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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.
| De-reddening of optical spectra Which extinction curve?
Spectral lines are used to determine a broad range of physicalproperties within HII regions and planetary nebulae (PNe). It istherefore important that we possess accurate intensities for thetransitions, and have the means by which observed results may beaccurately de-reddened. We point out that there are serious differencesbetween the “standard” extinction curves, and that these maylead to errors in line ratios of as high as ˜80%. It is noted thatthe variation in Balmer line ratios in planetary nebulae is mostconsistent with the extinction curves of Whitworth [Whitworth, A.E.,1958. AJ 63, 201] and Ardeberg and Virdefors [Ardeberg, A., Virdefors,B., 1982. A&A 115, 347], and that these are likely to represent themost reliable functions for spectral de-reddening.
| 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.
| Discovery of a Nearby Twin of SN 1987A's Nebula around the Luminous Blue Variable HD 168625: Was Sk -69 202 an LBV?
Spitzer Space Telescope images of the luminous blue variable (LBV)candidate HD 168625 reveal the existence of a bipolar nebula severaltimes larger than its previously known equatorial dust torus. The outernebula of HD 168625 has a full extent of ~80" or 0.85 pc, and one of thelobes has a well-defined polar ring. The nebula is a near twin of thetriple-ring system around SN 1987A. Because of these polar rings, andaccounting for stellar/progenitor luminosity, HD 168625 is an evencloser twin of SN 1987A than the B supergiant Sher 25 in NGC 3603. HD168625's nebula was probably ejected during a giant LBV eruption and notduring a red supergiant phase, so its similarity to the nebula around SN1987A may open new possibilities for the creation of SN 1987A's rings.Namely, the hypothesis that Sk -69 202 suffered an LBV-like eruptionwould avert the complete surrender of single-star models for its bipolarnebula by offering an alternative to an unlikely binary merger scenario.It also hints that LBVs are the likely progenitors of some Type IIsupernovae, and that HD 168625's nebula is a good example of apre-explosion environment.Based on observations made with the Spitzer Space Telescope, which isoperated by the Jet Propulsion Laboratory, California Institute ofTechnology, under NASA contract 1407.
| Compositional dependence of infrared absorption spectra of crystalline silicates. III. Melilite solid solution
The infrared optical properties and absorption spectra of the melilitesolid solution series are reported. Melilite is a high-temperaturecondensate, and is expected to condense during the early phase of thecondensation sequence. In this study, samples of the melilite solidsolution series were newly synthesized between the aluminium end member(gehlenite) and the magnesium end member (åkermanite) at ~10%intervals in chemical composition, and the infrared absorption spectraof their samples were measured. Variation in the absorption featureswere detected: such variations include changes in numbers of absorptionpeaks, peak intensity, peak position, and peak width. The prominentabsorption peaks appeared at 10-13 micron, 14, 15, 17, 19, 21, 24, 30,37 and in the 60 micron region. In particular, the 10 micron featurecomplex and the 60 micron broad features are very sensitive to chemicalcomposition. For application to and implications for astronomical data,we focused on the 60 micron feature, and carried out a comparison withISO data of NGC 6302. Our data set will supply the spectroscopic basisfor the interpretation of astronomical data accumulated by space andground based observatories such as ISO, the Spitzer Space Telescope, theAKARI (ASTRO-F) and the Subaru telescope.
| Silicate absorption in heavily obscured galaxy nuclei
Spectroscopy at 8-13 μm with T-ReCS on Gemini-S is presented forthree galaxies with substantial silicate absorption features, NGC 3094,NGC 7172 and NGC 5506. In the galaxies with the deepest absorptionbands, the silicate profile towards the nuclei is well represented bythe emissivity function derived from the circumstellar emission from thered supergiant, μ Cephei which is also representative of themid-infrared absorption in the diffuse interstellar medium in theGalaxy. There is spectral structure near 11.2 μm in NGC 3094 whichmay be due to a component of crystalline silicates. In NGC 5506, thedepth of the silicate absorption increases from north to south acrossthe nucleus, suggestive of a dusty structure on scales of tens ofparsecs. We discuss the profile of the silicate absorption band towardsgalaxy nuclei and the relationship between the 9.7-μm silicate and3.4-μm hydrocarbon absorption bands.
| 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.
| Mixed Chemistry Phenomenon during Late Stages of Stellar Evolution
We discuss phenomenon of simultaneous presence of O- and C-basedmaterial in the surroundings of evolutionary advanced stars. Weconcentrate on silicate carbon stars and present observations thatdirectly confirm for them the binary model scenario. We discuss also theclass of C-stars with OH emission detected, to which some [WR] planetarynebulae do belong.
| Binary Central Stars of Planetary Nebulae
This paper reviews our knowledge on binary central stars of planetarynebulae and presents some personal opinions regarding their evolution.Three types of interactions are distinguished: type I, where the binarycompanion induces the mass loss; type II, where it shapes the mass lossbut does not enhance it; type III, where a wide orbit causes the centerof mass to move, leading to a spiral embedded in the wind. Surveys forbinary central stars are discussed, and the separations are compared tothe distribution for binary post-AGB stars. The effect of close binaryevolution on nebular morphology is discussed. Post-common-envelopebinaries are surrounded by thin, expanding disks, expelled in theorbital plane. Wider binaries give rise to much thicker expanding torii.Type I binary evolution predicts a wide distribution of masses ofcentral stars, skewed towards low masses. Comparison with observed massdistributions suggests that this is unlikely to be the only channelleading to the formation of a planetary nebula. A new sample of compactBulge nebulae shows about 40 % of nebulae with binary-inducedmorphologies.
| Two new evolved bipolar planetary nebulae in the solar neighbourhood
We present AAO/UKST Hα+[N II] narrow-band imagery and low- andmedium-resolution optical spectroscopy of RCW24 and RCW69. These nebulaewere previously classified as HII regions, but we now show them to betwo of the largest and nearest bipolar Type I PNe yet discovered.Distances were estimated using extinction-distance and kinematicmethods, and via a new Hα surface brightness-radius relation. Theadopted distances are 1.0 +/- 0.3kpc for RCW24 and 1.3 +/- 0.2kpc forRCW69. Both objects have enhanced nitrogen abundances, withlogN/O~=+0.44 for RCW24, and logN/O=+0.33 for RCW 69. Systemicvelocities and |z| distances are VLSR = +5 km s-1and |z| ~ 23pc for RCW 24, and VLSR = -33 km s-1and only |z| ~ 7pc for RCW 69. Both PNe originated from massiveprogenitors (>2.0-2.5Msolar), as deduced from theirchemical abundances, large ionized masses, small |z| distances, lowpeculiar velocities and relatively hot central stars. These two objectsform an important addition to the small sample of evolved bipolar PNe inthe solar neighbourhood.
| An Infrared Imaging Study of the Bipolar Proto-Planetary Nebula IRAS 16594-4656
High-resolution mid-infrared images have been obtained in N and Q bandfor the proto-planetary nebula IRAS 16594-4656. A bright equatorialtorus and a pair of bipolar lobes can clearly be seen in the infraredimages. The torus appears thinner at the center than at the edges,suggesting that it is viewed nearly edge-on. The infrared lobescorrespond to the brightest lobes of the reflection nebula seen in theHubble Space Telescope (HST) optical image, but with no sign of thepoint-symmetric structure seen in the visible image. The lobe structureshows a close correspondence with a molecular hydrogen map obtained withHST, suggesting that the dust emission in the lobes traces thedistribution of the shocked gas. The shape of the bipolar lobes showsclearly that the fast outflow is still confined by the remnantcircumstellar envelope of the progenitor asymptotic giant branch (AGB)star. However, the nondetection of the dust outside of the lobessuggests that the temperature of the dust in the AGB envelope is too lowfor it to be detected at 20 μm.The paper is based on observations obtained at the Gemini Observatory.The Gemini Observatory is operated by the Association of Universitiesfor Research in Astronomy, Inc., under a cooperative agreement with theNational Science Foundation (NSF) on behalf of the Gemini partnership:the NSF (United States), the Particle Physics and Astronomy ResearchCouncil (United Kingdom), the National Research Council (Canada),CONICYT (Chile), the Australian Research Council (Australia), CNPq(Brazil), and CONICET (Argentina).
| 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.
| 3D continuum radiative transfer. A new adaptive grid construction algorithm based on the Monte Carlo method
Aims. The continuum radiative transfer is a common problem inAstrophysics, that must be solved to describe the physics and interpretthe observations of objects embedded (for instance) in a dusty medium(e.g. evolved stars, young stellar objects, ldots). Extreme high angularresolution observations reveal in general complex geometries (e.g.mediums with clumps of matter, warps ldots). It is thus necessary tosolve the radiative transfer problem making no simplifying assumptionson the geometry, i.e. resolve the full spatial three dimensionalproblem. Methods: . We use the flexible Monte Carlo (MC) method tosolve the continuum radiative transfer problem. We focus on thecomputation of the temperature structure of a dusty medium and theconstruction of the grid that samples this structure. We propose to usea natural by-product of the MC method, the locations of the photonpackage absorptions in the medium, to build an adaptive grid to reachthe finest resolution in the regions of interest. Results: . Wepresent the details of the new algorithm used for the construction ofthe grid. We also propose modifications to a widely used MC propagationframework. The reliability of our method is numerically tested againstwell established numerical results in one and two dimensions. In orderto illustrate the capabilities of our three dimensional methods, weexamine the observational evidence for the presence of warps incircumstellar discs.
| 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.
| Observed Planetary Nebulae as Descendants of Interacting Binary Systems
We examine recent studies on the formation rate of planetary nebulae andfind this rate to be about one-third of the formation rate of whitedwarfs. This implies that only about one-third of all planetary nebulaethat evolve to form white dwarfs are actually bright enough to beobserved. This finding corresponds with the claim that it is necessaryfor a binary companion to interact with the asymptotic giant branchstellar progenitor for the descendant planetary nebulae to be brightenough to be detected. The finding about the formation rate alsostrengthens O. De Marco's conjecture that the majority of observedplanetary nebulae harbor binary systems. In other words, single starsalmost never form observed planetary nebulae.
| 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.
| The structure of planetary nebulae: theory vs. practice
Context.This paper is the first in a short series dedicated to thelong-standing astronomical problem of de-projecting the bi-dimensional,apparent morphology of a three-dimensional mass of gas. Aims.Wefocus on the density distribution in real planetary nebulae (and alltypes of expanding nebulae). Methods. We introduce some basictheoretical notions, discuss the observational methodology, and developan accurate procedure for determining the matter radial profile withinthe sharp portion of nebula in the plane of the sky identified by thezero-velocity-pixel-column (zvpc) of high-resolution spectral images.Results. The general and specific applications of the method (andsome caveats) are discussed. Moreover, we present a series of evolutivesnapshots, combining illustrative examples of both model and trueplanetary nebulae. Conclusions. The zvpc radial-densityreconstruction - added to tomography and 3D recovery developed at theAstronomical Observatory of Padua (Italy) - constitutes a very usefultool for looking more closely at the spatio-kinematics, physicalconditions, ionic structure, and evolution of expanding nebulae.
| Morpho-Kinematic Modeling of Gaseous Nebulae with SHAPE
We present a powerful new tool to analyse and disentangle the 3-Dgeometry and kinematic structure of gaseous nebulae. The method consistsin combining commercially available digital animation software tosimulate the 3-D structure and expansion pattern of the nebula with adedicated, purpose-built rendering software that produces the finalimages and long slit spectra which are compared to the real data. Weshow results for the complex planetary nebulae NGC 6369 and Abell 30based on long slit spectra obtained at the San Pedro MártirObservatory.
| Mid-infrared, spatially resolved spectroscopy of the nucleus of the Circinus galaxy
High spatial resolution spectroscopy at 8-13 μm with T-ReCS onGemini-S has revealed striking variations in the mid-infrared emissionand absorption in the nucleus of the Circinus galaxy (hereafterCircinus) on subarcsecond scales. The core of Circinus is compact andobscured by a substantial column of cool silicate dust. Weak extendedemission to the east and west coincides with the coronal line region andarises from featureless dust grains which are probably heated by lineemission in the coronal emission zone. The extended emission on the eastside of the nucleus displays a much deeper silicate absorption than thaton the west, indicating significant columns of cool material along theline of sight and corresponding to an additional extinction ofAV~ 25 mag. Emission bands from aromatic hydrocarbons are notsubject to this additional extinction, are relatively weak in the coreand in the coronal line region, and are much more spatially extendedthan the continuum dust emission; they presumably arise in thecircumnuclear star-forming regions. These data are interpreted in termsof an inclined disc-like structure around the nucleus extending overtens of parsecs and possibly related to the inner disc found fromobservations of water masers by Greenhill et al..
| Quantitative infrared spectra of hydrosilicates and related minerals
Absorption coefficients associated with atomic motions of speciesexpected in astronomical environments are determined from infraredmeasurements of various hydrosilicates, hydrated magnesium oxide, andthe Al-bearing chain silicate, sapphirine. Band types measured includeO-H stretching modes near 3 μm, Si-O stretching motions near 10μm, Si-O-Si bends near 14 μm, O-Si-O bends near 20 μm, andtranslations of cations such as Mg and Ca near 50-200 μm. We obtaindata from films of varying thickness and use a ratioing method. First,bandstrengths of O-H fundamentals were determined from spectra obtainedfrom films of controlled thicknesses, generally 6 μm. The O-Habsorbance strength was then used to accurately determine thickness fora thinner film of each mineral (found to be <1 μm), thus providingbandstrengths of all other absorptions. Thin films were prepared suchthat the fundamental lattice modes showed intrinsic behaviour (i.e. bandshapes were unchanged upon further thinning) and O-H modes are wellresolved above the spectral noise. Bandstrengths were found to dependweakly on structure and should be applicable to other silicate minerals,allowing estimation of elemental concentrations independent of knowingthe speciation of dust in astronomical environments. Comparison withobservational data of NGC 6302 suggests that lizardite and saponitecould be present in addition to refractory minerals.
| Tentative Discovery of a New Supernova Remnant in Cepheus: Unveiling an Elusive Shell in the Spitzer Galactic First Look Survey
We have discovered an axially symmetric, well-defined shell of materialin the constellation of Cepheus, based on imaging acquired as part ofthe Galactic First Look Survey with the Spitzer Space Telescope. The86''×75'' object exhibits brightened limbson the minor axis and is clearly visible at 24 μm, but it is notdetected in the 3.6, 4.5, 5.8, 8.0, 70, or 160 μm images. Follow-up7.5-40 μm spectroscopy reveals that the shell is composed entirely ofionized gas and that the 24 μm imaging traces [O IV] 25.89 μmemission solely. The spectrum also exhibits weaker [Ne III] and [S III]emission, and very weak [Ne V] emission. No emission from warm dust isdetected. Spectral cuts through the center of the shell and at thenorthern limb are highly consistent with each other. The progenitor isnot readily identified, but with scaling arguments and comparison towell-known examples of evolved stellar objects, we find the observationsto be most straightforward to interpret in terms of a young supernovaremnant located at a distance of at least 10 kpc, some 400 pc above theGalactic disk. If confirmed, this would be the first supernova remnantdiscovered initially at infrared wavelengths.
| Chemical abundances in seven galactic planetary nebulae
An observational study of chemical abundances in the galactic planetarynebulae NGC 1535, NGC 2438,NGC 2440, NGC 3132, NGC3242, NGC 6302, and NGC7009 based on long-slit spectra of high signal-to-noise ratioin the 3100 to 6900 Å range is presented. We determined the N, O,Ne, S, and Cl abundances from collisionally excited lines and the He andO++ abundances from recombination lines. TheO++/H+ estimates derived from recombination linesare about a factor of four and two higher than those derived fromforbidden lines for NGC 7009 and NGC3242, respectively. Spatial profiles ofO++/H+ abundance from O II permitted lines andfrom [O III] forbidden lines were obtained for the planetary nebulaNGC 7009. The differences betweenO++/H+ derived from recombination and fromforbidden lines present smooth variations along the nebular surface ofNGC 7009, with the differences decreasing from thecenter to the edges of the nebula. If these abundance differences areexplained by the presence of electron temperature fluctuations,quantified by the parameter t2, a value of aboutt2=0.09 is required for NGC 3242 andNGC 7009.
| Water ice growth around evolved stars. II. Modeling infrared spectra
We present combined radiative transfer and H2O ice formationcalculations for the dusty envelopes of oxygen-rich evolved stars. Westudy the effects of various (circum-)stellar parameters on the spectralenergy distribution of these stars, their infrared spectral water icefeatures at 3 μm and in the 30-100 μm region, and the propertiesof (water ice on) the grains in their envelopes. We also study the iceformation process as a function of stellar evolution for a star with aninitial mass of 5 Mȯ, which is followed during the AGB,post-AGB and planetary nebula (PN) phase. We find that its water icefeatures probe its evolution. Both crystalline and amorphous water iceform in our models. The 43 and 62 μm crystalline water ice featuresare most prominent during the post-AGB phase, and only modestly or notpresent during the AGB and PN phase, in agreement with observations. Thestrength of the 3, 43 and 62 μm water ice features decreases withdecreasing initial mass of the star. The total amount of ice predicted(a few percent of the total dust mass) also agrees with observations,but the crystalline ice mass fraction is consistently about two ordersof magnitude lower. This is mainly due to efficient amorphization byinterstellar UV photons, and leads to weaker 43 and 62 μm crystallinewater ice features than observed. The intensity of the interstellar UVradiation field strongly influences the strength of these features. Wediscuss several means to increase the crystalline water ice mass, andhence their strength. The strength of the features increasesdramatically when the mass-loss rate over luminosity ratio of the star,dot{M}/L, is large in the AGB phase. In case of the post-AGB star HD161796 we demonstrate that this indeed leads to the correct crystallineice mass fraction and feature strengths. Also, the formation of clumpsin the AGB wind provides high densities to stimulate the formation of(crystalline) ice. For stars with high initial masses, it additionallyprovides sufficient shielding from interstellar UV radiation to keep icecrystalline during the post-AGB and PN phase. Axisymmetric mass loss onthe AGB provides favorable conditions for the formation and preservationof water ice, and crystalline water ice in particular, as well. Incontrast we find that post-AGB crystallization of AGB produced amorphousice is unimportant for increasing the crystalline water ice mass around5 Mȯ stars.
| New insights on the complex planetary nebula Hen 2-113
We report on infrared observations of the planetary nebula Hen 2-113obtained with VLT/NACO, VLTI/MIDI, VLT/ISAAC and TIMMI at the ESO 3.6 m.Hen 2-113 exhibits a clear ring-like structure superimposed to a morediffuse environment visible in the L' (3.8 μm), M' (4.78 μm) and8.7 μm bands. No clear core at 8.7 μm and no fringes through the Nband could be detected for this object with MIDI. A qualitativeinterpretation of the object structure is proposed using a diabolo-likegeometrical model. The PAH content of the nebula was also studied withISAAC and TIMMI observations. This indicates that the PAHs are mostlyconcentrated towards the lobes of the diabolo and the bipolar lobes ofthe nebula. In L' band, a void 0.3 arcsec in diameter was discoveredwith NACO around the central source. The L' and M' fluxes from thecentral source were derived from NACO data indicating an importantinfrared excess with respect to the expected stellar emission based onstellar models and short wavelength data. The observed flux from thissource in the L' and M' is about 300 and 800 times respectively thanthose expected from a model including only the central star. Moreover,the central object appears resolved in L' band with measured FWHM of 155mas. This infrared excess can be explained by emission from a cocoon ofhot dust (T1000 K) with a total mass~10-9~Mȯ.
| Raman-scattered He II λλ4850, 6545 in the Young and Compact Planetary Nebula IC 5117
We report the discovery of He II λλ4850 and 6545Raman-scattered by atomic hydrogen in the high-resolution spectrum ofthe very compact and young planetary nebula IC 5117 obtained with theESPaDOnS (Echelle Spectropolarimetric Device for the Observation ofStars) installed on the 3.6 m Canada-France-Hawaii Telescope. Byadopting case B recombination atomic data and considering all thefine-structure emission components, we calculate the weighted averagesfor the line centers of the Raman-scattered features, as well as thosefor the emission lines He II λλ4859 and 6560 to determineredward velocity shifts of Δv4850=+29 km s-1and Δv6545=+24 km s-1. We compute the Ramanconversion efficiency by a Monte Carlo technique adopting a simplescattering geometry, in which the hot central star is surrounded by ahollow cylindrical H I region characterized by thickness T that is inturn parameterized by the H I column density. It is proposed that thecentral far-UV emission region is covered significantly by the neutralscattering region characterized by the H I column densityNHI~=4×1021 cm-2. The H I columndensity is higher by an order of magnitude than the value proposed from21 cm radio observations. We briefly discuss the importance of Ramanscattering as a probe of neutral material around a newly formed hotwhite dwarf and the mass-loss process occurring in the late stage ofstellar evolution.
| Scorpius the Winter-Bug.
Not Available
| Discovery of Multiple Coaxial Rings in the Quadrupolar Planetary Nebula NGC 6881
We report the discovery of multiple two-dimensional rings in thequadrupolar planetary nebula NGC 6881. As many as four pairs of ringsare seen in the bipolar lobes, and three rings are seen in the centraltorus. While the rings in the lobes have the same axis as one pair ofthe bipolar lobes, the inner rings are aligned with the other pair. Thetwo pairs of bipolar lobes are likely to be carved out by two separatehigh-velocity outflows from the circumstellar material left over fromthe asymptotic giant branch (AGB) wind. The two-dimensional rings couldbe the results of dynamical instabilities or the consequence of a fastoutflow interacting with remnants of discrete AGB circumstellar shells.
| Abundances in planetary nebulae: Mz 3
ISO spectra of the bipolar planetary nebula Mz 3 areused to determine the element abundances in the bright lobes of thenebula. The ISO spectra alone are sufficient to determine nitrogen,neon, argon, sulfur and iron abundances. These spectra are combined withspectra in the visual wavelength region (taken from the literature) toobtain an extinction corrected spectrum which is used to determine theabundance of oxygen and some other elements using a classicaldetermination. We have tried abundance determination usingphotoionization modeling using cloudy, which is essential fordetermining the helium, silicon and chlorine abundances. It was founddifficult to model the entire spectrum. New information about thecentral star could be determined. The abundances determined are found todiffer somewhat from earlier results using only visual spectra. Thereasons for this difference are discussed. An elevated helium abundanceis found, agreeing with the determination of Smith 2003. Taken togetherwith the high nitrogen abundance found, it is concluded that theexciting star of Mz 3 had a high progenitor mass.
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