Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Near-infrared polarimetry and modelling of the dusty young planetary nebula IRAS 19306+1407
We present near-infrared polarimetric images of the dusty circumstellarenvelope (CSE) of IRAS 19306+1407, acquired at the United KingdomInfrared Telescope (UKIRT) using the UKIRT 1-5 μm Imager Spectrometer(UIST) in conjunction with the half-waveplate module IRPOL2. We presentadditional 450- and 850-μm photometry data obtained with theSubmillimetre Common-User Bolometer Array (SCUBA) at the James ClerkMaxwell Telescope (JCMT), as well as archived Hubble Space Telescope(HST) F606W- and F814W-filter images. The CSE structure in polarizedflux at J and K bands shows an elongation north of north-east and southof south-west with two bright scattering shoulders north-west andsouth-east. These features are not perpendicular to each other and couldsignify a recent `twist' in the outflow axis. We model the CSE using anaxisymmetric light scattering (ALS) code to investigate the polarizationproduced by the CSE, and an axisymmetric radiation transport (DART) codeto fit the spectral energy distribution. A good fit was achieved withthe ALS and DART models using silicate grains, 0.1-0.4 μm with apower-law size distribution of a-3.5, and an axisymmetricshell geometry with an equator-to-pole ratio of 7:1. The spectral typeof the central star is determined to be B1I supporting previoussuggestions that the object is an early planetary nebula. We haveconstrained the CSE and interstellar extinction as 2.0 and 4.2 mag,respectively, and have estimated a distance of 2.7 kpc. At thisdistance, the stellar luminosity is ~4500Lsolar and the massof the CSE is ~0.2Msolar. We also determine that the massloss lasted for ~5300 yr with a mass-loss rate of ~3.4 ×10-5Msolaryr-1.
| Low-mass binary-induced outflows from asymptotic giant branch stars
A significant fraction of planetary nebulae (PNe) and protoplanetarynebulae (PPNe) exhibit aspherical, axisymmetric structures, many ofwhich are highly collimated. The origin of these structures is notentirely understood, however, recent evidence suggests that manyobserved PNe harbour binary systems, which may play a role in theirshaping. In an effort to understand how binaries may produce suchasymmetries, we study the effect of low-mass (<0.3Msolar)companions (planets, brown dwarfs and low-mass main-sequence stars)embedded into the envelope of a 3.0-Msolar star during threeepochs of its evolution [red giant branch, asymptotic giant branch(AGB), interpulse AGB]. We find that common envelope evolution can leadto three qualitatively different consequences: (i) direct ejection ofenvelope material resulting in a predominately equatorial outflow, (ii)spin-up of the envelope resulting in the possibility of powering anexplosive dynamo-driven jet and (iii) tidal shredding of the companioninto a disc which facilitates a disc-driven jet. We study how thesefeatures depend on the secondary's mass and discuss observationalconsequences.
| Low-temperature single crystal reflection spectra of forsterite
The infrared reflectivities of crystalline forsterite(Mg2SiO4) were measured for the temperature range295-50 K for each crystal axis, between wavenumber 5000 and 100cm-1. The reflection spectra show clear dependence oftemperature; most of the bands become more intense, sharper and theirpeak positions shift to higher wavenumber with decreasing temperature.Reflection spectra were fitted with dispersion formula of dampedoscillator model of the dielectric constants and the oscillatorparameters in the model were derived. The absorption spectra offorsterite particle are calculated with the derived dielectric constantsto show that the forsterite features are good thermal indicator for coldtemperature range below 295 K.
| Blueshifted diffuse interstellar bands in the spectrum of HD 34078
In this paper, we report the very first observation of diffuseinterstellar bands (DIBs) that, in the spectrum of HD 34078 (AE Aur),are blueshifted with respect to the normal position that they have inother objects, where the rest-wavelength velocity frame is determinedusing very sharp interstellar atomic lines or molecular features. Onlyreasonably narrow DIBs seemingly show this effect, which is absent inbroader ones. The result is confirmed independently using threedifferent spectrographs attached to two different telescopes.
| Binary post-AGB stars and their Keplerian discs .
In this contribution we give a progress report on our systematic studyof a large sample of post-AGB stars. The sample stars were selected onthe basis of their infrared colours and the selection criteria weretuned to discover objects with hot dust in the system. We started a veryextensive, multi-wavelength programme which includes the analysis of ourradial velocity monitoring; our optical high-resolution spectra; ourgroundbased N-band spectral data as well as the Spitzer full spectralscans; the broad-band SED and the high spatial-resolutioninterferometric experiments with the VLTI. In this contribution wehighlight the main results obtained so far and argue that all systems inour sample are indeed binaries, which are surrounded by dusty Kepleriancircumbinary discs. The discs play a lead role in the evolution of thesystems.
| Identification and Spectral Properties of Polycyclic Aromatic Hydrocarbons in Carbonaceous Soot Produced by Laser Pyrolysis
Carbon soot has been prepared by laser-induced pyrolysis of a mixture ofethylene (C2H4) and benzene(C6H6) vapor. The soluble part of the carbonaceouspowder has been separated from its insoluble counterpart by soxhletextraction in toluene. Several techniques were applied to obtaininformation on the composition of the extract. These included UV/visibleand IR spectroscopy in solid and liquid phase, gas chromatographycombined with mass spectrometry, gas-phase laser spectroscopy in asupersonic jet, and matrix spectroscopy in helium droplets, the latterbeing also combined with mass spectrometry. The analysis revealed thatthe carbonaceous powder contained various polycyclic aromatichydrocarbons (PAHs). The highest concentration was found for thethree-membered catacondensed PAHs, phenanthrene and anthracene. Theresults are discussed in view of the possible role of these molecules asinterstellar dust components.
| Optical Emission Band Morphologies of the Red Rectangle
We present narrowband images of the Red Rectangle (RR) Nebula thatreveal the distinct morphologies of this intriguing nebula in differentoptical emission bands. The morphology of the RR nebula in blueluminescence (BL) and extended red emission (ERE) are almost mutuallyexclusive. We also present the optical detection of the circumbinarydisk of the RR in the light of the BL. The total intensities from thetwo optical band emissions (BL and ERE) when summed over the nebula areof comparable magnitude. Their spatial distributions with respect to theembedded illumination sources lead us to suggest that they may beattributed to different ionization stages of the same family ofcarriers.
| A Massive Bipolar Outflow and a Dusty Torus with Large Grains in the Preplanetary Nebula IRAS 22036+5306
We report high angular resolution (~1") CO J=3-2 interferometric mappingusing the Submillimeter Array (SMA) of IRAS 22036+5306 (I22036), abipolar preplanetary nebula (PPN) with knotty jets discovered in our HSTsnapshot survey of young PPNs. In addition, we have obtained supportinglower resolution (~10") CO and 13CO J=1-0 observations withthe Owens Valley Radio Observatory (OVRO) interferometer, as well asoptical long-slit echelle spectra at the Palomar Observatory. The COJ=3-2 observations show the presence of a very fast (~220 kms-1), highly collimated, massive (0.03 Msolar)bipolar outflow with a very large scalar momentum (about 1039g cm s-1), and the characteristic spatiokinematic structureof bow shocks at the tips of this outflow. The Hα line shows anabsorption feature blueshifted from the systemic velocity by ~100 kms-1, which most likely arises in neutral interface materialbetween the fast outflow and the dense walls of the bipolar lobes at lowlatitudes. The fast outflow in I22036, as in most PPNs, cannot be drivenby radiation pressure. We find an unresolved source of submillimeter(and millimeter-wave) continuum emission in I22036, implying a verysubstantial mass (0.02-0.04 Msolar) of large (radius >~1mm), cold (<~50 K) dust grains associated with I22036's toroidalwaist. We also find that the 13C/12C ratio inI22036 is very high (0.16), close to the maximum value achieved inequilibrium CNO nucleosynthesis (0.33). The combination of the highcircumstellar mass (i.e., in the extended dust shell and the torus) andthe high 13C/12C ratio in I22036 provides strongsupport for this object having evolved from a massive (>~4Msolar) progenitor in which hot-bottom-burning has occurred.
| A Post-AGB Star in the Small Magellanic Cloud Observed with the Spitzer Infrared Spectrograph
We have observed an evolved star with a rare combination of spectralfeatures, MSX SMC 029, in the Small Magellanic Cloud (SMC) using thelow-resolution modules of the Infrared Spectrograph on the Spitzer SpaceTelescope. A cool dust continuum dominates the spectrum of MSX SMC 029.The spectrum also shows both emission from polycyclic aromatichydrocarbons (PAHs) and absorption at 13.7 μm fromC2H2, a juxtaposition seen in only two othersources, AFGL 2688 and IRAS 13416-6243, both post-asymptotic giantbranch (AGB) objects. As in these sources, the PAH spectrum has theunusual trait that the peak emission in the 7-9 μm complex liesbeyond 8.0 μm. In addition, the 8.6 μm feature has an intensity asstrong as the CC modes that normally peak between 7.7 and 7.9 μm. Therelative flux of the feature at 11.3 μm to that at 8 μm suggeststhat the PAHs in MSX SMC 029 either have a low-ionization fraction orare largely unprocessed. The 13-16 μm wavelength region shows strongabsorption features similar to those observed in the post-AGB objectsAFGL 618 and SMP LMC 11. This broad absorption may arise from the samemolecules that have been identified in those sources:C2H2, C4H2, HC3N,and C6H6. The similarities between MSX SMC 029,AFGL 2688, and AFGL 618 lead us to conclude that MSX SMC 029 has evolvedoff the AGB in only the past few hundred years, making it the thirdpost-AGB object identified in the SMC.
| 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.
| Detecting Interstellar Alkanes
Infrared spectra of long-chain n-alkane molecules, CH3(CH2)mCH3, with m>8, havebeen obtained in absorption and emission and are compared to spectra ofa variety of interstellar sources. Absorption spectra of these moleculesin the 3.4 μm region are remarkably similar, consisting of narrowfeatures at 3.384, 3.425, and 3.508 μm, independent of chain length.These features are not seen in absorption in interstellar clouds. Weestimate that the column density of CH2 groups in long-chainn-alkanes is typically N<(1.2-2.9)×1016cm-2 in VI Cyg 12. Given the narrow width of these features,this limit could be improved with higher resolution observations.Thermal spectra of these molecules show a number of characteristicfeatures near 6.8, 7.3, and 13.9 μm that should be useful indetermining whether large n-alkane molecules are present in interstellaremission sources.
| Accreting White Dwarfs among the Planetary Nebulae Most Luminous in [O III] λ5007 Emission
I propose that some of the most luminous planetary nebulae (PNs) areactually proto-PNs, where a companion white dwarf (WD) accretes mass ata relatively high rate from the post-asymptotic giant branch star thatblew the nebula. The WD sustains a continuous nuclear burning andionizes the nebula. The WD is luminous enough to make the dense nebulaluminous in the [O III] λ5007 line. In young stellar populationsthese WD accreting systems account for a small fraction of [OIII]-luminous PNs, but in old stellar populations these binaries mightaccount for most, or even all, of the [O III]-luminous PNs. This mightexplain the puzzling constant cutoff (maximum) [O III] λ5007luminosity of the planetary nebula luminosity function across differentgalaxy types.
| Revealing the Mid-Infrared Emission Structure of IRAS 16594-4656 and IRAS 07027-7934
TIMMI2 diffraction-limited mid-infrared images of a multipolarproto-planetary nebula IRAS 16594-4656 and a young [WC] ellipticalplanetary nebula IRAS 07027-7934 are presented. Their dust shells arefor the first time resolved (only marginally in the case of IRAS07027-7934) by applying the Lucy-Richardson deconvolution algorithm tothe data, taken under exceptionally good seeing conditions (<=0.5").IRAS 16594-4656 exhibits a two-peaked morphology at 8.6, 11.5, and 11.7μm, which is mainly attributed to emission from PAHs. Ourobservations suggest that the central star is surrounded by a toroidalstructure, observed edge-on, with a radius of 0.4" (~640 AU at anassumed distance of 1.6 kpc) and with its polar axis atP.A.~80deg, coincident with the orientation defined by onlyone of the bipolar outflows identified in the HST optical images. Wesuggest that the material expelled from the central source is currentlybeing collimated in this direction and that the multiple outflowformation has not been coeval. IRAS 07027-7934 shows a bright,marginally extended emission (FWHM=0.3") in the mid-infrared with aslightly elongated shape along the north-south direction, consistentwith the morphology detected by HST in the near-infrared. Themid-infrared emission is interpreted as the result of the combinedcontribution of small, highly ionized PAHs and relatively hot dustcontinuum. We propose that IRAS 07027-7934 may have recently experienceda thermal pulse (likely at the end of the AGB) which has produced aradical change in the chemistry of its central star.Based on observations collected at the European Southern Observatory (LaSilla, Chile), on observations made with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, Netherlands, and the United Kingdom) with theparticipation of ISAS and NASA, and on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy (AURA), Inc., under NASAcontract NAS5-26555.
| Nanodiamond as a Possible Carrier of Extended Red Emission
Diamond nanocrystals (size ~100 nm) emit bright luminescence at 600-800nm when exposed to green and yellow photons. The photoluminescence,arising from excitation of the nitrogen-vacancy defect centers createdby proton-beam irradiation and thermal annealing, closely resembles theextended red emission (ERE) bands observed in reflection nebulae andplanetary nebulae. The central wavelength of the emission is ~700 nm,and it blueshifts to ~660 nm as the excitation wavelength decreases from535 to 470 nm as the result of a combined excitation of two differentdetect centers [(N-V)- and (N-V)0]. Ourobservations lend support to the suggestion that nanodiamond is apossible carrier for the ERE band.
| A Plasmon Resonance in Dehydrogenated Coronene (C24Hx) and Its Cations and the Origin of the Interstellar Extinction Band at 217.5 Nanometers
Analysis of the wavelength dependence of extinction in the ultravioletis shown to provide additional insight into the physical properties ofthe carrier of the interstellar absorption feature at 217.5 nm (5.7 eV).In particular, it is found that this band has a cutoff at both long andshort wavelengths, supporting the assignment of this feature to aplasmon band. The photon energies corresponding to these cutoffs arehighly diagnostic of the carrier of this feature and suggest that theabsorber is a seven-ring aromatic molecule. Comparison with laboratorydata, together with theoretical calculations of the plasmon resonance insmall dehydrogenated aromatic-ring structures, supports the assignmentof this feature to the π-π* plasmon resonance in dehydrogenatedcoronene molecules, C24Hx and its cations, wherex<=3. It is shown that this assignment is consistent with availableobservational data on the 217.5 nm feature, including such properties asa constant central wavelength, variable width, its relation toE(1250-V)/E(B-V), and the carbon abundance in the interstellar medium.The π and π* energy levels in the 217.5 nm carrier, derived froman analysis of interstellar extinction, have been used to construct itsenergy-level diagram, enabling comparison with other observational data
| Sequence Structure Emission in the Red Rectangle Bands
We report high-resolution (R~37,000) integral field spectroscopy of thecentral region (r<14'') of the Red Rectangle nebulasurrounding HD 44179. The observations focus on the λ5800emission feature, the bluest of the yellow/red emission bands in the RedRectangle. We propose that the emission feature, widely believed to be amolecular emission band, is not a molecular rotation contour, but avibrational contour caused by overlapping sequence bands from a moleculewith an extended chromophore. We model the feature as arising in apolycyclic aromatic hydrocarbon (PAH) with 45-100 carbon atoms.
| Polycyclic Aromatic Hydrocarbons Orbiting HD 233517, an Evolved Oxygen-rich Red Giant
We report spectra obtained with the Spitzer Space Telescope in theλ = 5-35 μm range of HD 233517, an evolved K2 III giant withcircumstellar dust. For λ > 13 μm, the flux is a smoothcontinuum that varies approximately as ν-5/3. For λ< 13 μm, although the star is oxygen-rich, PAH features producedby carbon-rich species at 6.3, 8.2, 11.3, and 12.7 μm are detectedalong with likely broad silicate emission near 20 μm. These resultscan be explained if there is a passive, flared disk orbiting HD 233517.Our data support the hypothesis that organic molecules in orbiting disksmay be synthesized in situ as well as having been incorporated from theinterstellar medium.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| The Excitation of Extended Red Emission: New Constraints on Its Carrier from Hubble Space Telescope Observations of NGC 7023
The carrier of the dust-associated photoluminescence process causing EREin many dusty interstellar environments remains unidentified. Severalcompeting models are more or less able to match the observed broad,unstructured ERE band. We now constrain the character of the ERE carrierfurther by determining the wavelengths of the radiation that initiatesthe ERE. Using the imaging capabilities of the HST, we have resolved thewidth of narrow ERE filaments appearing on the surfaces of externallyilluminated molecular clouds in the bright reflection nebula NGC 7023and compared them with the depth of penetration of radiation of knownwavelengths into the same cloud surfaces. We identify photons withwavelengths shortward of 118 nm as the source of ERE initiation, not tobe confused with ERE excitation, however. There are strong indicationsfrom the well-studied ERE in the Red Rectangle Nebula and in thehigh-|b| Galactic cirrus that the photon flux with wavelengths shortwardof 118 nm is too small to actually excite the observed ERE, even with100% quantum efficiency. We conclude, therefore, that ERE excitationresults from a two-step process. The first, involving far-UV photonswith E>10.5 eV, leads to the creation of the ERE carrier, most likelythrough photoionization or photodissociation of an existing precursor.The second, involving more abundant near-UV/optical photons, consists ofthe optical pumping of the previously created carrier, followed bysubsequent deexcitation via photoluminescence. The latter process canoccur many times for a single particle, depending upon the lifetime ofthe ERE carrier in its active state. While none of the previouslyproposed ERE models can match these new constraints, we note that underinterstellar conditions most PAH molecules are ionized to the dicationstage by photons with E>10.5 eV and that the electronic energy levelstructure of PAH dications is consistent with fluorescence in thewavelength band of the ERE. Therefore, PAH dications deserve furtherstudy as potential carriers of the ERE.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555. These observations are associated with program9471.
| A general model for the identification of specific PAHs in the far-IR
Context: .In the framework of the interstellar PAH hypothesis, far-IRskeletal bands are expected to be a fingerprint of single species inthis class. Aims: .A detailed model of the photo-physics ofinterstellar PAHs is required for such single-molecule identification oftheir far-IR features in the presently available Infrared SpaceObservatory data and in those of the forthcoming Herschel SpaceObservatory mission. Methods: .We modelled the detailedphotophysics of a vast sample of species in different radiation fields,using a compendium of Monte-Carlo techniques and quantum-chemicalcalculations. This enabled us to validate the use of purely theoreticaldata and assess the expected accuracy and reliability of the resultingsynthetic far-IR emission spectra. Results: .We produce positionsand intensities of the expected far-IR features that ought to be emittedby each species in the sample in the considered radiation fields. Acomposite emission spectrum for our sample is computed for one of themost favourable sources for detection, namely the Red Rectangle nebula.The resulting spectrum is compared with the estimated dust emission inthe same source to assess the dependence of detectability on keymolecular parameters. Conclusions: .Identifying specific PAHs fromtheir far-IR features is going to be a difficult feat in general; still,it may well be possible under favourable conditions.
| Diagnostics for specific PAHs in the far-IR: searching neutral naphthalene and anthracene in the Red Rectangle
Context: .In the framework of the interstellar polycyclic aromatichydrocarbons (PAHs) hypothesis, far-IR skeletal bands are expected to bethe fingerprints of single species in this class. Aims: .Weaddress the question of detectability of low energy PAH vibrationalbands, with respect to spectral contrast and intensity ratio with"classical" Aromatic Infrared Bands (AIBs). Methods: .We extendour established Monte Carlo model of the photophysics of specific PAHsin astronomical environments to include rotational and anharmonic bandstructure. The required molecular parameters were calculated in theframework of the Density Functional Theory. Results: .We calculatethe detailed spectral profiles of three low-energy vibrational bands ofneutral naphthalene and four low-energy vibrational bands of neutralanthracene. They are used to establish detectability constraints basedon intensity ratios with "classical" AIBs. A general procedure issuggested to select promising diagnostics and tested on availableInfrared Space Observatory data for the Red Rectanglenebula. Conclusions: .The search for single, specific PAHs in thefar-IR is a challenging, but promising task, especially in view of theforthcoming launch of the Herschel Space Observatory.
| A close look into the carbon disk at the core of the planetary nebula CPD-56°8032
Aims.We present high spatial resolution observations of the dusty coreof the Planetary Nebula with Wolf-Rayet central star CPD-56°8032,for which indications of a compact disk have been found by HST/SITSobservations. Methods: .These observations were taken with themid-infrared interferometer VLTI/MIDI in imaging mode providing atypical 300 mas resolution and in interferometric mode using UT2-UT3 47mbaseline providing a typical spatial resolution of 20 mas. We also madeuse of unpublished HST/ACS images in the F435W and F606W filters.Results: .The visible HST images exhibit a complex multilobal geometrydominated by faint lobes. The farthest structures are located at 7arcsec from the star. The mid-IR environment of CPD-56°8032 isdominated by a compact source, barely resolved by a single UT telescopein a 8.7 μm filter (Δ λ=1.6~μm, contaminated by PAHemission). The infrared core is almost fully resolved with the three40-45 m projected baselines ranging from -5° to 51° but smoothoscillating fringes at low level have been detected in spectrallydispersed visibilities. This clear signal is interpreted in terms of aring structure which would define the bright inner rim of the equatorialdisk. Geometric models allowed us to derive the main geometricalparameters of the disk. For instance, a reasonably good fit is reachedwith an achromatic and elliptical truncated Gaussian with a radius of97±11 AU, an inclination of 28±7° and a PA for themajor axis at 345° ±7°. Furthermore, we performed someradiative transfer modeling aimed at further constraining the geometryand mass content of the disk, by taking into account the MIDI dispersedvisibilities, spectra, and the large aperture SED of the source. Thesemodels show that the disk is mostly optically thin in the N band andhighly flared. As a consequence of the complex flux distribution, anedge-on inclination is not excluded by the data.
| Properties of dust in the high-latitude translucent cloud L1780. I. Spatially distinct dust populations and increased dust emissivity from ISO observations
We have analyzed the properties of dust in the high galactic latitudetranslucent cloud Lynds 1780 using ISOPHOT maps at 100 μm and 200μm and raster scans at 60 μm, 80 μm, 100 μm, 120 μm, 150μm and 200 μm. In far-infrared (FIR) emission, the cloud has asingle core that coincides with the maxima of visual extinction and 200μm optical depth. At the resolution of 3.0 arcmin, the maximum visualextinction is 4.0 mag. At the cloud core, the minimum temperature andthe maximum 200 μm optical depth are 14.9 ± 0.4 K and 2.0± 0.2 × 10-3, respectively, at the resolution of1.5 arcmin. The cloud mass is estimated to be 18 Mȯ. TheFIR observations, combined with IRAS observations, suggest the presenceof different, spatially distinct dust grain populations in the cloud:the FIR core region is the realm of the "classical" large grains,whereas the very small grains and the PAHs have separate maxima on theEastern side of the cold core, towards the "tail" of thiscometary-shaped cloud. The color ratios indicate an overabundance ofPAHs and VSGs in L1780. Our FIR observations combined with the opticalextinction data indicate an increase of the emissivity of the big graindust component in the cold core, suggesting grain coagulation or someother change in the properties of the large grains. Based on ourobservations, we also address the question, to what extent the 80 μmemission and even the 100 μm and the 120 μm emission contain acontribution from the small-grain component.
| Post-AGB stars as testbeds of nucleosynthesis in AGB stars
We construct a data base of 125 post-AGB objects (including R CrB andextreme helium stars) with published photospheric parameters (effectivetemperature and gravity) and chemical composition. We estimate themasses of the post-AGB stars by comparing their position in the (logT{eff}, log g) plane with theoretical evolutionary tracks ofdifferent masses. We construct various diagrams, with the aim of findingclues to AGB nucleosynthesis. This is the first time that a large sampleof post-AGB stars has been used in a systematic way for such a purposeand we argue that, in several respects, post-AGB stars should be morepowerful than planetary nebulae to test AGB nucleosynthesis. Our mainfindings are that: the vast majority of objects which do not showevidence of N production from primary C have a low stellar mass(Mstar < 0.56 Mȯ); there is no evidencethat objects which did not experience 3rd dredge-up have a differentstellar mass distribution than objects that did; there is clear evidencethat 3rd dredge-up is more efficient at low metallicity. The sample ofknown post-AGB stars is likely to increase significantly in the nearfuture thanks to the ASTRO-F and follow-up observations, making theseobjects even more promising as testbeds for AGB nucleosynthesis.
| Resolving the compact dusty discs around binary post-AGB stars using N-band interferometry
We present the first mid-IR long baseline interferometric observationsof the circumstellar matter around binary post-AGB stars. Two objects,SX Cen and HD 52961, were observedusing the VLTI/MIDI instrument during Science Demonstration Time. Bothobjects are known binaries for which a stable circumbinary disc isproposed to explain the SED characteristics. This is corroborated by ourN-band spectrum showing a crystallinity fraction of more than 50% forboth objects, pointing to a stable environment where dust processing canoccur. Surprisingly, the dust surrounding SX Cen is not resolved in theinterferometric observations providing an upper limit of 11 mas (or 18AU at the distance of this object) on the diameter of the dust emission.This confirms the very compact nature of its circumstellar environment.The dust emission around HD 52961 originates from avery small but resolved region, estimated to be ~35 mas at 8 μm and~55 mas at 13 μm. These results confirm the disc interpretation ofthe SED of both stars. In HD 52961, the dust is nothomogeneous in its chemical composition: the crystallinity is clearlyconcentrated in the hotter inner region. Whether this is a result of theformation process of the disc, or due to annealing during the longstorage time in the disc is not clear.
| Temperature effects on the mid-and far-infrared spectra of olivine particles
The absorption spectra of the olivine particles of different Mg/Fecontent were measured in the infrared spectral region between 5 and 100μm, while the particles were continuously cooled down to 10 K.Measurements independently carried out on different samples of syntheticforsterite, natural olivine, and synthetic fayalite at laboratories inKyoto and Jena. The positions of the olivine infrared bands weremeasured for these samples in detail at up to seven individualtemperatures in the interval between 300 K and 10 K. According to thedifferent widths of the olivine bands in different wavelength regions,spectral resolutions of 2, 1, 0.5, 0.25, 0.2, and 0.125 cm-1were used in order to measure the band positions with high accuracy.While in general the band positions and their temperature-dependentshift agree very well for the Kyoto and Jena samples, the positions ofsome very strong bands differ, which is probably a consequence ofdifferent particle shapes. For the two long-wavelength forsterite bandsat 49 and 69 μm, the sharpening and strengthening of the bands werequantified. The widths of these bands differ for the Kyoto and Jenasamples, which is discussed in terms of different crystal quality andparticle coagulation of the samples. Our new data can be used to derivedust temperatures from the observed peak positions for crystallinesilicate dust in circumstellar regions.
| Keplerian discs around post-AGB stars: a common phenomenon?
Aims.We aim at showing that the broad-band SED characteristics of oursample of post-AGB stars are best interpreted, assuming thecircumstellar dust is stored in Keplerian rotating passivediscs.Methods.We present a homogeneous and systematic study of theSpectral Energy Distributions (SEDs) of a sample of 51 post-AGB objects.The selection criteria to define the whole sample were tuned to coverthe broad-band characteristics of known binary post-AGB stars. The wholesample includes 20 dusty RV Tauri stars from the General Catalogue ofVariable Stars (GCVS). We supplemented our own Geneva optical photometrywith literature data to cover a broad range of fluxes from the UV to thefar-IR.Results.All the SEDs display very similar characteristics: alarge IR excess with a dust excess starting near the sublimationtemperature, irrespective of the effective temperature of the centralstar. Moreover, when available, the long wavelength fluxes show ablack-body slope indicative of the presence of a component of large mmsized grains.Conclusions.We argue that in all systems, gravitationallybound dusty discs are present. The discs must be puffed-up to cover alarge opening angle for the central star and we argue that the discshave some similarity with the passive discs detected around youngstellar objects. We interpret the presence of a disc to be a signaturefor binarity of the central object, but this will need confirmation bylong-term monitoring of the radial velocities. We argue that dusty RVTauri stars are those binaries which happen to be in the Population IIinstability strip.
| Estimated IR and phosphorescence emission fluxes for specific polycyclic aromatic hydrocarbons in the Red Rectangle
Following tentative identification of the blue luminescence in theRed Rectangle by Vijh et al., we computed absolutefluxes for the vibrational IR emission and phosphorescence bands ofthree small polycyclic aromatic hydrocarbons. The calculated IR spectrawere compared with available ISO observations. A subset of the emissionbands are predicted to be observable using presently availablefacilities, and can be used for an immediate, independent,discriminating test of their alleged presence in this well-knownastronomical object.
| 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.
| 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).
| The dark lane of the planetary nebula NGC 6302*†
The butterfly-shaped planetary nebula, NGC 6302, shows a unique, denseequatorial dark lane, which is presumably a dusty disc, obscuring anunobserved, very hot central star. We trace the structure of this discusing Hubble Space Telescope Hα and [N II] images, Very LargeTelescope L-and M-band images at 0.4-arcsec resolution, includingBrα and polycyclic aromatic hydrocarbon (PAH) images, and a JamesClerk Maxwell Telescope 450-μm image. Extinction maps are derivedfrom these images. Within the disc, the extinction isAHα= 5-7 mag and ABrα= 1-2 mag. The450-μm map shows a north-south elongated central core, tracing themassive dust disc, and extended emission from dust in the bipolar flows.A fit to the spectral energy distribution yields the disc dust mass of0.03 Msolar. The innermost region shows an ionized shell. Theorientation of the polar axis shows a marked change between shell, discand inner and outer outflow. The structures are well described by thewarped-disc model of Icke (2003). PAH images are presented: PAH emissionis found in the shell but avoids the disc. An infrared source is foundclose to the expected location of the central star.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Једнорог |
| Ректацензија: | 06h19m58.22s |
| Deклинација: | -10°38'14.7" |
| Apparent магнитуда: | 9.047 |
| Даљина: | 381.679 parsecs |
| Proper motion RA: | -11.2 |
| Proper motion Dec: | -25.8 |
| B-T magnitude: | 9.462 |
| V-T magnitude: | 9.082 |
Каталог и designations:
|
