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And in the Darkness Bind Them: Equatorial Rings, B[e] Supergiants, and the Waists of Bipolar Nebulae
We report the discovery of two new circumstellar ring nebulae in thewestern Carina Nebula, and we discuss their significance in stellarevolution. The brighter of the two new objects, SBW 1, resembles alidless staring eye and encircles a B1.5 Iab supergiant. Although seenin Carina, its luminosity class and radial velocity imply a largerdistance of ~7 kpc in the far Carina arm. At that distance its size andshape are nearly identical to the equatorial ring around SN 1987A, butSBW 1's low N abundance indicates that the ring was excreted without itsstar passing through a red supergiant phase. The fainter object, SBW 2,is a more distorted ring, is N-rich, and is peculiar in that its centralstar seems to be invisible. We discuss the implications of these two newnebulae in context with other circumstellar rings such as those aroundSN 1987A, Sher 25, HD 168625, RY Scuti, WeBo 1, SuWt 2, and others. Thering bearers fall into two groups: Five rings surround hot supergiants,and it is striking that all except for the one known binary are carboncopies of the ring around SN 1987A. We propose a link between these Bsupergiant rings and B[e] supergiants, where the large spatiallyresolved rings derive from the same material that would have given riseto emission lines during the earlier B[e] phase, when it was much closerto the star. The remaining four rings surround evolved intermediate-massstars; all members of this ring fellowship are close binaries, hintingthat binary interactions govern the forging of such rings. Two-thirds ofour sample are found in or near giant H II regions. We estimate thatthere may be several thousand more dark rings in the Galaxy, but we arescarcely aware of their existence-either because they are onlyilluminated in precious few circumstances or because of selectioneffects. For intermediate-mass stars, these rings might be thepreexisting equatorial density enhancements invoked to bind the waistsof bipolar nebulae.Based in part on observations made at the Clay Telescope of the MagellanObservatory, a joint facility of the Carnegie Observatories, HarvardUniversity, the Massachusetts Institute of Technology, the University ofArizona, and the University of Michigan.
| 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.
| 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.
| XMM-Newton Observations of the Bipolar Planetary Nebulae NGC 2346 and NGC 7026
We have obtained X-ray observations of the bipolar planetary nebulae(PNe) NGC 2346 and NGC 7026 with XMM-Newton. These observations detecteddiffuse X-ray emission from NGC 7026 but not from NGC 2346. The X-rayemission from NGC 7026 appears to be confined within the bipolar lobesof the PN and has spectral properties suggesting a thermal plasmaemitting at a temperature of1.1+0.5-0.2×106 K. The X-rayspectrum of NGC 7026 is modeled using nebular and stellar abundances toassess whether a significant amount of nebular material has been mixedinto the shocked wind, but the results of this comparison are notconclusive owing to the small number of counts detected. Observations ofbipolar PNe indicate that diffuse X-ray emission is much less likelydetected in open-lobed nebulae than closed-lobed nebulae, possiblybecause open-lobed nebulae do not have strong fast winds or are unableto retain hot gas.
| 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.
| Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79northern Galactic planetary nebulae (PNe) whose morphological classeshave been uniformly determined. Ionic abundances and plasma diagnosticswere derived from selected optical line strengths in the literature, andelemental abundances were estimated with the ionization correctionfactor developed by Kingsbourgh & Barlow in 1994. We compare theelemental abundances to the final yields obtained from stellar evolutionmodels of low- and intermediate-mass stars, and we confirm that mostbipolar PNe have high nitrogen and helium abundance and are the likelyprogeny of stars with main-sequence mass greater than 3Msolar. We derive =0.27 and discuss the implication of such ahigh ratio in connection with the solar neon abundance. We determine theGalactic gradients of oxygen and neon and foundΔlog(O/H)/ΔR=-0.01 dex kpc-1 andΔlog(Ne/H)/ΔR=-0.01 dex kpc-1. These flat PNgradients are irreconcilable with Galactic metallicity gradientsflattening with time.
| The new pre-cataclysmic binary PG 2200+085
Aims.We present the results of spectroscopic- and orbit-sampledphotometric observations of the faint UV-excess object PG 2200+085. Methods: .The optical CCD photometry observations of this object wereperformed by the Russian-Turkish 1.5-m telescope RTT150 at the TUBITAKNational Observatory (Turkey). The long-slit optical spectroscopyobservations with 2.6 Å resolution were carried out by 6-mtelescope BTA at the Special Astrophysical Observatory (Russia).Results: .The photometric variations over two nights are almostsinusoidal with an amplitude Δ mV = 0.04m and a periodof P = 0.3186 d. Such a light curve is typical of a detached closebinary with an illumination effect or the ellipsoidal deformation of asecondary star. The observed spectrum clearly displays a featurelessblue continuum of a hot component and a rich absorption-line andmolecular band K-star spectrum. The CaII line profiles with strongemission cores are remarkably similar to those of V471 Tau.Conclusions: .We tentatively classify PG 2200+085 as a pre-cataclysmicbinary of the V471 Tau type.
| 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.
| Precatalysmic binary systems.
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| Some astronomical niches with 3D spectroscopy
An overview of some of the most interesting results obtained with theuse of 3D spectrometers working in 4m-class telescopes is given with thepurpose of taking advantage of those experiences in the definition ofscientific programs for telescopes of larger diameter as the GTC.
| 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.
| 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.
| 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 Multitude of Molecular Hydrogen Knots in the Helix Nebula
We present Hubble Space Telescope NICMOS imaging of the H22.12 μm emission in five fields in the Helix Nebula ranging in radialdistance from 250" to 450" from the central star. The images revealarcuate structures with their apexes pointing toward the central star.These molecular hydrogen knots are most highly structured in the fieldsclosest to the central star and become increasingly less structured withincreasing radius. Comparison of these images with ground-based imagesof comparable resolution reveals that the molecular gas is more highlyclumped than the ionized gas line tracers. From our images, we determinean average number density of knots in the molecular gas ranging from 162knots arcmin-2 in the denser regions to 18 knotsarcmin-2 in the lower density outer regions. The decreasingnumber density of H2 knots in the outer regions creates alower filling factor of neutral and molecular gas emission in the radioobservations of CO and H I and may explain why these outer regions,where we clearly detect H2 2.12 μm, fall below thedetection limit of the radio observations. Using this new numberdensity, we estimate the total number of knots in the Helix to be~23,000, which is a factor of 6.5 larger than previous estimates. Thetotal neutral gas mass in the Helix is 0.35 Msolar assuming amass of ~1.5×10-5 Msolar for the individualknots. The H2 emission structure of the entire Helix Nebulasupports the recent interpretation of the Helix as a nearly pole-onpolypolar planetary nebula (PN). The H2 intensity,(5-9)×10-5 ergs s-1 cm-2sr-1, remains relatively constant with projected distancefrom the central star, suggesting a heating mechanism for the moleculargas that is distributed almost uniformly in the knots throughout thenebula. The temperature and H2 2.12 μm intensity of theknots can be approximately explained by photodissociation regions (PDRs)in the individual knots; however, theoretical PDR models of PNsunderpredict the intensities of some knots by a factor of 10. Thebrightest H2 emission (~3×10-4 ergss-1 cm-2 sr-1) may be enhanced by alarger than unity area filling factor of H2 knots or may bean individual H2 knot exposed to direct starlight, causingrapid photoevaporation compared with the more embedded knots of thedisk.Based in part on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| The mean properties of planetary nebulae as a function of Peimbert class
Planetary nebulae are known to possess a broad range of abundances, andthese (with other characteristics) have been used to define five classesof outflow. Peimbert Type I sources, for instance, possess high N and Heabundances, filamentary structures, and low mean scaleheights above theGalactic plane, whilst those of Type III have much lower abundances,high peculiar velocities, and belong to the Galactic thick disc. Apartfrom some rather ill-defined indications, however, very little is knownconcerning their mean physical, spatial, structural, kinematic andthermal characteristics.We have performed a comprehensive study of all of these properties, andfind evidence for strong variations between the various Peimbertclasses. Certain of these differences are consistent with Type I sourceshaving the highest progenitor masses, although it seems that thesenebulae also possess the lowest rms densities and 5-GHz brightnesstemperatures. The latter results are in conflict with a range of recentmodelling.
| A Multi-Instrument Study of the Helix Nebula Knots with the Hubble Space Telescope
We have conducted a combined observational and theoretical investigationof the ubiquitous knots in the Helix Nebula (NGC 7293). We haveconstructed a combined hydrodynamic + radiation model for the ionizedportion of these knots and have accurately calculated a static model fortheir molecular regions. Imaging observations in optical emission lineswere made with the Hubble Space Telescope's STIS, operating in a``slitless'' mode, complemented by WFPC2 images in several of the samelines. The NICMOS camera was used to image the knots in H2.These observations, when combined with other studies of H2and CO, provide a complete characterization of the knots. They possessdense molecular cores of densities about 106 cm-3surrounded on the central star side by a zone of hot H2. Thetemperature of the H2-emitting layer defies explanationeither through detailed calculations for radiative equilibrium orthrough simplistic calculations for shock excitation. Farther away fromthe core is the ionized zone, whose peculiar distribution of emissionlines is explained by the expansion effects of material flowing throughthis region. The shadowed region behind the core is the source of mostof the CO emission from the knot and is of the low temperature expectedfor a radiatively heated molecular region.Based in part on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.Based in part on observations obtained at the Cerro TololoInter-American Observatory, which is operated by AURA, Inc., under acooperative agreement with the National Science Foundation.
| A Survey of Large Molecules toward the Proto-Planetary Nebula CRL 618
We present the results of our survey toward the proto-planetary nebulaCRL 618 for several large, highly saturated, oxygen-bearing organicmolecules of biological importance including acetaldehyde(CH3CHO), acetic acid (CH3COOH), dimethyl ether(CH3OCH3), ethanol(CH3CH2OH), formic acid (HCOOH) and methyl formate(HCOOCH3); large carbon chain molecules including methylcyanide (CH3CN), methylcyanoacetylene(CH3C3N), cyanoacetylene (HC3N),cyanodiacetylene (HC5N), and C6H; and, finally,smaller molecules including 34SO, SO2,OC34S, and MgNC. No biologically important organic moleculeswere detected. However, we report the first interferometric detectionsof CH3CN and vibrationally excited HC3N andHC5N toward this source. The temperature and distribution ofCH3CN toward CRL 618 indicates that it is formed in the outerenvelope surrounding the ultracompact H II region. Furthermore, the PCygni line profile and corresponding channel maps of vibrationallyexcited HC5N support its distribution in the extendedenvelope expanding radially from the central star. The detection ofvibrationally excited HC3N confirmed the temperaturestructure and column density of HC3N in the inner envelopefound by Wyrowski and colleagues. Finally, our observations clearlyindicate that CRL 618 is a good source of large carbon chain species buta very poor source for detecting or producing organic species ofbiological importance.
| CO J = 2-1 and 4-3 Observations of Proto-planetary Nebulae: Time-variable Mass Loss
Observations made with the Heinrich Hertz Telescope of CO millimeter andsubmillimeter emission toward a sample of 22 proto-planetary nebula(PPN) candidates resulted in detections of 12 sources in the CO J=2-1line. Of these 12, seven sources were also detected in the J=4-3 line.These 4-3 transitions are the highest yet observed in all but one ofthese PPNs. Statistical equilibrium/radiative transfer models werecalculated for the CO emission in the circumstellar envelopes (CSEs),assuming various power-law density distributions. These models werecompared with the intensity and profile shape of the observed spectra.For the region of the CSE probed by CO emission, the density laws mustbe steeper than inverse squared and are consistent with power lawsbetween ρ~r-3 and r-4. These radial densitydistributions imply that the mass loss was not constant but increasedduring the last part of the asymptotic giant branch (AGB) phase.Mass-loss rates at the end of the AGB for the three best-constrainedsources are found to be 7.7×10-5 Msolaryr-1 (IRAS 22272+5435), 2.3×10-5Msolar yr-1 (IRAS 07134+1005), and1.3×10-5 Msolar yr-1 (IRAS17436+5003) for the case of ρ~r-3. These time-varyingmass-loss rates can be integrated to calculate the enclosed envelopemasses ejected in the past ~10,000 yr. The ejected envelope masses closeto the star lie in the range 0.02-0.30 Msolar these valuesare consistent with theoretical models, which indicate that <20% ofthe stellar mass loss occurs in the last 10,000 years of the AGB. Theseresults are in contrast to some recent dust studies based on infraredemission, however, in which much higher envelope masses are determined.The density laws, mass-loss rates, and enclosed envelope masses that wederive furnish important constraints for evolutionary models of stars inthe late AGB and during the transition to the planetary nebula phase.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| Molecular hydrogen kinematics in the ring-like planetary nebula NGC 6781
This paper presents the radial velocity mapping of the molecularhydrogen in the ring-like planetary nebula NGC 6781. Observations of theH2 v=1{-}0 S(1) emission line (2.122 μm, Δ λ=0.02 μm)were obtained by Fabry-Pérot spectroscopy. These observationsprovide a very detailed map of the kinematic structure of the molecularhydrogen envelope in NGC 6781. The kinematic structure is explained witha model consisting of a thin hollow cylinder whose axis is tilted withrespect to the line of sight, and gas expanding radially outward with avelocity proportional to the distance to the central star. Molecularhydrogen emission was detected at LSR velocities ranging from -6.2 to+57.1 km s-1, with peak intensities between +15.7 and +25.5km s-1. There is evidence that the molecular hydrogenenvelope is excited by shocks from the ionization front. The mass ofmolecular gas is estimated to be of the order of 0.2 M_ȯ. Theemergent picture from the H2 kinematic structure supports the scenariowhere the structure of NGC 6781 was formed from the ionization anddestruction of an ellipsoidal molecular envelope that began with thedestruction and ionization of the least dense polar caps and willcontinue until the densest molecular material at nebular waist is fullyionized.
| The Binarity of η Carinae and Its Similarity to Related Astrophysical Objects
I examine some aspects of the interaction between the massive star ηCarinae and its companion, in particular during the eclipse-like event,known as the spectroscopic event or the shell event. The spectroscopicevent is thought to occur when near periastron passages the stellarcompanion induces much higher mass-loss rate from the primary starand/or enters into a much denser environment around the primary star. Ifind that enhanced mass-loss rate during periastron passages, if itoccurs, might explain the high eccentricity of the system. However,there is not yet a good model to explain the presumed enhanced mass-lossrate during periastron passages. In the region where the winds from thetwo stars collide, a dense slow flow is formed, such that large dustgrains may be formed. Unlike the case during the 19th century GreatEruption, the companion does not accrete mass during most of its orbitalmotion. However, near periastron passages short accretion episodes mayoccur, which may lead to pulsed ejection of two jets by the companion.The companion may ionize a nonnegligible region in its surrounding,resembling the situation in symbiotic systems. I discuss the relation ofsome of these processes to other astrophysical objects, by incorporatingη Car into a large class of astrophysical bipolar nebulae.
| Reconstructing the evolution of white dwarf binaries: further evidence for an alternative algorithm for the outcome of the common-envelope phase in close binaries
We determine the possible masses and radii of the progenitors of whitedwarfs in binaries from fits to detailed stellar evolution models anduse these to reconstruct the mass-transfer phase in which the whitedwarf was formed. We confirm the earlier finding that in the first phaseof mass transfer in the binary evolution leading to a close pair ofwhite dwarfs, the standard common-envelope formalism (theα-formalism) equating the energy balance in the system (implicitlyassuming angular momentum conservation) does not work. An algorithmequating the angular momentum balance (implicitly assuming energyconservation) can explain the observations. This conclusion is now basedon 10 observed systems rather than three. With the latter algorithm (theγ-algorithm) the separation does not change much for approximatelyequal-mass binaries. Assuming constant efficiency in the standardα-formalism and a constant value of γ, we investigate theeffect of both methods on the change in separation in general andconclude that when there is observational evidence for strong shrinkageof the orbit, the γ-algorithm also leads to this. We then extendour analysis to all close binaries with at least one white dwarfcomponent and reconstruct the mass-transfer phases that lead to thesebinaries. In this way we find all possible values of the efficiency ofthe standard α-formalism and of γ that can explain theobserved binaries for different progenitor and companion masses. We findthat all observations can be explained with a single value of γ,making the γ-algorithm a useful tool to predict the outcome ofcommon-envelope evolution. We discuss the consequences of our findingsfor different binary populations in the Galaxy, including massivebinaries, for which the reconstruction method cannot be used.
| XMM-Newton Observations of Hot Gas in Two Bipolar Planetary Nebulae: NGC 2346 and NGC 7026
Planetary Nebulae (PNe) consist of material ejected by stars with masses≤8-10 Mȯ and form through the interaction between thecurrent fast stellar wind and previous asymptotic giant branch (AGB)wind. PNe exhibit a large variety of shapes but most can be classifiedas either elliptical or bipolar. Different scenarios have been suggestedfor these two broad classes of PNe, including non-isotropic mass loss inthe AGB phase, early shaping of the nebula through fast collimatedoutflows, and binarity. The impact of the fast stellar winds orcollimated outflows, with velocities often >1000 km s-1,produces shock-heated gas that fills the interior of the PN and isexpected to emit X-rays. Indeed, XMM-Newton and Chandra have detecteddiffuse X-ray emission from several elliptical PNe, but the large numberof Chandra observations of bipolar PNe have yielded positive detectionsin only two young nebulae - NGC 7027 and Mz 3.We have used XMM-Newton to observe two evolved bipolar PNe: NGC 2346 andNGC 7026. These two PNe are selected because of their different bipolarlobe structures, open lobes in NGC 2346 and closed lobes in NGC 7026.Diffuse X-ray emission is detected from only NGC 7026. The X-rayemission fills its bipolar lobes and shows a spectrum consistent withthin plasma emission at a temperature of 1.5×106 K. ItsX-ray luminosity is ˜1×1032 ergs s-1for a distance of 1.9 kpc.We have examined all available Chandra and XMM-Newton observations ofbipolar PNe and find that diffuse X-ray emission is detected only fromthree nebulae with closed lobe structures. No bipolar PNe with openlobes have been detected in X-rays. Evidently, hot gas is dispersed inopen lobes but can be confined within closed lobes to produce detectableX-ray emission.
| High Spatial Resolution Mid- and Far-infrared Imaging Study of NGC 2346
We present the first high spatial resolution mid-IR and far-IR images ofthe planetary nebula NGC 2346 using the Multiband Imaging Photometer forSpitzer (MIPS) on board the Spitzer Space Telescope. The morphology at24 μm is strikingly similar to that seen in the optical image, withan additional unresolved hot dust component at the center. The 70 μmimage reveals a torus-like structure near the narrow waist of thebipolar nebula, while the 160 μm image shows a very extended colddust envelope around the nebula. Our new MIPS data provide directevidence that the mass loss in the end of the asymptotic giant branch(AGB) phase diverts from spherical symmetry to axisymmetry and that thebinary central star has played an important role in the bipolar nebularevolution.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the Jet Propulsion Laboratory, California Institute ofTechnology, under NASA contract 1407.
| Why a Single-Star Model Cannot Explain the Bipolar Nebula of η Carinae
I examine the angular momentum evolution during the 1837-1856 GreatEruption of the massive star η Carinae. I find that the new estimateof the mass blown during that eruption implies that the envelope ofη Car substantially spun down during the 20 yr eruption. Single-starmodels, most of which require the envelope to rotate at close to thebreak-up velocity, cannot account for the bipolar nebula-theHomunculus-formed from matter expelled in that eruption. The kineticenergy and momentum of the Homunculus further constrain single-starmodels. I discuss how η Car can fit into a unified model for theformation of bipolar lobes in which two oppositely ejected jets inflatetwo lobes (or bubbles). These jets are blown by an accretion disk, whichrequires stellar companions in the case of bipolar nebulae aroundstellar objects.
| Me 1-1: A PN containing a cool star
We report the detection of a cool stellar component at the center of theplanetary nebula (PN) Me 1-1 and present optical spectra of the system.From measurements of nebular emission lines, we have derived electrontemperature, density and chemical composition. Heavy elementalabundances deduced from collisionally excited lines (CELs) are comparedwith those derived from optical recombination lines (ORLs). The electrontemperature and density deduced from the nebular analysis were used tocalculate the nebular continuum emission, which was then subtracted fromthe observed spectrum in order to obtain the spectrum of the coolstellar component apparent in the observed spectrum. We calculate B andV magnitudes of the cool companion and obtain a color index of B-V=1.20.By comparing the spectrum of the cool star with standard spectra inPickles's 1998 Stellar Flux Library, we find that the cool component hasthe spectral type of a K3-4 bright giant. Our analysis suggests that Me1-1 is probably a yellow symbiotic system.
| Energy and Angular Momentum Deposition During Common Envelope Evolution
I consider three processes which enhance mass loss rate from a commonenvelope of a giant star with a main sequence or a white dwarf companionspiraling-in inside its envelope. I consider deposition of orbitalenergy and orbital angular momentum to the giant's envelope, and theformation of jets by an accreting companion and their propagation in theenvelope. I find that in many cases the deposition of orbital angularmomentum to the envelope may be more important to the mass loss processthan the deposition of orbital energy. Jets blown by an accretingcompanion, in particular a white dwarf, orbiting inside the outerregions of the giant's envelope may also dominate over orbital energydeposition at early stage of the common envelope evolution. These implythat studies which ignore the deposition of angular momentum to theenvelope and the effects of the accreting companion may reach wrongconclusions.
| Energy and angular momentum deposition during common envelope evolution
I consider three processes which enhance mass loss rate from a commonenvelope of a giant star with a main sequence or a white dwarf companionspiraling-in inside its envelope. I consider deposition of orbitalenergy and orbital angular momentum to the giant's envelope, and in moredetail the formation of jets by an accreting companion and theirpropagation in the envelope. I find that in many cases the deposition oforbital angular momentum to the envelope may be more important to themass loss process than the deposition of orbital energy. Jets blown byan accreting companion, in particular a white dwarf, orbiting inside theouter regions of the giant's envelope also dominate over orbital energydeposition at early stages of the common envelope evolution. These implythat studies which ignore the deposition of angular momentum to theenvelope and the effects of the accreting companion may reach wrongconclusions.
| A reanalysis of chemical abundances in galactic PNe and comparison with theoretical predictions
New determinations of chemical abundances for He, N, O, Ne, Ar and Sare derived for all galactic planetary nebulae (PNe) so far observedwith a relatively high accuracy, in an effort to overcome differences inthese quantities obtained over the years by different authors usingdifferent procedures. These include: ways to correct for interstellarextinction, the atomic data used to interpret the observed line fluxes,the model nebula adopted to represent real objects and the ionizationcorrections for unseen ions. A unique `good quality' classical-typeprocedure, i.e. making use of collisionally excited forbidden lines toderive ionic abundances of heavy ions, has been applied to allindividual sets of observed line fluxes in each specific position withineach PN. Only observational data obtained with linear detectors, andsatisfying some `quality' criteria, have been considered. Suchobservations go from the mid-1970s up to the end of 2001. Theobservational errors associated with individual line fluxes have beenpropagated through the whole procedure to obtain an estimate of theaccuracy of final abundances independent of an author's `prejudices'.Comparison of the final abundances with those obtained in relevantmulti-object studies on the one hand allowed us to assess the accuracyof the new abundances, and on the other hand proved the usefulness ofthe present work, the basic purpose of which was to take full advantageof the vast amount of observations done so far of galactic PNe, handlingthem in a proper homogeneous way. The number of resulting PNe that havedata of an adequate quality to pass the present selection amounts to131. We believe that the new derived abundances constitute a highlyhomogeneous chemical data set on galactic PNe, with realisticuncertainties, and form a good observational basis for comparison withthe growing number of predictions from stellar evolution theory. Owingto the known discrepancies between the ionic abundances of heavyelements derived from the strong collisonally excited forbidden linesand those derived from the weak, temperature-insensitive recombinationlines, it is recognized that only abundance ratios between heavyelements can be considered as satisfactorily accurate. A comparison withtheoretical predictions allowed us to assess the state of the art inthis topic in any case, providing some findings and suggestions forfurther theoretical and observational work to advance our understandingof the evolution of low- and intermediate-mass stars.
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