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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.
| 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.
| X-ray emission from planetary nebulae calculated by 1D spherical numerical simulations
We calculate the X-ray emission from both constant and time-evolvingshocked fast winds blown by the central stars of planetary nebulae (PNe)and compare our calculations with observations. Using sphericallysymmetric numerical simulations with radiative cooling, we calculate theflow structure and the X-ray temperature and luminosity of the hotbubble formed by the shocked fast wind. We find that a constant fastwind gives results that are very close to those obtained from theself-similar solution. We show that in order for a fast shocked wind toexplain the observed X-ray properties of PNe, rapid evolution of thewind is essential. More specifically, the mass-loss rate of the fastwind should be high early on when the speed is ~300-700 kms-1, and then it needs to drop drastically by the time the PNage reaches ~1000 yr. This implies that the central star has a veryshort pre-PN (post-asymptotic giant branch) phase.
| Germanium Production in Asymptotic Giant Branch Stars: Implications for Observations of Planetary Nebulae
Observations of planetary nebulae (PNe) in the work of Sterling,Dinerstein, & Bowers have revealed abundances in the neutron-captureelement germanium (Ge) from solar to factors of 3-10 above solar. Theenhanced Ge is an indication that the slow neutron-capture process (thes-process) operated in the parent star during the thermally pulsingasymptotic giant branch (TP-AGB) phase. We compute the detailednucleosynthesis of a series of AGB models to estimate the surfaceenrichment of Ge near the end of the AGB. A partial mixing zone ofconstant mass is included at the deepest extent of each dredge-upepisode, resulting in the formation of a 13C pocket in thetop approximately one-tenth of the He-rich intershell. All of the modelsshow surface increases of [Ge/Fe] <~ 0.5, except the 2.5Msolar, Z=0.004 case, which produced a factor of 6enhancement of Ge. Near the tip of the TP-AGB, a couple of extra thermalpulses (TPs) could occur to account for the composition of the mostGe-enriched PNe. Uncertainties in the theoretical modeling of AGBstellar evolution might account for larger Ge enhancements than wepredict here. Alternatively, a possible solution could be provided bythe occurrence of a late TP during the post-AGB phase. Difficultiesrelated to spectroscopic abundance estimates also need to be taken intoconsideration. Further study is required to better assess how the modeluncertainties affect the predictions and, consequently, if a late TPshould be invoked.
| Detailed Far-Ultraviolet to Optical Analysis of Four [WR] Stars
We present far-UV to optical analyses of four hydrogen-deficient centralstars of planetary nebulae: BD +30 3639, NGC 40, NGC 5315, and NGC 6905.Using the radiative transfer code CMFGEN, we determined new physicalparameters and chemical abundances for these stars. The results wereanalyzed in the context of the [WR]-->PG 1159 evolution via thetransformed radius-temperature (RT×T*) andH-R diagrams. NGC 5315 showed itself as an odd object among thepreviously analyzed central stars. Its temperature (~76 kK) isconsiderably lower than other early-type [WR] stars (~120-150 kK). Fromour models for NGC 5315 and NGC 6905, it is unclear if early-type [WR]stars have smaller C/He mass ratios than other spectral classes, asclaimed in the literature. A ratio of ~0.8 is found for NGC 6905. Weanalyzed FUSE spectra of these stars for the first time and identifiedphosphorus in the spectra of BD +30 3639, NGC 40, and NGC 5315 throughthe doublet transition P V λλ1118, 1128 (3p2Po-3s 2S). The Fe, Si, P, S, and Neabundances were analyzed in the context of the nucleosynthesis occurringin previous evolutionary phases. We found evidence for Fe deficiency inBD +30 3639 and NGC 5315, and we determined a solar Si abundance for BD+30 3639 and NGC 40. Regarding P, an oversolar abundance in the NGC 5315model was preferred. Upper limits for the S abundance were estimated. Wefound that Ne is overabundant in BD +30 3639. In the other stars, Ne isweak or undetectable. Our results are in agreement with theoreticalpredictions and show the usefulness of [WR] stars as test beds fornucleosynthesis calculations in the AGB and post-AGB phases.
| Infrared Mapping of Four Interesting Planetary Nebulae
We present Two Micron All Sky Survey near-infrared (NIR) photometricmapping of the planetary nebulae A30, A78, NGC 7027, and BD +30 3639. Itis shown that the NIR envelopes of both NGC 7027 and BD +30 3639 extendto much greater distances than has previously been supposed. It alsoappears that the dust in NGC 7027 has extinction properties similar tothose of interstellar grains, and that the radii of the particles mustbe reasonably large. An analysis of emission in A78 and A30 shows theinfrared indices to be reasonably large and implies grain temperaturesTgr of order ~950 K. Similarly, the angular extensions of thecontinua (>18" and >22", respectively) suggest that they may arisefrom very small grains indeed. By contrast, while BD +30 3639 is alsocharacterized by a high-temperature continuum, much of this emissionappears to be confined to the core. This may imply that grain heatingarises through direct stellar irradiation.
| Molecules in (proto)planetary Nebulae
This contribution reviews results on the structure and dynamics ofplanetary and protoplanetary nebulae from observations of molecular lineemission. Molecular line data have been particularly useful to study thevery fast evolution of these nebulae. Our knowledge on the molecularcontent of these objects, i.e., the molecule abundances, is alsosummarized.
| Evolution of the Circumstellar Molecular Envelope. I. A BIMA CO Survey of Evolved Stars
This paper reports the results of a small imaging survey of eightevolved stars including two AGB stars (IRC +10216 and Mira), fiveproto-planetary nebula (PPN) candidates (AFGL 2688, IRAS 22272+5435, HD161796, 89 Her, and HD 179821), and a planetary nebula (PN, NGC 7027).We present high-resolution 12CO J=1-->0 maps of their fullmolecular envelopes made by combining BIMA Millimeter Array and NRAO 12m telescope observations. For the PPNe and PN, the neutral molecularenvelopes are compared with images taken at optical, near-IR, and mid-IRwavelengths. Drawing from the literature, we augmented our BIMA surveysample to 38 well-studied sources with CO emission maps. We classifiedthis sample of sources based on the kinematics and morphologies of theCO emission into three types: spherical/elliptical/shell sources, disksources, and structured outflow sources. Confirming previous studies, wefind strong evidence for the photodissociation of the molecular envelopeas an object evolves from the AGB to PN stages. While the spherical AGBstars follow theoretical expectations for mass-loss rate versus envelopesize, the post-AGB structured outflow sources have significantly highermass-loss rates than expected probably because of their recentsuperwinds. We find evidence that the structured outflows are clearlyyounger than the AGB wind. The disk sources have little correlationbetween mass-loss rate and envelope size because their properties aredetermined more by the properties of the central stars and diskevolution than by the mass-loss rate history that shapes the sphericaland structured-outflow sources.
| Infrared Observations of the Helix Planetary Nebula
We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRACinstrument on the Spitzer Space Telescope. The Helix is one of theclosest bright PNs and therefore provides an opportunity to resolve thesmall-scale structure in the nebula. The emission from this PN in the5.8 and 8 μm IRAC bands is dominated by the pure rotational lines ofmolecular hydrogen, with a smaller contribution from forbidden lineemission such as [Ar III] in the ionized region. The IRAC images resolvethe ``cometary knots,'' which have been previously studied in this PN.The ``tails'' of the knots and the radial rays extending into the outerregions of the PN are seen in emission in the IRAC bands. IRS spectra onthe main ring and the emission in the IRAC bands are consistent withshock-excited H2 models, with a small (~10%) component fromphotodissociation regions. In the northeast arc, the H2emission is located in a shell outside the Hα emission.
| X-Ray Emission from Planetary Nebulae. I. Spherically Symmetric Numerical Simulations
The interaction of a fast wind with a spherical asymptotic giant branch(AGB) wind is thought to be the basic mechanism for shapingpre-planetary nebulae (PPNs) and later planetary nebulae (PNs). Due tothe large speed of the fast wind, one expects extended X-ray emissionfrom these objects, but X-ray emission has only been detected in a smallfraction of PNs and only in one PPN. Using numerical simulations weinvestigate the constraints that can be set on the physical propertiesof the fast wind (speed, mass flux, opening angle) in order to producethe observed X-ray emission properties of PPNs and PNs. We combinenumerical hydrodynamic simulations including radiative cooling using thecode FLASH with calculations of the X-ray properties of the resultingexpanding hot bubble using the atomic database ATOMDB. In this firststudy, we compute X-ray fluxes and spectra using one-dimensional models.Comparing our results with analytical solutions, we find some agreementsand many disagreements. These disagreements, which are both qualitativeand quantitative in nature, argue for the necessity of using numericalsimulations for understanding the X-ray properties of PNs. We apply ourspherical models to the objects BD +30 3639 and NGC 40. We find that themodel values of the X-ray temperature and luminosity for these objectsare significantly higher than observed values and discuss severalmechanisms for resolving the discrepancies.
| 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.
| Suzaku Reveals Helium-burning Products in the X-Ray-emitting Planetary Nebula BD +30 3639
BD +30 3639, the brightest planetary nebula at X-ray energies, wasobserved with Suzaku, an X-ray observatory launched on 2005 July 10.Using the X-ray Imaging Spectrometer, the K lines from C VI, O VII, andO VIII were resolved for the first time, and the C/O, N/O, and Ne/Oabundance ratios were determined. The C/O and Ne/O abundance ratiosexceed the solar value by factors of at least 30 and 5, respectively.These results indicate that the X-rays are emitted mainly byhelium-shell-burning products.
| 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.
| X-ray emission by a shocked fast wind from the central stars of planetary nebulae
We calculate the X-ray emission from the shocked fast wind blown by thecentral stars of planetary nebulae (PNe) and compare with observations.Using spherically symmetric self-similar solutions, we calculate theflow structure and X-ray temperature for a fast wind slamming into apreviously ejected slow wind. We find that the observed X-ray emissionof six PNe can be accounted for by shocked wind segments that wereexpelled during the early-PN phase, if the fast wind speed is moderate,v2~ 400-600kms-1, and the mass-loss rate is a fewtimes 10-7Msolaryr-1. We find, asproposed previously, that the morphology of the X-ray emission is in theform of a narrow ring inner to the optical bright part of the nebula.The bipolar X-ray morphology of several observed PNe, which indicates animportant role of jets, rather than a spherical fast wind, cannot beexplained by the flow studied here.
| 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.
| Planetary nebulae with emission-line central stars
The kinematic structure of a sample of planetary nebulae, consisting of23 [WR] central stars, 21 weak emission line stars (wels), and 57non-emission line central stars, is studied. The [WR] stars are shown tobe surrounded by turbulent nebulae, a characteristic shared by some welsbut almost completely absent from the non-emission line stars. Thefraction of objects showing turbulence for non-emission-line stars,wels, and [WR] stars is 7%, 24%, and 91%, respectively. The [WR] starsshow a distinct IRAS 12-micron excess, indicative of small dust grains,which is not found for wels. The [WR]-star nebulae are on average morecentrally condensed than those of other stars. On the age-temperaturediagram, the wels are located on tracks of both high and low stellarmass, while [WR] stars trace a narrow range of intermediate masses.Emission-line stars are not found on the cooling track. One group ofwels may form a sequence wels-[WO] stars with increasing temperature.For the other groups, both the wels and the [WR] stars appear torepresent several, independent evolutionary tracks. We find adiscontinuity in the [WR] stellar temperature distribution and suggestdifferent evolutionary sequences above and below the temperature gap.One group of cool [WR] stars has no counterpart among any other group ofPNe and may represent binary evolution. A prime factor distinguishingwels and [WR] stars appears to be stellar luminosity. We find noevidence for an increase in the nebular expansion velocity with time.
| 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.
| An X-Ray Search for Compact Central Sources in Supernova Remnants. II. Six Large-Diameter SNRs
We present the second in a series of studies in which we have searchedfor undiscovered neutron stars in supernova remnants (SNRs). This paperdeals with the six largest SNRs in our sample, too large for Chandra orXMM-Newton to cover in a single pointing. These SNRs are nearby, withtypical distances of <1 kpc. We therefore used the ROSAT BrightSource Catalog and past observations in the literature to identify X-raypoint sources in and near the SNRs. Out of 54 sources, we wereimmediately able to identify optical/IR counterparts to 41 from existingdata. We obtained Chandra snapshot images of the remaining 13 sources.Of these, 10 were point sources with readily identified counterparts,two were extended, and one was not detected in the Chandra observationbut is likely a flare star. One of the extended sources may be a pulsarwind nebula, but if so it is probably not associated with the nearbySNR. We are then left with no identified neutron stars in these six SNRsdown to luminosity limits of ~1032 ergs s-1. Theselimits are generally less than the luminosities of typical neutron starsof the same ages, but are compatible with some lower luminosity sourcessuch as the neutron stars in the SNRs CTA 1 and IC 443.
| Iron Depletion in the Hot Bubbles in Planetary Nebulae
We have searched for the emission from [Fe X] λ6374 and [Fe XIV]λ5303 that is expected from the gas emitting in diffuse X-rays inBD +30°3639, NGC 6543, NGC 7009, and NGC 7027. Neither line wasdetected in any object. Models that fit the X-ray spectra of theseobjects indicate that the [Fe X] λ6374 emission should be belowour detection thresholds, but the predicted [Fe XIV] λ5303emission exceeds our observed upper limits (1 σ) by factors of atleast 3.5-12. The best explanation for the absence of [Fe XIV]λ5303 is that the X-ray plasma is depleted in iron. In principle,this result provides a clear chemical signature that may be used todetermine the origin of the X-ray gas in either the nebular gas or thestellar wind. At present, although various lines of evidence appear tofavor a nebular origin, the lack of atmospheric and nebular ironabundances in the objects studied here precludes a definitiveconclusion.Based on observations obtained at the Observatorio AstronómicoNacional in San Pedro Mártir, Baja California, Mexico.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.
| A Spectrophotometric Survey of K-Band Emission Lines in Planetary Nebulae
We present observations of 16 planetary nebulae (PNs) in the 2 μm (Kband) spectral region, obtained with a long-slit near-infraredspectrometer at McDonald Observatory. In general, the strongest featuresin our spectra are recombination lines of H I, He I, and (in some cases)He II. Half the sample shows emission from vibrationally excitedH2. Some of the observed PNs (e.g., M 1-13) displayH2 line ratios characteristic of shocked, thermalized gas,while others (e.g., BD +30 3639) have ratios intermediate between pureradiative (UV) and shock excitation, consistent with either acombination of the mechanisms or UV illumination of dense material. Ourspectra of J900 and M 1-13 confirm that published narrowband imagestrace the H2 emission, and we find that the H2emission in SwSt 1 has a larger spatial extent than previously reported.In IC 5117, SwSt 1, and NGC 40 we detect the [Kr III] 2.199 μm lineidentified by Dinerstein in 2001, with strengths indicating that kryptonis enriched relative to the solar abundance, most markedly so in NGC 40.We also detect several lines from the 3G term of [Fe III] inVy 2-2, SwSt 1, and marginally in Cn 3-1. The [Kr III] and [Fe III]lines fall near in wavelength to H2 transitions, which areoften used as diagnostics for UV excitation because they arise fromhigher vibrationally excited levels (v=2, 3). For moderate spectralresolving power, R<=600, these lines may be blended with, or evenmistaken for, the corresponding H2 lines, leading tomisinterpretation of the H2 emission. The strength of boththe Kr and Fe nebular emission lines can be enhanced by specialcircumstances, Kr because of nucleosynthetic self-enrichment in theprogenitor star and Fe due to inefficient initial dust condensation orpartial destruction of the dust after formation, causing a largerfraction of the elemental iron to reside in the gas phase.
| The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
We review the observed properties of extremely hot, hydrogen-deficientpost-asymptotic giant branch (AGB) stars of spectral type [WC] andPG1159. Their H deficiency is probably caused by a (very) latehelium-shell flash or an AGB final thermal pulse, laying bare interiorstellar regions that are usually kept hidden below the hydrogenenvelope. Thus, the photospheric elemental abundances of these starsallow us to draw conclusions about details of nuclear burning and mixingprocesses in the precursor AGB stars. We summarize the state of the artof stellar evolution models that simulate AGB evolution and theoccurrence of a late He-shell flash. We compare predicted elementalabundances to those determined by quantitative spectral analysesperformed with advanced non-LTE model atmospheres. Good qualitative andquantitative agreement is found. Future work can contribute to an evenmore complete picture of the nuclear processes in AGB stars.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| An Ultraviolet to Mid-Infrared Study of the Physical and Wind Properties of HD 164270 (WC9) and Comparison to BD +30 3639 ([WC9])
We present new Spitzer IRS observations of HD 164270 (WC9, WR103). Aquantitative analysis of the UV, optical, near-, and mid-IR spectra ofHD 164270 is presented, allowing for line blanketing and wind clumping,revealing T*~48 kK, logL/Lsolar~4.9, andM˙~10-5 Msolar yr-1 for a volumefilling factor of f~0.1. Our models predict that He is partiallyrecombined in the outer stellar wind, such that recent radio-derivedmass-loss rates of WC9 stars have been underestimated. We obtainC/He~0.2 and O/He~0.01 by number from optical diagnostics. Mid-IRfine-structure lines of [Ne II] 12.8 μm and [S III] 18.7 μm areobserved, with [Ne III] 15.5 μm and [S IV] 10.5 μm absent. Fromthese we obtain Ne/He~Ne+/He=2.2×10-3 bynumber, 7 times higher than the solar value (as recently derived byAsplund et al.), and S/He~S2+/He=5.1×10-5 bynumber. From a comparison with similar results for other WC subtypes weconclude that WC9 stars are as chemically advanced as earlier subtypes.We consider why late WC stars are exclusively observed inhigh-metallicity environments. In addition, we compare theUV/optical/mid-IR spectroscopic morphology of HD 164270 with theplanetary nebula central star BD +30 3639 ([WC9]). Their UV and opticalsignatures are remarkably similar, such that our quantitativecomparisons confirm similarities in stellar temperature, wind densities,and chemistry first proposed by Smith & Aller, in spite ofcompletely different evolutionary histories, with HD 164270 presently afactor of 10 more massive than BD +30 3639. At mid-IR wavelengths, thedust from the dense young nebula of BD +30 3639 completely dominates itsappearance, in contrast with HD 164270.
| 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.
| 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.
| X-Ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40
We present the results of Chandra X-Ray Observatory observations of theplanetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-typeWolf-Rayet central stars that are currently driving fast (~1000 kms-1), massive winds into denser, slow-moving (~10 kms-1) material ejected during recently terminated asymptoticgiant branch (AGB) evolutionary phases. Hence, these observationsprovide key tests of models of wind-wind interactions in PNs. In NGC 40,we detect faint, diffuse X-ray emission distributed within a partialannulus that lies nested within a ~40" diameter ring of nebulosityobserved in optical and near-infrared images. Hen 2-99 is not detected.The inferred X-ray temperature (TX~106 K) andluminosity (LX~2×1030 ergs s-1)of NGC 40 are the lowest measured thus far for any PN displaying diffuseX-ray emission. These results, combined with the ringlike morphology ofthe X-ray emission from NGC 40, suggest that its X-ray emission arisesfrom a ``hot bubble'' that is highly evolved and is generated by ashocked, quasi-spherical fast wind from the central star, as opposed toAGB or post-AGB jet activity. In contrast, the lack of detectable X-rayemission from Hen 2-99 suggests that this PN has yet to enter a phase ofstrong wind-wind shocks.
| 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.
| Dust Extinction in Compact Planetary Nebulae
The effects of dust extinction on the departure from axisymmetry in themorphology of planetary nebulae (PNs) are investigated through acomparison of the radio free-free emission and hydrogen recombinationline images. The dust extinction maps from five compact PNs are derivedusing high-resolution (~0.1") Hα and radio maps of the HST andVLA. These extinction maps are then analyzed by an ellipsoidal shellionization model including the effects of dust extinction to infer thenebulae's intrinsic structure and orientation in the sky. This methodprovides a quantitative analysis of the morphological structure of PNsand represents a step beyond qualitative classification of morphologicaltypes of PNs.
| The evolution of planetary nebulae. III. Internal kinematics and expansion parallaxes
A detailed theoretical study of the basic internal kinematics ofplanetary nebulae is presented, based on 1D radiation-hydrodynamicssimulations of circumstellar envelopes around central stars of 0.595 and0.696 Mȯ. By means of observable quantities like radialsurface-brightness distributions and emission-line profiles computedfrom the models, a comparison with real objects was performed andrevealed a reasonable agreement. This allowed to draw importantconclusions by investigating the kinematics of these models in detail.Firstly, it is shown that the determination of kinematical ages,normally considered to be simple if size and expansion rate of an objectare given, can seriously be flawed. Secondly, the expansion law of aplanetary nebula is different from what is assumed for derivingspatio-kinematical models. Thirdly and most importantly, ourhydrodynamical models help to correctly use existing angular expansionmeasurements for distance determinations. The mere combination of theangular expansion rates with the spectroscopic expansion velocitiesleads always to a serious underestimate of the distance, the degree ofwhich depends on the evolutionary state of the object. The necessarycorrection factor varies between 3 and 1.3. Individual correctionfactors can be estimated with an accuracy of about 10% by matching ourhydrodynamical models to real objects. As a result, revised distancesfor a few objects with reliable angular expansion rates are presented.But even these corrected distances are not always satisfying: they stillappear to be inconsistent with other distance determinations and, evenmore disturbing, with the accepted theory of post-asymptotic giantbranch evolution. As a byproduct of the angular expansion measurements,the transition times from the vicinity of the asymptotic giant branch tothe planetary-nebula regime could be estimated. They appear to beshorter than assumed in the present evolutionary calculations.
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Observation and Astrometry data
| Constellation: | Schwan |
| Right ascension: | 19h34m45.24s |
| Declination: | +30°30'58.9" |
| Apparent magnitude: | 10.432 |
| Proper motion RA: | -2.8 |
| Proper motion Dec: | -8.4 |
| B-T magnitude: | 10.409 |
| V-T magnitude: | 10.431 |
Catalogs and designations:
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