Contents
Images
Upload your image
DSS Images Other Images
Related articles
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).
| The abundance discrepancy - recombination line versus forbidden line abundances for a northern sample of galactic planetary nebulae
We present deep optical spectra of 23 galactic planetary nebulae, whichare analysed in conjunction with archival infrared and ultravioletspectra. We derive nebular electron temperatures based on standardcollisionally excited line (CEL) diagnostics as well as the hydrogenBalmer jump and find that, as expected, the Balmer jump almost alwaysyields a lower temperature than the [OIII] nebular-to-auroral lineratio. We also make use of the weak temperature dependence of helium andOII recombination line ratios to further investigate the temperaturestructure of the sample nebulae. We find that, in almost every case, thederived temperatures follow the relation , which is the relationpredicted by two-component nebular models in which one component is coldand hydrogen-deficient. Te(OII) may be as low as a fewhundred Kelvin, in line with the low temperatures found for thehydrogen-deficient knots of Abell 30 by Wesson, Liu and Barlow.Elemental abundances are derived for the sample nebulae from both CELsand optical recombination lines (ORLs). ORL abundances are higher thanCEL abundances in every case, by factors ranging from 1.5 to 12. Fiveobjects with O2+ abundance discrepancy factors greater than 5are found. DdDm 1 and Vy 2-2 are both found to have a very largeabundance discrepancy factor of 11.8.We consider the possible explanations for the observed discrepancies.From the observed differences between Te(OIII) andTe(BJ), we find that temperature fluctuations cannot resolvethe abundance discrepancies in 22 of the 23 sample nebulae, implyingsome additional mechanism for enhancing ORL emission. In the oneambiguous case, the good agreement between abundances derived fromtemperature-insensitive infrared lines and temperature-sensitive opticallines also points away from temperature fluctuations being present. Theobserved recombination line temperatures, the large abundancediscrepancies and the generally good agreement between infrared andoptical CEL abundances all suggest instead the existence of a coldhydrogen-deficient component within the `normal' nebular gas. The originof this component is as yet unknown.
| Absolute spectrophotometry of northern compact planetary nebulae
We present medium-dispersion spectra and narrowband images of sixnorthern compact planetary nebulae (PNe): BoBn 1,DdDm 1, IC 5117, M1-5, M 1-71, and NGC6833. From broad-slit spectra, total absolute fluxes andequivalent widths were measured for all observable emission lines. Highsignal-to-noise emission line fluxes of Hα, Hβ, [Oiii],[Nii], and HeI may serve as emission line flux standards for northernhemisphere observers. From narrow-slit spectra, we derive systemicradial velocities. For four PNe, available emission line fluxes weremeasured with sufficient signal-to-noise to probe the physicalproperties of their electron densities, temperatures, and chemicalabundances. BoBn 1 and DdDm 1,both type IV PNe, have an Hβ flux over three sigma away fromprevious measurements. We report the first abundance measurements ofM 1-71. NGC 6833 measured radialvelocity and galactic coordinates suggest that it is associated with theouter arm or possibly the galactic halo, and its low abundance([O/H]=1.3× 10-4) may be indicative of low metallicitywithin that region.
| 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.
| Helium recombination spectra as temperature diagnostics for planetary nebulae
Electron temperatures derived from the HeI recombination line ratios,designated Te(HeI), are presented for 48 planetary nebulae(PNe). We study the effect that temperature fluctuations inside nebulaehave on the Te(HeI) value. We show that a comparison betweenTe(HeI) and the electron temperature derived from the Balmerjump of the HI recombination spectrum, designated Te(HI),provides an opportunity to discriminate between the paradigms of achemically homogeneous plasma with temperature and density variations,and a two-abundance nebular model with hydrogen-deficient materialembedded in diffuse gas of a `normal' chemical composition (i.e.~solar), as the possible causes of the dichotomy between the abundancesthat are deduced from collisionally excited lines and those deduced fromrecombination lines. We find that Te(HeI) values aresignificantly lower than Te(HI) values, with an averagedifference of = 4000 K. Theresult is consistent with the expectation of the two-abundance nebularmodel but is opposite to the prediction of the scenarios of temperaturefluctuations and/or density inhomogeneities. From the observeddifference between Te(HeI) and Te(HI), we estimatethat the filling factor of hydrogen-deficient components has a typicalvalue of 10-4. In spite of its small mass, the existence ofhydrogen-deficient inclusions may potentially have a profound effect inenhancing the intensities of HeI recombination lines and thereby lead toapparently overestimated helium abundances for PNe.
| 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.
| Electron temperatures and densities of planetary nebulae determined from the nebular hydrogen recombination spectrum and temperature and density variations
A method is presented to derive electron temperatures and densities ofplanetary nebulae (PNe) simultaneously, using the observed hydrogenrecombination spectrum, which includes continuum and line emission. Bymatching theoretical spectra to observed spectra around the Balmer jumpat about 3646 Å, we determine electron temperatures and densitiesfor 48 Galactic PNe. The electron temperatures based on this method -hereafter Te(Bal) - are found to be systematically lower thanthose derived from [OIII] λ4959/λ4363 and [OIII] (88 μm+ 52 μm)/λ4959 ratios - hereafterTe([OIII]na) andTe([OIII]fn). The electron densities based on thismethod are found to be systematically higher than those derived from[OII] λ3729/λ3726, [SII] λ6731/λ6716,[ClIII] λ5537/λ5517, [ArIV] λ4740/λ4711 and[OIII] 88 μm/52 μm ratios. These results suggest that temperatureand density fluctuations are generally present within nebulae. Thecomparison of Te([OIII]na) and Te(Bal)suggests that the fractional mean-square temperature variation(t2) has a representative value of 0.031. A majority oftemperatures derived from the Te([OIII]fn) ratioare found to be higher than those of Te([OIII]na),which is attributed to the existence of dense clumps in nebulae - those[OIII] infrared fine-structure lines are suppressed by collisionalde-excitation in the clumps. By comparingTe([OIII]fn), Te([OIII]na)and Te(Bal) and assuming a simple two-density-componentmodel, we find that the filling factor of dense clumps has arepresentative value of 7 × 10-5. The discrepanciesbetween Te([OIII]na) and Te(Bal) arefound to be anticorrelated with electron densities derived from variousdensity indicators; high-density nebulae have the smallest temperaturediscrepancies. This suggests that temperature discrepancy is related tonebular evolution. In addition, He/H abundances of PNe are found to bepositively correlated with the difference betweenTe([OIII]na) and Te(Bal), suggestingthat He/H abundances might have been overestimated generally because ofthe possible existence of H-deficient knots. Electron temperatures anddensities deduced from spectra around the Paschen jump regions at 8250Åare also obtained for four PNe: NGC 7027, NGC 6153, M 1-42 andNGC 7009. Electron densities derived from spectra around the Paschenjump regions are in good agreement with the corresponding values derivedfrom spectra around the Balmer jump, whereas temperatures deduced fromthe spectra around the Paschen jump are found to be lower than thecorresponding values derived from spectra around the Balmer jump for allthe four cases. The reason remains unclear.
| 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}
| Comparative Absorption and Emission Abundance Analyses of Nebulae: Ion Emission Densities for IC 418
Recent analyses of nebular spectra have resulted in discrepantabundances from CNO forbidden and recombination lines. We considerindependent methods of determining ion abundances for emission nebulae,comparing ion emission measures with column densities derived fromresonance absorption lines viewed against the central star continuum.Separate analyses of the nebular emission lines and the stellar UVabsorption lines yield independent abundances for ions, and their ratiocan be expressed in terms of a parameterem, the ``emission density'' for eachion. Adequate data for this technique are still scarce, but separateanalyses of spectra of the planetary nebula and central star of IC 418do show discrepant abundances for several ions, especially Fe II. Thediscrepancies are probably due to the presence of absorbing gas thatdoes not emit and/or to uncertain atomic data and excitation processes,and they demonstrate the importance of applying the technique ofcombining emission- and absorption-line data in deriving abundances fornebulae.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the STScI, which is operated by AURA, Inc., under NASAcontract NAS 5-26555, and on observations made at CTIO/NOAO, which isoperated by AURA, Inc., under cooperative agreement with the NSF.
| 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
| Gravity distances of planetary nebulae II. Aplication to a sample of galactic objects.
Not Available
| 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.
| Spectral trends in planetary nebulae: The roles of radiative and shock excitation
We have investigated de-reddened spectral line ratios for some 538planetary nebulae. As a result, it has proved possible to definecomparative variations between differing transitions, evaluate theviability of radiative modeling for the generality of nebulae, andassess the importance of shocks in modifying low- excitation linestrengths. Whilst most transitions are well represented in terms ofradiative excitation, the [OI] lambda 6300 Angstroms line appears to beappreciably too strong in most of the present sample; a deviation whichmay arise through shock interaction between the primary outflow shelland enveloping superwind material. Comparison between shock modeling andline excesses also suggests that an appreciable proportion of [SII]lambda 6716/31 Angstroms emission may arise through shock excitation; aconclusion which, if confirmed, may have serious consequences fornebular density estimations. Some 14 nebulae are identified as likelyshock candidates, whilst it is proposed that the majority of bipolarnebulae may also show spectral deviations associated with shockexcitation. Line excesses for these latter sources are most consistentwith shock velocities V_s ~ 80 => 100 km s(-1) ; values which arealso comparable to observed wind velocities. Finally, sources containingFLIERs (\cite[Balick et al. 1993]{ba93}) are shown to be confined tohighly specific spectral regimes; a result which permits us to identifythree further possible FLIER sources, and propose characteristic lineratio diagnostics for the further discovery of such features.
| A self-consistent determination of distances, physical parameters, and chemical composition for a large sample of galactic planetary nebulae: chemical composition
The relative abundances of He, C, N, O, Ne, Mg, Si, S, and Ar arepresented for, respectively, 185, 65, 212, 221, 180, 13, 41, 197, and205 Galactic planetary nebulae. The observed stages of ionization weretaken into account using the relations between the relative abundancesof different ions derived from a grid of photoionization models for thenebular emission. The chemical compositions of all the planetary nebulaewere determined using the same method and the same atomic data, so thatthe results have a high degree of uniformity; this is the first timethis has been done for such a large sample of Galactic planetary nebulae(221 objects).
| Inhomogeneous planetary nebulae: carbon and oxygen abundances
We reconsider the problem of the difference between the abundances ofcarbon and oxygen in galactic planetary nebulae (PN) derived from theintensities of the recombination and collisionally excited lines. Thisdiscrepancy can be explained by an inhomogeneity of the PNe and anoverestimation of the weak line intensities. The formulae forcalculation of the nebular line intensities in presence of bothtemperature and density fluctuations are given. The intensities of theforbidden [OIII] lines, the CII, CIII and CIV recombination lines andthe CIII]lambda 1909 UV intercombination doublet for different values ofthe mean electron temperature T_0 in PNe and the rms temperaturevariation t^2, are calculated. Results of these calculations are used tofind the values of T_0 and t^2 which allow to provide the best fit ofthe observed and calculated line intensities (taking into account theobservational errors). In most cases, the obtained values of T_0 appearto be significantly smaller than ordinarily used for the abundancedeterminations T_e([OIII]), while t^2<0.16. The carbon and oxygenabundances for more than 70 PNe are calculated. For these PNe averagechemical abundances are evaluated separately for nebulae of type I, IIand III. For the first, we found C/H=6.67*10(-4) cm(-3) andO/H=5.74*10(-4) 4cm(-3) . For the second they are C/H=8.94*10(-4) cm(-3)and O/H=6.36*10(-4) cm(-3) . For the third we obtained C/H=3.94*10(-4)cm(-3) and O/H=4.79*10(-4) cm(-3) . Results of the fitting of the lineintensities for the NIII lambda 4640 and NIV] lambda 1486 lines are alsogiven.
| A self-consistent determination of the distances, physical parameters, and chemical composition for a large sample of galactic planetary nebulae: The distances and parameters of central stars and the optical depths of envelopes
The distances and parameters of the central stars and the optical depthsof the envelopes in the Lyman limits of neutral hydrogen and neutralhelium were determined in a self-consistent way for 170 Galacticplanetary nebulae (PNe). The distance to each PN was so chosen that thetheoretically calculated evolutionary age of its nucleus was equal tothe dynamical age of its expanding envelope. The effective temperatureof the central star and its related parameters were determined either bythe generalized energy-balance method or, where appropriate, byZanstra's method. The derived distance estimates lend support to a`long' distance scale for PNe and are generally in agreement withcurrent individual and statistical estimates of the distances to PNeavailable in the literature. The mean distance to the bulge PNe is 7.9+/- 0.3 kpc, in agreement with the distance to the Galactic center. Themasses of the central stars of PNe corresponding to the deriveddistances are closely correlated with the nebular nitrogen-to-oxygenabundance ratio.
| On the mass distribution of planetary nebulae central stars.
We apply a method, described in Gorny et al. (1997A&A...318..256G),to derive the masses of 125 central stars of planetary nebulae (PN).This method is self-consistent and distance-independent. It requires theknowledge of the nebular Hβ fluxes, angular radii and expansionvelocities, as well as the stellar visual magnitudes. This method isbased on a simple model for the evolution of planetary nebulae, in whichthe central stars evolve according to the theoretical models of Bloecker(1995A&A...299..755B) and Schoenberner (1983ApJ...272..708S). Theresults are dependent on the assumed total nebular mass. Nevertheless,for any reasonable total nebular mass distribution, we find that therange in planetary nebulae central star masses is very restricted: morethan 80% of the objects have a central star mass between 0.55 and0.65Msun_. We show how to convert, in this mass range, theobserved PN central star mass distribution into a zero-age post-AGB starmass distribution.
| The molecular envelopes of planetary nebulae.
We report the results of a survey of millimeter CO emission in 91planetary nebulae using the IRAM 30 m and SEST 15 m telescopes. Theobservations provide new detections or improved data for 23 nebulae inthe CO(2-1) and/or CO(1-0) line, and sensitive limits for those not seenin CO. Analysis of the results together with previous observationsconfirms the existence of an important class of planetary nebulae withmassive (10^-2^-a few M_sun__) envelopes of molecular gas.These nebulae typically have abundance ratios of N/O>0.3 and bipolarmorphologies indicative of a young disk population. The column densitythrough the envelopes and their mass relative to the mass of ionized gasshow dramatic decreases with increasing nebular size, documenting theexpansion of the envelopes and the growth of the optical nebulae at theexpense of the molecular gas. The molecular envelopes remain a majormass component in these objects until the nebulae reach a radius ofR=~0.1pc. The nebulae not detected in CO have little or no molecular gas(<~10^-2^-10^-3^Msun_), and their envelopes must berapidly photo-dissociated before or during the compact phase. The largedifferences in the molecular gas content of the nebulae highlight thedifferent evolutionary paths for planetary nebula formation which resultfrom the range in mass of the progenitors and the structure of theircircumstellar envelopes.
| [N II] and [O III] Mean Electron Temperatures in Planetary Nebulae
Mean electron tempertures for 106 planetary nebulae are presented, whichhave been derived using calculations of the values of electrontemperature-sensitive line ratios involving forbidden transitions amongthe 2s^2 2p^2 3P, 1D, and 1S levels of N+ and O++, based on new electronimpact rates and transition probabilities. Comparison of these resultswith values of T_e[N II] and T_e[O III] determined previously by Kalerreveal that the present electron temperatures are systematically lowerfor both ions, and that this discrepancy is correlated with the electrondensity in the nebula. It is also shown that the average differencetween T_e[N II] and T_e[O III] in a planetary nebula is somewhat smallerthan that derived by Kaler, with the present results implying that the NII and O III temperatures disagree on average by 2070 K as opposed tothe 2210 K average found by Kaler. (SECTION: Interstellar Medium andNebulae)
| Classification of planetary nebulae by cluster analysis and artificial neural networks.
According to the chemical composition, a sample of 192 Planetary Nebulaeof different types has been re-classified, and 41 others have beenclassified for the first time, by means of two methods not employed sofar in this field: hierarchical cluster analysis and supervisedartificial neural network. The cluster analysis reveals itself as a goodfirst guess for grouping Planetary Nebulae, while an artificial neuralnetwork provides reliable automated classification of this kind ofobjects.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| 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.
| Abundances, planetary nebulae, and stellar evolution
Among Henry Norris Russell's many achievements were his contributions tosolar and stellar spectroscopy, in particular, to an analysis of thechemical composition of the solar atmosphere. The question ofcomposition differences between stars was hotly debated; somedistinguished astronomers argued that all stars had the solarcomposition. Some early challenges to this doctrine are described.Determinations of chemical compositions of gaseous nebulae were muchmore difficult. If we observe the lines of a given chemical element inone ionization stage in a stellar spectrum, we can deduce readily theabundance of that element. No such luxury is available for a planetaryor diffuse gaseous nebula. We must measure lines of as many ionizationstages as we can. Furthermore, a nebula is an extended object. Oftendetailed spectroscopy is at hand only for narrow pencil columns takenthrough the image. Different observers use a variety of apertures.Fortunately it is possible to calculate theoretical spectra for anyarbitrary cross section taken through a symmetrical model, so UV,optical, and IR observations all can be compared properly with aprediction. The value of high-resolution spectra obtained withinstruments such as the Hamilton Echelle Spectrograph at LickObservatory is emphasized. Improved fluxes for weak but importanttransitions are found. Close blends of lines of different ions can beresolved, and checks can be made on predictions of atomic parameterssuch as Einstein A-values and collision strengths. High spectralresolution data have been obtained and reduced for 22 planetary nebulaeof varying size, structure, stellar population membership, dustiness,level of excitation, evolutionary status, and chemical compositions. Thepromise seems justified that with such extensive, high quality data,additional insights on nebular genesis and late states of stellarevolution can be found. The present survey is confined to nebulae ofhigh surface brightness, but among these are found some engaging objectssuch as NGC 7027, IC 4997, and NGC 6572.
| Abundances and radial gradients from disk planetary nebulae: He, N, C, and CL
Chemical abundances of the elements He, N, C, and Cl are presented fordisk planetary nebulae, comprising Peimbert types I, II, and III.Average abundances for these classes are determined and compared withthe remaining abundances available. The presence of abundance gradientsrelative to hydrogen for disk nebulae is investigated in a region ofabout 8 kpc centered in the solar system. It can be concluded that thegradients of the ratios N/H, Cl/H, and probably C/H are similar to theO/H gradient, especially for type II objects.
| 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).
| A near infrared survey of northern planetary nebulae
We have observed 77 evolved planetary nebulae in the J, H, K, L', and Mphotometric bands. As a consequence, we note that most PN fall into arange -0.6 < (J-H)_0_ < 0.2 mag , and 0.0 < (H-K)_0_ <1.2mag , whilst a few sources have also been detected in the range 1.0< (K-L)_0_ < 5.3 mag . Most indices (J-H)_0_ are almost certainlydepressed through the influence of HeI 2^3^ S 2^3^ P emission, whilststellar continua appear to dominate ~ 6 sources, and components of hightemperature dust enhance (J-H)_0_ in several others. Fully 60% ofsources having (J-H)_0_ >=0.0 appear to be type I nebulae. Finally,we note that H_2_S(1) and Q band quadrupole emission may be important inenhancing K band fluxes for several of the nebulae, and the mean value(H-K)_0_ =~ 0.88 mag for H_2_S(1) emission sources is significantly inexcess of the mean. As a consequence, the parameter (H-K)_0_ may prove auseful tool in the prior selection of nebulae having shocked, neutralgas. By contrast, the various L' band detections are likely to derivefrom a mix of mechanisms, including emission from grains having a broadrange of temperatures.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
