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Nonblind Catalog of Extragalactic Point Sources from the Wilkinson Microwave Anisotropy Probe (WMAP) First 3 Year Survey Data
We have used the MHW2 filter to obtain estimates of the flux densitiesat the WMAP frequencies of a complete sample of 2491 sources, mostlybrighter than 500 mJy at 5 GHz, distributed over the whole sky excludinga strip around the Galactic equator (|b|<=5deg). Afterhaving detected 933 sources at the >=3 σ level in the MHW2filtered maps-our New Extragalactic WMAP Point Source Catalogue(NEWPS3σ)-we are left with 381 sources at >=5σ in at least one WMAP channel, 369 of which constitute ourNEWPS5σ catalog. It is remarkable to note that 98(i.e., 26%) sources detected at >=5 σ are ``new,'' they are notpresent in the WMAP catalog. Source fluxes have been corrected for theEddington bias. Our flux density estimates before such correction aregenerally in good agreement with the WMAP ones at 23 GHz. At higherfrequencies WMAP fluxes tend to be slightly higher than ours, probablybecause WMAP estimates neglect the deviations of the point spreadfunction from a Gaussian shape. On the whole, above the estimatedcompleteness limit of 1.1 Jy at 23 GHz we detected 43 sources missed bythe blind method adopted by the WMAP team. On the other hand, ourlow-frequency selection threshold left out 25 WMAP sources, only 12 ofwhich, however, are >=5 σ detections and only three haveS23GHz>=1.1 Jy. Thus, our approach proved to becompetitive with and complementary to the WMAP one.
| White dwarf masses derived from planetary nebula modelling
Aims.We compare the mass distribution of central stars of planetarynebulae (CSPNe) with those of their progeny, white dwarfs (WD). Methods: We use a dynamical method to measure masses with an uncertaintyof 0.02 M_ȯ. Results: The CSPN mass distribution is sharplypeaked at 0.61~M_ȯ. The WD distribution peaks at lower masses(0.58~M_ȯ) and shows a much broader range of masses. Some of thedifference can be explained if the early post-AGB evolution is fasterthan predicted by the Blöcker tracks. Between 30 and 50 per cent ofWD may avoid the PN phase because they have too low a mass. However, thediscrepancy cannot be fully resolved and WD mass distributions may havebeen broadened by observational or model uncertainties.Data is only available in electronic form at http://www.aanda.org
| A search for OB stars in the field of the galactic OB association Bochum 7. I. UBV-IR photometry and optical spectroscopy
Context: The presence of a Wolf-Rayet star, WR12, at α=8^h44^m47.2^s, δ = - 45°58 arcmin55.5 arcsec (J2000.0),triggered the search for a surrounding OB association now known asBochum 7. Several papers have dealt with the existence of Bo 7, but noneactually included spectral types of the stars. To assure the spectraltypes of members of the association, it is essential to have spectralimages of them. Aims: In this paper we present previously unpublishedspectra of OB stars over a 30 arcmin field covering the estimated fieldof the galactic OB association. Methods: From UBV aperture photometry,OB star candidates were selected, and optical spectroscopic observationswere obtained for these in CASLEO, Argentina. Results: 63 stars wereclassified as OB type, including 8 stars already present in the LSScatalogue, and 37 of these 63 stars did not have a well-known spectraltype. The OB stars were used to test the properties of the interstellardust in the line of sight and the relation between the diffuseinterestelar band at 4428 Å, which grows linearly withE(B-V) until 1 mag. Combining our data with 2MASS IRphotometry we tested and confirmed that derivation of stellar parametersfrom our photometry, using the CHORIZOS code, is highly reliable.Figures 6-15 are only available in electronic form athttp://www.aanda.org
| An Atlas of [N II] and [O III] Images and Spectra of Planetary Nebulae
We present an atlas of Hubble Space Telescope images and ground-based,long-slit, narrowband spectra centered on the 6584 Å line of [NII] and the 5007 Å line of [O III]. The spectra were obtained fora variety of slit positions across each target (as shown on the images)in an effort to account for nonspherical nebular geometries in a robustmanner. We have extended the prolate ellipsoidal shell model originallydevised by Aaquist, Zhang, and Kwok to generate synthetic images, aswell as long-slit spectra. Using this model, we have derived basicparameters for the subsample of PNe that present ellipsoidal appearancesand regular kinematic patterns. We find differences between ourparameters for the target PNe as compared to those of previous studies,which we attribute to increased spatial resolution for our image dataand the inclusion of kinematic data in the model fits. The data andanalysis presented in this paper can be combined with detections ofnebular angular expansion rates to determine precise distances to the PNtargets.
| s-Process Abundances in Planetary Nebulae
The s-process should occur in all but the lower mass progenitor stars ofplanetary nebulae, and this should be reflected in the chemicalcomposition of the gas that is expelled to create the current planetarynebula shell. Weak forbidden emission lines are expected from severals-process elements in these shells and have been searched for and insome cases detected in previous investigations. Here we extend thesestudies by combining very high signal-to-noise ratio echelle spectra ofa sample of PNe with a critical analysis of the identification of theemission lines of Z>30 ions. Emission lines of Br, Kr, Xe, Rb, Ba,and Pb are detected with a reasonable degree of certainty in at leastsome of the objects studied here, and we also tentatively identify linesfrom Te and I, each in one object. The strengths of these lines indicateenhancement of s-process elements in the central star progenitors, andwe determine the abundances of Br, Kr, and Xe, elements for which atomicdata relevant for abundance determination have recently becomeavailable. As representative elements of the ``light'' and ``heavy''s-process peaks, Kr and Xe exhibit similar enhancements over solarvalues, suggesting that PN progenitors experience substantial neutronexposure.
| The Abundances of Light Neutron-Capture Elements in Planetary Nebulae. I. Photoionization Modeling and Ionization Corrections
We have conducted a large-scale survey of 120 planetary nebulae (PNe) tosearch for the near-infrared emission lines [Kr III] 2.199 μm and [SeIV] 2.287 μm. The neutron (n)-capture elements Se and Kr may beenriched in a PN if its progenitor star experienced s-processnucleosynthesis and third dredge-up. In order to determine Se and Krabundances, we have added these elements to the atomic databases of thephotoionization codes Cloudy and XSTAR, which we use to deriveionization correction factors (ICFs) to account for the abundances ofunobserved Se and Kr ions. However, much of the atomic data governingthe ionization balance of these two elements are unknown, and have beenapproximated from general principles. We find that uncertainties in theatomic data can lead to errors approaching 0.3 dex in the derived Seabundances and up to 0.2-0.25 dex for Kr. To reduce the uncertainties inthe Kr ionization balance stemming from the approximate atomic data, wehave modeled 10 bright PNe in our sample, selected because they exhibitemission lines from multiple Kr ions in their optical and near-infraredspectra. We have empirically adjusted the uncertain Kr atomic data untilthe observed line intensities of the various Kr ions are adequatelyreproduced by our models. Using the adjusted Kr atomic data, we havecomputed a grid of models over a wide range of physical parameters(central star temperature, nebular density, and ionization parameter)and derived formulae that can be used to compute Se and Kr ICFs. In thesecond paper of this series, we will apply these ICFs to our full sampleof 120 PNe, which comprises the first large-scale survey of n-captureelements in PNe.This paper includes data taken at the McDonald Observatory of theUniversity of Texas at Austin.
| Radio-continuum spectrum, brightness temperature, and planetary nebulae properties
Context: .Radio-continuum spectra are frequently used to infer thedensity distribution of ionized gaseous regions, while observedcorrelations between the brightness temperature and otherdistance-independent parameters are used to test evolutionary models ofplanetary nebulae. Aims: .We check empirical correlations amongfeatures derived from the observed radio data and the inferredconclusions available in the literature, using self-consistentphotoionization models. Methods: .Photoionization models arecomputed for the physical conditions of planetary nebulae in order toderive self-consistent radio-continuum spectra, as well as thebrightness temperature. Results: .The temperature and ionizationdistributions throughout the nebulae explain the observed range ofspectral indexes in the thick region of the spectrum, even for a uniformdensity distribution, usually challenged in the literature. The obtainedmodels fit the observed radio spectra for planetary nebulae in a largerange of spectral indexes. Our calculations show a correlation betweenthe spectral index obtained in a given frequency range and the nebulasize, as well as reproduce the observed relations between the brightnesstemperature and other distance-independent parameters. Such diagrams arefrequently used to check evolutionary models of the central star and/orof the nebula. Conclusions: .Since PNe images clearly show thatthe density is not constant inside the nebulae, and models with uniformand non-uniform density distributions can both reproduce the observedradio spectra, we conclude that it is not possible to favour one of themfrom the radio data, or to infer a particular density distribution forplanetary nebulae.
| 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.
| 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.
| On near Chandrasekhar mass central stars of planetary nebulae
Aims.A recent spectroscopic analysis of central stars of planetarynebulae (CSPNe) claims that the sample studied includes five CSPNe withmasses very close to the Chandrasekhar limit of white dwarfs. This claimis tested using available kinematical and chemical abundanceinformation. Methods.Kinematical parameters are extracted fromGalactic orbits and compared with parameters expected for populations ofdifferent ages. The chemistry of the nebulae is compared with averagevalues for different types. Results.The reported high masses arenot supported by our investigation. The claimed high central star massesare in contradiction with all other evidence. A more consistent pictureemerges if CSPN masses close to the peak of the white dwarf massdistribution are assumed.
| Galactic Planetary Nebulae with Wolf-Rayet Nuclei III. Kinematical Analysis of a Large Sample of Nebulae
Expansion velocities (V_{exp}) of different ions and line widths at thebase of the lines are measured and analyzed for 24 PNe with [WC]-typenuclei (WRPNe), 9 PNe ionized by WELS (WLPNe) and 14 ordinary PNe. Acomparative study of the kinematical behavior of the sample clearlydemonstrates that WRPNe have on average 40-45% larger V_{exp}, andpossibly more turbulence than WLPNe and ordinary PNe. WLPNe havevelocity fields very much like the ones of ordinary PNe, rather than theones of WRPNe. All the samples (WRPNe, WLPNe and ordinary PNe) showexpansion velocities increasing with age indicators, for example is larger for low-density nebulae and also it is largerfor nebulae around high-temperature stars. This age effect is muchstronger for evolved WRPNe, suggesting that the [WC] winds have beenaccelerating the nebulae for a long time, while for non-WRPNe theacceleration seems to stop at some point when the star reaches atemperature of about 90,000 - 100,000. Non-WR nebulae reach a maximumV_{exp} ≤ 30 km s(-1) evolved WRPNe reach maximum V_{exp} about 40km s(-1) . For all kinds of objects (WRPNe and non-WRPNe) it is foundthat on average V_{exp}(N(+) ) is slightly larger than V_{exp}(O(++) ),indicating that the nebulae present acceleration of the external shells.
| 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.
| Infrared photometry and evolution of mass-losing AGB stars. I. Carbon stars revisited
As part of a reanalysis of galactic Asymptotic Giant Branch (AGB) starsat infrared (IR) wavelengths, we discuss a sample (357) of carbon starsfor which mass loss rates, near-IR photometry and distance estimatesexist. For 252 sources we collected mid-IR fluxes from the MSX (6C) andthe ISO-SWS catalogues. Most stars have spectral energy distributions upto 21 μm, and some (1/3) up to 45 μm. This wide wavelengthcoverage allows us to obtain reliable bolometric magnitudes. Theproperties of our sample are discussed with emphasis on ~70 stars withastrometric distances. We show that mid-IR fluxes are crucial toestimate the magnitude of stars with dusty envelopes. We construct HRdiagrams and show that the luminosities agree fairly well with modelpredictions based on the Schwarzschild's criterion, contrary to what iswidely argued in the literature. A problem with the brightness of Cstars does not appear to exist. From the relative number of Mira andSemiregular C-variables, we argue that the switch between these classesis unlikely to be connected to thermal pulses. The relevance of the twopopulations varies with the evolution, with Miras dominating the finalstages. We also analyze mass loss rates, which increase for increasingluminosity, but with a spread that probably results from a dependence ona number of parameters (like e.g. different stellar masses and differentmechanisms powering stellar winds). Instead, mass loss rates are wellmonitored by IR colours, especially if extended to 20 μm and beyond,where AGB envelopes behave like black bodies. From these colours theevolutionary status of various classes of C stars is discussed.
| Precatalysmic binary systems.
Not Available
| Hot dust haloes in planetary nebulae
We point out that many planetary nebulae (PNe) have large infraredindices (H-KS), and that this is likely to result from thepresence of hot grains, and/or H2 S(1) line emission atλ= 2.122μm. We are able to identify two groups of sourcesassociated with each of these mechanisms, both of which appear topossess distinct physical characteristics. One difference between thesegroups concerns the near-infrared dimensions of the sources. It appearsthat hot dust outflows frequently have sizes θ(KS)> θ(H) > θ(J). Four of the sources are particularlyextreme in this regard, and show emission extending well outside of theprimary shells. We propose that this is likely to arise fromhigh-temperature grains located in low-density haloes. The location ofsuch grains at large distances from the central stars represents achallenge for any mechanism purporting to explain this phenomenon. Themost likely explanation appears to be in terms of photon heating of verysmall grains.
| The Fine-Structure Lines of Hydrogen in H II Regions
The 2s1/2 state of hydrogen is metastable and overpopulatedin H II regions. In addition, the 2p states may be pumped by ambientLyα radiation. Fine-structure transitions between these states maybe observable in H II regions at 1.1 GHz(2s1/2-2p1/2) and/or 9.9 GHz(2s1/2-2p3/2), although the details of absorptionversus emission are determined by the relative populations of the 2s and2p states. The n=2 level populations are solved with a parameterizationthat allows for Lyα pumping of the 2p states. The Lyαpumping rate has long been considered uncertain, as it involves solutionof the difficult Lyα transfer problem. The density of Lyαphotons is set by their creation rate, easily determined from therecombination rate, and their removal rate. Here we suggest that thedominant removal mechanism of Lyα radiation in H II regions isabsorption by dust. This circumvents the need to solve the Lyαtransfer problem and provides an upper limit to the rate at which the 2pstates are populated by Lyα photons. In virtually all cases ofinterest, the 2p states are predominantly populated by recombination,rather than Lyα pumping. We then solve the radiative transferproblem for the fine-structure lines in the presence of free-freeradiation. In the likely absence of Lyα pumping, the2s1/2-->2p1/2 lines will appear in stimulatedemission, and the 2s1/2-->2p3/2 lines inabsorption. Because the final 2p states are short lived, these lines aredominated by intrinsic line width (99.8 MHz). In addition, eachfine-structure line is a multiplet of three blended hyperfinetransitions. Searching for the 9.9 GHz lines in high emission measure HII regions offers the best prospects for detection. The lines arepredicted to be weak; in the best cases, line-to-continuum ratios ofseveral tenths of a percent might be expected with line strengths oftens to a hundred mK with the Green Bank Telescope. Predicted linestrengths, at both 1.1 and 9.9 GHz, are given for a number of H IIregions, high emission measure components, and planetary nebulae, basedon somewhat uncertain emission measures, sizes, and structures. Theextraordinary width of these lines and their blended structure willcomplicate detection.
| Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio
Airborne and space-based low-resolution spectroscopy in the 1980sdiscovered tantalizing quantitative relationships between the gas phaseC/O abundance ratio in planetary nebulae (PNe) and the fractions oftotal far-infrared (FIR) luminosity radiated by the 7.7- and 11.3-μmbands (the C = C stretch and C-H bend, respectively), of polycyclicaromatic hydrocarbons (PAHs). Only a very small sample of nebulae wasstudied in this context, limited by airborne observations of the7.7-μm band, or the existence of adequate IRAS Low ResolutionSpectrometer data for the 11.3-μm band. To investigate these trendsfurther, we have expanded the sample of planetaries available for thisstudy using Infrared Space Observatory (ISO) low-resolution spectrasecured with the Short Wavelength Spectrometer and the Long WavelengthSpectrometer. The new sample of 43 PNe, of which 17 are detected in PAHemission, addresses the range from C/O = 0.2-13 with the objective oftrying to delineate the pathways by which carbon dust grains might haveformed in planetaries. For the 7.7-μm and 11.3-μm bands, weconfirm that the ratio of band strength to total infrared (IR)luminosity is correlated with the nebular C/O ratio. Expressed inequivalent width terms, the cut-on C/O ratio for the 7.7-μm band isfound to be 0.6+0.2-0.4, in good accord with thatfound from sensitive ground-based measurements of the 3.3-μ band.
| The 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).
| Fluorine Abundances in Planetary Nebulae
We have determined fluorine abundances from the [F II] λ4789 and[F IV] λ4060 nebular emission lines for a sample of planetarynebulae (PNe). Our results show that fluorine is generally overabundantin PNe, thus providing new evidence for the synthesis of fluorine inasymptotic giant branch (AGB) stars. [F/O] is found to be positivelycorrelated with the C/O abundance ratio, in agreement with thepredictions of theoretical models of fluorine production in thermallypulsing AGB stars. A large enhancement of fluorine is observed in theWolf-Rayet PN NGC 40, suggesting that high mass-loss rates probablyfavor the survival of fluorine.
| The NII spectrum of the Orion nebula
The predicted emission spectrum of NII is compared with observations ofpermitted lines in the Orion nebula. Conventional nebular models showthat the intensities of the more intense lines can be explained byfluorescence of starlight absorption with a N abundance that isconsistent with forbidden lines. Lines excited mostly by recombination,on the other hand, predict high N abundances. The effects of stellar andnebular parameters and of the atomic data on the predicted intensitiesare examined.
| 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.
| Distance to NGC 253 based on the planetary nebula luminosity function
We identify a sample of 14 planetary nebulae in the nearby starburstdisc galaxy NGC 253, using broad- and narrow-band imaging. A simplemodel of the stellar and dust layers in the galaxy shows that planetarynebula magnitudes are not statistically affected much by extinction. Theluminosity function of the planetary nebulae is fitted with modelsaccounting for dust, which leads to a distance to NGC 253 ofμ0= 27.62+0.16-0.26 mag (d=3.34+0.26-0.38 Mpc). This new estimate is found tobe in agreement with the few existing distance estimates for this galaxywhen they are all set to a common Large Magellanic Cloud distance of18.50 mag. A weighted average of the most reliable distance estimatesyields a distance of μ0= 27.7 +/- 0.2 mag (d= 3.5 +/- 0.2Mpc).
| The 3-D shaping of NGC 6741: A massive, fast-evolving Planetary Nebula at the recombination-reionization edge
We infer the gas kinematics, diagnostics and ionic radial profiles,distance and central star parameters, nebular photo-ionization model,spatial structure and evolutionary phase of the Planetary Nebula NGC6741 by means of long-slit ESO NTT+EMMI high-resolution spectra at nineposition angles, reduced and analysed according to the tomographic and3-D methodologies developed at the Astronomical Observatory of Padua(Italy). NGC 6741 (distance≃2.0 kpc, age≃ 1400 yr, ionizedmass Mion≃ 0.06 Mȯ) is a dense(electron density up to 12 000 cm-3), high-excitation,almost-prolate ellipsoid (0.036 pc × 0.020 pc × 0.018 pc,major, intermediate and minor semi-axes, respectively), surrounded by asharp low-excitation skin (the ionization front), and embedded in aspherical (radius≃ 0.080 pc), almost-neutral, high-density (n(HI)≃ 7 ×103 atoms cm-3) halo containinga large fraction of the nebular mass (Mhalo≥ 0.20Mȯ). The kinematics, physical conditions and ionicstructure indicate that NGC 6741 is in a deep recombination phase,started about 200 years ago, and caused by the rapid luminosity drop ofthe massive (M*=0.66{-}0.68 Mȯ), hot (logT* ≃ 5.23) and faint (logL*/Lȯ ≃ 2.75) post-AGB star, which hasexhausted the hydrogen-shell nuclear burning and is moving along thewhite dwarf cooling sequence. The general expansion law of the ionizedgas in NGC 6741, Vexp(km s-1)=13 × R arcsec,fails in the innermost, highest-excitation layers, which move slowerthan expected. The observed deceleration is ascribable to the luminositydrop of the central star (the decreasing pressure of the hot-bubble nolonger balances the pressure of the ionized gas), and appears instriking contrast to recent reports inferring that acceleration is acommon property of the Planetary Nebulae innermost layers. A detailedcomparative analysis proves that the "U"-shaped expansion velocity fieldis a spurious, incorrect result due to a combination of: (a) simplisticassumptions (spherical shell hypothesis for the nebula); (b) unfitreduction method (emission profiles integrated along the slit); and (c)inappropriate diagnostic choice (λ4686 Å of He II, i.e. athirteen fine-structure components recombination line). Some generalimplications for the shaping mechanisms of Planetary Nebulae arediscussed.
| The Chemical Composition of Galactic Planetary Nebulae with Regard to Inhomogeneity in the Gas Density in Their Envelopes
The results of a study of the chemical compositions of Galacticplanetary nebulae taking into account two types of inhomogeneity in thenebular gas density in their envelopes are reported. New analyticalexpressions for the ionization correction factors have been derived andare used to determine the chemical compositions of the nebular gas inGalactic planetary nebulae. The abundances of He, N, O, Ne, S, and Arhave been found for 193 objects. The Y Z diagrams for various Heabundances are analyzed for type II planetary nebulae separately andjointly with HII regions. The primordial helium abundance Y p andenrichment ratio dY/dZ are determined, and the resulting values arecompared with the data of other authors. Radial abundance gradients inthe Galactic disk are studied using type II planetary nebulae.
| The FUSE Spectrum of the Planetary Nebula SwSt 1: Evidence for Inhomogeneities in the Gas and Dust
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofthe young, compact planetary nebula (PN) SwSt 1 along the line of sightto its central star HD 167362. We detect circumstellar absorption linesfrom several species against the continuum of the central star. Thephysical parameters of the nebula derived from the FUSE data differsignificantly from those found from emission lines. We derive anelectron density ne=8800+4800-2400cm-3 from the column density ratio of the excited S IIIfine-structure levels, which is at least a factor of 3 lower than allprior estimates. The gaseous iron abundance derived from the UV lines isquite high ([Fe/S]=-0.35+/-0.12), which implies that iron is notsignificantly depleted into dust. In contrast, optical and near-infraredemission lines indicate that Fe is more strongly depleted:[Fe/H]=-1.64+/-0.24 and [Fe/S]=-1.15+/-0.33. We do not detect nebularH2 absorption, to a limitN(H2)<7×1014 cm-2, at least 4orders of magnitude lower than the column density estimated frominfrared H2 emission lines. Taken together, the lack ofH2 absorption, low ne, and high gaseous Feabundance derived from the FUSE spectrum provide strong evidence thatdense structures (which can shield molecules and dust from thedestructive effects of energetic stellar photons) are not present alongthe line of sight to the central star. On the other hand, there issubstantial evidence for dust, molecular material, and dense gaselsewhere in SwSt 1. Therefore, we conclude that the nebula must have aninhomogeneous structure.We detect nebular absorption at 1040.94 and 1041.69 Å from the twoexcited fine-structure levels of neutral oxygen. These levels give riseto far-infrared emission lines at 63 and 145 μm, which are often usedto infer gas properties, particularly temperature, under the assumptionthat they are collisionally excited. We find that the O I fine-structurelevels in SwSt 1 have an inverted population ratio. This requires anonthermal excitation mechanism, which we identify as fluorescentexcitation by the stellar continuum. To the extent that fluorescenceaffects the level populations, the far-infrared [O I] line strengthscannot be directly used as diagnostics of density and temperature.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| The evolution of planetary nebulae. II. Circumstellar environment and expansion properties
We investigate and discuss the expansion properties of planetary nebulaeby means of 1D radiation-hydrodynamics models computed for differentinitial envelope configurations and central star evolutionary tracks. Inparticular, we study how the expansion depends on the initial densitygradient of the circumstellar envelope and show that it is possible toderive information on the very last mass-loss episodes during the star'sfinal evolution along and off the asymptotic giant branch. To facilitatethe comparison of the models with real objects, we have also computedobservable quantities like surface brightness and emission-lineprofiles. With the help of newly acquired high-resolution emission-lineprofiles for a sample of planetary nebulae we show that models withinitial envelopes based on the assumption of a stationary wind outflowfail to explain the observed expansion speeds of virtually all of theobserved planetary nebulae. Instead it must be assumed that during thevery last phase of evolution along the final asymptotic giant branchevolution the mass-loss rate increases in strength, resulting in a muchsteeper slope of the circumstellar radial density distribution. Underthese conditions, the expansion properties of the nebular gas differconsiderably from the self-similar solutions found for isothermalconditions. Furthermore, the mass loss must remain at a rather highlevel until the stellar remnant begins to evolve quickly towards thecentral star regime. Current theoretical computations of dust-drivenmass-loss which are restricted to rather low temperatures cannot beapplied during the star's departure from the asymptotic giant branch.Based on observations obtained at the 3.5 μm NTT and the 1.2 μmCAT telescope of the European Southern Observatory, La Silla, and at the2.6 μm NOT telescope operated on the island of La Palma by NOTSA, inthe Spanish Observatorio del Roque de Los Muchachos of the InstitutodeAstrofísica de Canarias.Dedicated to Prof. V. Weidemann on the occasion of his 80th birthday,October 3, 2004.
| Unresolved Hα Enhancements at High Galactic Latitude in the WHAM Sky Survey Maps
We have identified 85 regions of enhanced Hα emission at|b|>10deg subtending approximately 1° or less on theWisconsin Hα Mapper (WHAM) sky survey. These high-latitude ``WHAMpoint sources'' have Hα fluxes of 10-11-10-9ergs cm-2 s-1, radial velocities within about 70km s-1 of the LSR, and line widths that range from less than20 to about 80 km s-1 (FWHM). Twenty-nine of theseenhancements are not identified with either cataloged nebulae or hotstars and appear to have kinematic properties that differ from thoseobserved for planetary nebulae. Another 14 enhancements are near hotevolved low-mass stars that had no previously reported detections ofassociated nebulosity. The remainder of the enhancements are catalogedplanetary nebulae and small, high-latitude H II regions surroundingmassive O and early B stars.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| Hunting post-AGB/RSG objects using Virtual Observatories and other internet-based technology.
After the Asymptotic Giant Branch or Red Supergiant stage of evolution,stars evolve rapidly from cool M-type to F-type and hotter before theirfinal fates. We describe the use of the European Virtual Observatory toderive a set of properties based on published data which identifypost-AGB transition objects. We then apply these selection criteria tofind new candidate transition objects and investigate their images andspectra in more detail. Heterogenous data collections from all over theworld can be tracked down, manipulated and compared using the internet.The internet is itself part of the telescope in a separate development,real-time correlation of E-VLBI observations of hydroxyl masers from thepost-RSG star IRC+10420.
| 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]).
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