperature and radius. Here wepresent a catalog of distance determinations calculated using thismethod.
| Spectrophotometry of southern planetary nebulae.
Twenty-seven southern planetary nebulae of moderate to low surfacebrightness have been observed in one or various positions across thenebula with the 152cm and the 3.6m ESO telescopes equipped with the IDS(image dissector scanner) attached to Boller & Chivens spectrographsat spectral resolution between 8 and 13A. The purpose was to search forobjects chemically enriched in helium and nitrogen, according to theprediction of the stellar evolution theory of intermediate mass stars.The chemical abundances of He, N, O, Ne, S and Ar have been determinedin the individual positions, and averaged into values representative ofthe whole nebulae. No cases of objects displaying chemical gradientsacross the nebula have been detected. Eleven nebulae enriched in heliumand/or in nitrogen have been found, six of which detected as such forthe first time. They are: A 37, A 70, Fg 1, MZ 1, MZ 2 and NGC 6578.They increase the number of the objects, which can be classified of typeI of Peimbert.
| A catalogue HeII 4686 line intensities in Galactic planetary nebulae.
We have compiled the intensities of the HeII 4686 lines measured inGalactic planetary nebulae. We present a few observational diagramsrelated to this parameter, and discuss them with the help of theoreticaldiagrams obtained from simple model planetary nebulae surroundingevolving central stars of various masses. We determine the hydrogen andhelium Zanstra temperature for all the objects with accurate enoughdata. We argue that, for Galactic planetary nebulae as a whole, the maincause for the Zanstra discrepancy is leakage of stellar ionizing photonsfrom the nebulae.
| Confrontation of theoretical tracks for post-AGB stars with observations of planetary nebulae
We have constructed a distance-independent diagram to test publishedtheoretical tracks for the evolution of post-AGB stars by comparing themwith the Galactic planetary nebulae data base. We have found noinconsistency between observations and the set of tracks computed bySchoenberner (1981, 1983) and Bloecker & Schoenberner (1990). On theother hand, observations do not seem support the large transition timesbetween the end of the AGB superwind and the beginning of the planetarynebula ionization phase adopted in the models of Vassiliadis & Wood(1994).
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