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Optically Thick Radio Cores of Narrow-Waist Bipolar Nebulae
We report our search for optically thick radio cores in 16 narrow-waistbipolar nebulae. Optically thick cores are a characteristic signature ofcollimated ionized winds. Eleven northern nebulae were observed with theVery Large Array (VLA) at 1.3 and 0.7 cm, and five southern nebulae wereobserved with the Australia Telescope Compact Array (ATCA) at 6 and 3.6cm. Two northern objects, 19W32 and M1-91, and three southern objects,He 2-25, He 2-84, and Mz 3, were found to exhibit a compact radio corewith a rising spectrum consistent with an ionized jet. Such jets havebeen seen in M2-9 and may be responsible for shaping bipolar structurein planetary nebulae.
| Density gradients in Galactic planetary nebulae
Certain hydrodynamic models of planetary nebulae (PNe) suggest thattheir shells possess appreciable radial density gradients. However, theobservational evidence for such gradients is far from clear. On the onehand, Taylor et al. claim to find evidence for radio spectral indices0.6 < α < 1.8, a trend which is taken to imply a variationne ~ r-2 in most of their sample of PNe. On theother hand, Siódmiak & Tylenda find no evidence for any suchvariations in density; shell inhomogeneities, where they occur, areprimarily attributable to `blobs or condensations'.It will be suggested that both of these analyses are unreliable, andshould be treated with a considerable degree of caution. A new analysiswithin the log(F(5GHz)/F(1.4GHz))-log(TB(5GHz)) plane will beused to show that at least 10-20 per cent of PNe are associated withstrong density gradients. We shall also show that the ratioF(5GHz)/F(1.4GHz) varies with nebular radius; an evolution that can beinterpreted in terms of varying shell masses, and declining electrondensities.
| Magnetic fields in planetary nebulae and post-AGB nebulae
Magnetic fields are an important but largely unknown ingredient ofplanetary nebulae. They have been detected in oxygen-rich asymptoticgiant branch (AGB) and post-AGB stars, and may play a role in theshaping of their nebulae. Here we present SCUBA submillimetrepolarimetric observations of four bipolar planetary nebulae and post-AGBstars, including two oxygen-rich and two carbon-rich nebulae, todetermine the geometry of the magnetic field by dust alignment. Three ofthe four sources (NGC 7027, 6537 and 6302) present a well-definedtoroidal magnetic field oriented along their equatorial torus or disc.NGC 6302 may also show field lines along the bipolar outflow. CRL 2688shows a complex field structure, where part of the field aligns with thetorus, whilst an other part approximately aligns with the polar outflow.It also presents marked asymmetries in its magnetic structure. NGC 7027shows evidence for a disorganized field in the south-west corner, wherethe SCUBA shows an indication for an outflow. The findings show a clearcorrelation between field orientation and nebular structure.
| Proof of polar ejection from the close-binary core of the planetary nebula Abell 63
We present the first detailed kinematical analysis of the planetarynebula Abell 63, which is known to contain the eclipsing close-binarynucleus UU Sge. Abell 63 provides an important test case ininvestigating the role of close-binary central stars on the evolution ofplanetary nebulae.Longslit observations were obtained using the Manchester echellespectrometer combined with the 2.1-m San Pedro Martir Telescope. Thespectra reveal that the central bright rim of Abell 63 has a tube-likestructure. A deep image shows collimated lobes extending from thenebula, which are shown to be high-velocity outflows. The kinematic agesof the nebular rim and the extended lobes are calculated to be 8400 +/-500 and 12900 +/- 2800 yr, respectively, which suggests that the lobeswere formed at an earlier stage than the nebular rim. This is consistentwith expectations that disc-generated jets form immediately after thecommon envelope phase.A morphological-kinematical model of the central nebula is presented andthe best-fitting model is found to have the same inclination as theorbital plane of the central binary system; this is the first proof thata close-binary system directly affects the shaping of its nebula. AHubble-type flow is well-established in the morphological-kinematicalmodelling of the observed line profiles and imagery.Two possible formation models for the elongated lobes of Abell 63 areconsidered, (i) a low-density, pressure-driven jet excavates a cavity inthe remnant asymptotic giant branch (AGB) envelope; (ii) high-densitybullets form the lobes in a single ballistic ejection event.
| Time variation of radial gradients in the Galactic disk: electron temperatures and abundances
Aims.We investigate the electron temperature gradient in the galacticdisk as measured by young HII regions on the basis of radiorecombination lines and the corresponding gradient in planetary nebulae(PN) based on [OIII] electron temperatures. The main goal is toinvestigate the time evolution of the electron temperature gradient andof the radial abundance gradient, which is essentially a mirror image ofthe temperature gradient. Methods: The recently derived electrontemperature gradient from radio recombination lines in HII regions iscompared with a new determination of the corresponding gradient fromplanetary nebulae for which the progenitor star ages have beendetermined. Results: The newly derived electron temperature gradientfor PN with progenitor stars with ages in the 4-5 Gyr range is muchsteeper than the corresponding gradient for HII regions. These electrontemperature gradients are converted into O/H gradients in order to makecomparisons with previous estimates of the flattening rate of theabundance gradient. Conclusions: .It is concluded that the O/H gradienthas flattened out in the past 5 Gyr at a rate of about 0.0094 dexkpc-1 Gyr-1, in good agreement with our previousestimates.
| Abundances in planetary nebulae: Hb 5
The ISO spectra of the bilobal planetary nebula Hb 5are presented. These spectra are combined with the spectra in the visualwavelength region to obtain a complete, extinction corrected, spectrum.The chemical composition of the nebula is then calculated in severalways. First by directly calculating and adding individual ionabundances, assuming that all the ionic lines are formed in an ionizedregion surrounding the ionizing star. Secondly by building an"end-to-end model" nebula in which we have included a neutral region anda photodissociation region (PDR) beyond the ionized nebula. In this waywe attempt to interpret the molecular hydrogen lines observed by ISO ina more self-consistent way. In the final analysis, the model is found tobe basically heuristic, but gives new insights about the PDR and the PN.The implications of these are discussed.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France,Germany, the Netherlands and the UK) and with theparticipation of ISAS and NASA.
| 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.
| IC 5217 as a double-shell, point-symmetric planetary nebula with a very narrow waist
Aims.Identification of the structural components and analysis of theinternal kinematics in the planetary nebula IC 5217. Methods:.Narrow-band images and high resolution long-slit spectra in theHα, [N ii] and [O iii] emission lines, and VLA 6 cm continuumdata. Results: .IC 5217 is composed of a very bright equatorialring, open bipolar lobes, off-axis point-symmetric features, on-axisdistant faint regions, and an off-axis very elongated bipolar structure.The ring, open lobes, point-symmetric features and distant faint regionsappear as elements of a single point-symmetric bipolar shell with a verynarrow waist. This shell presents an axis ratio of ~37 and an aspectratio of ~5. The 6 cm data show that the ring is an extremely flat diskwith a central hole. Expansion velocity in the ring ranges from ≤10km s-1 in He ii up to ≃27 km s-1 in [N ii],whereas a velocity of ~460 km s-1 is estimated for the polarregions of the bipolar shell. Strong acceleration of the outer regionsof the ring is observed. The elongated bipolar structure probablyrepresents a highly collimated (aspect ratio ~12), high velocitycylindrical-like shell. A collimated agent (wind or jet) would accountfor the shaping of the bipolar shell if this agent has operated in thedirection perpendicular to the equatorial disk. The point-symmetricfeatures and cylindrical shell are probably related to collimatedejections but that occurred when the basic nebular shape had alreadybeen established.
| Low-temperature single crystal reflection spectra of forsterite
The infrared reflectivities of crystalline forsterite(Mg2SiO4) were measured for the temperature range295-50 K for each crystal axis, between wavenumber 5000 and 100cm-1. The reflection spectra show clear dependence oftemperature; most of the bands become more intense, sharper and theirpeak positions shift to higher wavenumber with decreasing temperature.Reflection spectra were fitted with dispersion formula of dampedoscillator model of the dielectric constants and the oscillatorparameters in the model were derived. The absorption spectra offorsterite particle are calculated with the derived dielectric constantsto show that the forsterite features are good thermal indicator for coldtemperature range below 295 K.
| A SCUBA imaging survey of ultracompact HII regions. The environments of massive star formation
We present a SCUBA submillimetre (450 and 850 μm) survey of theenvironment of 105 IRAS point sources, selected from the Wood &Churchwell (1989a) and Kurtz et al. (1994) radio ultracompact (UC) Hiiregion surveys. We detected a total of 155 sub-mm clumps associated withthe IRAS point sources and identified three distinct types of object:ultracompact cm-wave sources that are not associated with any sub-mmemission (sub-mm quiet objects), sub-mm clumps that are associated withultracompact cm-wave sources (radio-loud clumps); and sub-mm clumps thatare not associated with any known ultracompact cm-wave sources(radio-quiet clumps). 90% of the sample of IRAS point sources were foundto be associated with strong sub-mm emission. We consider the sub-mmcolours, morphologies and distance-scaled fluxes of the sample of sub-mmclumps and show that the sub-mm quiet objects are unlikely to representembedded UC Hii regions unless they are located at large heliocentricdistances. Many of the 2.5 arcmin SCUBA fields contain more than onesub-mm clump, with an average number of companions (the companion clumpfraction) of 0.90. The clumps are more strongly clustered than othercandidate HMPOs and the mean clump surface density exhibits a brokenpower-law distribution with a break at 3 pc. We demonstrate that thesub-mm and cm-wave fluxes of the majority of radio-loud clumps are inexcellent agreement with the standard model of ultracompact Hii regions.We speculate on the nature of the radio-quiet sub-mm clumps and, whilstwe do not yet have sufficient data to conclude that they are in a pre-UCHii region phase, we argue that their characteristics are suggestive ofsuch a stage.
| Further 2MASS mapping of hot dust in planetary nebulae
We have used 2 Micron All Sky Survey (2MASS) mapping results toinvestigate the distribution of hot dust continua in 12 planetarynebulae (PNe). The nature of this emission is unclear, but it ispossible that where the continuum is extended, as is the case for M 1-12and NGC 40, then the grains concerned may be very small indeed. Theabsorption of individual photons by such grains may lead to sharp spikesin temperature, as has previously discussed for several other suchoutflows. Other sources (such as MaC 1-4, He 2-25, B1 2-1 and K 3-15)appear to be relatively compact, and the high temperatures observed areunderstandable in terms of more normal heating processes. It is possiblethat the grains in these cases are experiencing high radiant fluxlevels.Finally, it is noted that whilst the core of M 2-2 appears to show hotgrain emission, this is less the case for its more extended envelope.The situation may, in this case, be similar to that of NGC 2346, inwhich much of the emission is located within an unresolved nucleus.Similarly, it is noted that in addition to hot dust and gas thermalcontinua, the emission in the interior of NGC 40 may be enhanced throughrotational-vibrational transitions of H2, and/or the2p3P0-2s3S transition of HeI.
| Accreting White Dwarfs among the Planetary Nebulae Most Luminous in [O III] λ5007 Emission
I propose that some of the most luminous planetary nebulae (PNs) areactually proto-PNs, where a companion white dwarf (WD) accretes mass ata relatively high rate from the post-asymptotic giant branch star thatblew the nebula. The WD sustains a continuous nuclear burning andionizes the nebula. The WD is luminous enough to make the dense nebulaluminous in the [O III] λ5007 line. In young stellar populationsthese WD accreting systems account for a small fraction of [OIII]-luminous PNs, but in old stellar populations these binaries mightaccount for most, or even all, of the [O III]-luminous PNs. This mightexplain the puzzling constant cutoff (maximum) [O III] λ5007luminosity of the planetary nebula luminosity function across differentgalaxy types.
| Temperature effects on the mid-and far-infrared spectra of olivine particles
The absorption spectra of the olivine particles of different Mg/Fecontent were measured in the infrared spectral region between 5 and 100μm, while the particles were continuously cooled down to 10 K.Measurements independently carried out on different samples of syntheticforsterite, natural olivine, and synthetic fayalite at laboratories inKyoto and Jena. The positions of the olivine infrared bands weremeasured for these samples in detail at up to seven individualtemperatures in the interval between 300 K and 10 K. According to thedifferent widths of the olivine bands in different wavelength regions,spectral resolutions of 2, 1, 0.5, 0.25, 0.2, and 0.125 cm-1were used in order to measure the band positions with high accuracy.While in general the band positions and their temperature-dependentshift agree very well for the Kyoto and Jena samples, the positions ofsome very strong bands differ, which is probably a consequence ofdifferent particle shapes. For the two long-wavelength forsterite bandsat 49 and 69 μm, the sharpening and strengthening of the bands werequantified. The widths of these bands differ for the Kyoto and Jenasamples, which is discussed in terms of different crystal quality andparticle coagulation of the samples. Our new data can be used to derivedust temperatures from the observed peak positions for crystallinesilicate dust in circumstellar regions.
| The symmetric dust shell and the central star of the bipolar planetary nebula NGC6537*†
We present high-resolution images of the strongly bipolar planetarynebula NGC6537, obtained with the Hubble Space Telescope (HST) and withthe infrared adaptive optics system on the Very Large Telescope. Thecentral star is detected for the first time. Using the multibandphotometry and constraints from the dynamical age of the nebula, wederive a temperature in the range 1.5-2.5 × 105 K, aluminosity ~103Lsolar and a core massMc~ 0.7-0.9Msolar. The progenitor mass is probablyin the range Mi= 3-7Msolar. The extinction mapshows a largely symmetric, and compact dust structure, which is mostlikely a shell, located at the neck of the bipolar flow, only 2-4arcsecfrom the star. The dust shell traces a short-lived phase of very highmass loss at the end of the asymptotic giant branch (AGB). The dynamicalage of the shell and bipolar lobes are very similar but the morphologiesare very different. The data suggests that the mass loss during theejection of the compact shell was largely spherically symmetric, and thepronounced bipolarity formed afterwards. The dynamical ages of thebipolar lobes and dust shell are similar, which is consistent withsuggestions that bipolar structures form in a run-away event at the verylast stages of the AGB mass loss. The inner edge of the dust shell isionized, and polycyclic aromatic hydrocarbon (PAH) emission is seen justoutside the ionized gas. We associate the PAH emission with thephotodissociation region of the molecular shell.
| 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.
| Filaments as Possible Signatures of Magnetic Field Structure in Planetary Nebulae
We draw attention to the extreme filamentary structures seen inhigh-resolution optical images of certain planetary nebulae. Wedetermine the physical properties of the filaments in the nebulae IC418, NGC 3132, and NGC 6537, and based on their large length-to-widthratios, longitudinal coherence, and morphology, we suggest that they maybe signatures of the underlying magnetic field. The fields needed forthe coherence of the filaments are probably consistent with thosemeasured in the precursor circumstellar envelopes. The filaments suggestthat magnetic fields in planetary nebulae may have a localized andthreadlike geometry.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Institute. STScIis operated by the association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| 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.
| A general catalogue of 6.7-GHz methanol masers. I. Data.
Methanol masers are often detected in regions of intense star formation.Several studies in the last decade indicate that they may even be theearliest signpost of a high-mass star-forming region. Their powerfulemission make them very good candidates for observations using bothsingle-dish telescopes and interferometers, the latter allows detailedstructural and dynamical studies of these objects. We have prepared acatalogue of all known 6.7-GHz methanol masers, discovered both bysurveys that targeted possible associated objects and unbiased surveyscovering a large fraction of galactic longitudes across the Galacticplane (-0.5 ° ≤ b ≤ 0.5 ° for most of the regions). Thecatalogue contains 519 sources which are listed with their kinematic(galactocentric and heliocentric) distances as well as possiblyassociated IR objects. We find that 6.7-GHz methanol masers clearlytrace the molecular ring of our Galaxy, where most of the OBassociations are located. The present list of masers also reportsdetections of other masing transitions of methanol as furtherinformation for the study of the maser phenomenon. In a furtherpublication we will address some statistical considerations.The complete catalogue (Table [see full text]) is available inelectronic form at the 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/432/737 Figures 1 and 2are only available in electronic form at http://www.edpsciences.org
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/565
| Mineral formation in stellar winds. V. Formation of calcium carbonate
An emission band around 92 μm found in a few IR spectra from highlyevolved stars was proposed to be due to the presence of carbonate dustgrains in the circumstellar material (Kemper et al. \cite{Kem02a},Nature, 415, 295). This contribution presents the results of a modelcalculation for the condensation of calcite (CaCO_3) in the stellar windof AGB stars. It is shown that the quantities of carbonate dust formedrelative to the quantities of silicate dust are negligibly small. Thisresults from the fact that carbonates form at a much lower temperaturethan the silicate dust components. Carbonate dust formation then issuppressed by the strong acceleration of the wind material by radiationpressure on the silicate dust and the subsequent rapid dilution of thewind material. This makes it highly improbable that carbonate dust canbe formed in stellar outflows.
| 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]).
| K 4-47: a planetary nebula excited by photons and shocks
K 4-47 is an unusual planetary nebula (PN) composed of a compacthigh-ionization core and a pair of low-ionization knots. Long-slitmedium-resolution spectra of the knots and core are analyzed in thispaper. Assuming photoionization from the central star, we have derivedphysical parameters for all the nebular components, and the ionizationcorrection factors (icf) chemical abundances of the core, which appearsimilar to Type I PNe for He and N/O but significantly deficient inoxygen. The nebula has been further modelled using both photoionization(CLOUDY) and shock (MAPPINGS) codes. From the photoionization modellingof the core, we find that both the strong auroral [OIII] 4363-Åand[NII] 5755-Åemission lines observed and the optical size of thecore cannot be accounted for if a homogeneous density is adopted. Wesuggest that a strong density stratification, matching the high-densitycore detected at radio wavelengths and the much lower density of theoptical core, might solve the problem. From the bow-shock modelling ofthe knots, on the other hand, we find that the chemistry of the knots isalso represented by Type I PN abundances, and that they would move withvelocities of 250-300kms-1.
| 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.
| Spitzer Infrared Spectrograph (IRS) Observations of Large Magellanic Cloud Planetary Nebula SMP 83
The first observations of the infrared spectrum of the LMC planetarynebula SMP 83 as observed by the recently launched Spitzer SpaceTelescope are presented. The high-resolution (R~600) spectrum showsstrong emission lines but no significant continuum. The infraredfine-structure lines are used, together with published optical spectra,to derive the electron temperature of the ionized gas for several ions.A correlation between the electron temperature and the ionizationpotential is found. Ionic abundances for the observed infrared ions havebeen derived, and the total neon and sulfur abundances have beendetermined. These abundances are compared to average LMC abundances of HII regions in order to better understand the chemical evolution of theseelements. The nature of the progenitor star is also discussed.
| 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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Pairs of Bubbles in Planetary Nebulae and Clusters of Galaxies
I point to an interesting similarity in the morphology and somenondimensional quantities between pairs of X-ray-deficient bubbles inclusters of galaxies and pairs of optical-deficient bubbles in planetarynebulae (PNs). This similarity leads me to postulate a similar formationmechanism. This postulate is used to strengthen models for PN shaping byjets (or collimated fast winds [CFWs]). The presence of dense materialin the equatorial plane observed in the two classes of bubblesconstrains the jets and CFW activity in PNs to occur while theasymptotic giant branch star still blows its dense wind, or very shortlyafter. I argue that only a stellar companion can account for such jetsand CFWs.
| Observations of warm dust near methanol masers
Continuum emission at 450 and 850 mu m from warm dust has been mapped inthe fields of 71 methanol masers. Within these fields lie 30centimetre-wave radio continuum sources and an additional 13 methanolmaser sites. Sub-mm emission is detected at all but one of the masersites, confirming the association of methanol maser emission with deeplyembedded objects. Measured bolometric luminosities confirm that methanolmaser emission is an excellent signpost of high-mass star formation.Examples of nearby isolated maserless dust cores may be harbouringmassive protostars at an earlier evolutionary stage.Full version of Fig. 1 is only available in electronic form athttp://www.edpsciences.org
| 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.
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