Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Spectroscopic observations of the rapid rotating post-AGB star IRAS 05381+1012
We report on the high-resolution stellar parameters and abundanceanalysis of the rapidly rotating post-AGB star IRAS 05381+1012. Analysisof high-resolution spectra shows that IRAS 05381+1012 has an effectivetemperature of T_eff=5200± 100 K and a surface gravity of logg=1.0± 0.5 corresponding to a spectral type G(2-3)I. Theseparameters result in an estimated luminosity of 970 Lȯand a distance of 2700 pc. We also show that IRAS 05381+1012 has aprojected rotational velocity vsin i=40± 10 km s-1.The abundance analysis based on a few available lines reveals that thisstar is an iron-deficient object with [Fe/H]=-0.8. We also analyze theabundance pattern and compare it to other classes of stars with similarstellar parameters.
| 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.
| 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.
| The Vertical Stellar Kinematics in Face-On Barred Galaxies: Estimating the Ages of Bars
In order to perform a detailed study of the stellar kinematics in thevertical axis of bars, we obtained high signal-to-noise spectra alongthe major and minor axes of the bars in a sample of 14 face-on galaxiesand used them to determine the line-of-sight stellar velocitydistribution, parameterized as a Gauss-Hermite series. With these data,we developed a diagnostic tool that allows one to distinguish betweenrecently formed and evolved bars, as well as to estimate their ages,assuming that bars form in vertically thin disks that are recognizableby low values for the vertical velocity dispersion σz.Through N-body realizations of bar unstable disk galaxies we were alsoable to check the timescales involved in the processes that give bars animportant vertical structure. We show that σz inevolved bars is roughly 100 km s-1, which translates to aheight scale of about 1.4 kpc, giving support to scenarios in whichbulges form through disk material. Furthermore, the bars in ournumerical simulations have values for σz generallysmaller than 50 km s-1, even after evolving for 2 Gyr,suggesting that a slow process is responsible for making bars asvertically thick as we observe. We verify theoretically that theSpitzer-Schwarzschild mechanism is quantitatively able to explain theseobservations if we assume that giant molecular clouds are twice asconcentrated along the bar as in the rest of the disk.
| UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
| Polarization Measurements of Post-Asymptotic Giant Branch Candidates and Related Stars
We have obtained UBVRI polarization measurements of 26 post-asymptoticgiant branch (post-AGB) candidates and related stars. The extremelymetal-poor post-AGB star HR 4049 has been observed several times. Inmost cases we find the objects to be intrinsically polarized. Thepolarization measurements presented in this paper indicate asymmetriccircumstellar dust shells and disks around these stars. For some objectsthe steep percent polarization λ-dependence and large degree ofpolarization suggest that scattering by circumstellar dust grains may beresponsible for the observed polarizations in the blue.
| Chemical composition of evolved stars of high galactic latitude
We have carried out abundance analysis for a sample of high galacticlatitude supergiants in search of evolved stars.We find that HD 27381 has atmospheric parameters and an abundancepatternvery similar to that of the post-AGB star HD 107369.HD 10285 and HD 25291 are moderately metal-poor andshow the influence of mixing that has brought the productsof NeNa cycle to the surface.The high galactic latitude B supergiant HD 137569 shows selectivedepletion of refractory elements normally seen in post-AGB stars.We find that the high velocity B typestar HD 172324 shows moderate deficiency of Fe group elements butthe CNO abundances are verysimilar to that of disk B supergiants. The observed variations inthe radial velocities, transient appearance of emission componentsin hydrogen line profiles and doublingof O I lines at 7774 Å support the possibility of this star beinga pulsating variable or a binary star.
| The evolutionary status of the bright high-latitude supergiant HD 190390
Despite its mean apparent magnitude of mV = 6.39, theevolutionary status of HD 190390 (HR7671), a luminous F-type supergiant at high galactic latitude,is still not very clear, but in most papers a post-AGB classification isassumed. New observational material has been obtained with fourdifferent instruments and is presented here. An extensive abundanceanalysis based on high resolution, high signal-to-noise NTT+EMMI spectraconfirms the metal deficiency of this object ([Fe/H] = -1.6), togetherwith a high lithium content (log ɛ(Li) = 1.9). A variabilityanalysis based on Geneva photometry over seven years reveals beatingwith a period of ~3000 days. It is, however, not clear whether thisbeating is caused by a stable triplet, or it is the consequence of smallchanges in the main frequency. More recent data obtained with theHIPPARCOS satellite and the Mercator telescope not only confirm the mainperiod, but also support the presence of a second periodicity of 11days, which was also found in the Geneva photometry. A conclusiveevolutionary status of this object is not given, but alternative to theUU Her (i.e. post-AGB) status, a W Vir classification is discussed.Based on observations collected at the European Southern Observatory, LaSilla, Chile (programme 61.E-0426), and at the Observatorio del Roque delos Muchachos, La Palma, Spain.
| Molecules in bipolar proto-planetary nebulae
Two bipolar proto-planetary nebulae, IRAS 16594-4656and IRAS 17150-3224, have been detected in variousmolecular lines, namely CO, 13CO, HCN and CN, and remainundetected in several other species. CO(J=2-1) and CO(J=3-2) lineprofiles are compared to new spectra of similar PPN candidates,previously undetected in CO(J=2-1): CPD-53°5736,IRAS 17106-3046, IRAS 17245-3951and IRAS 17441-2411. CO(J=2-1) maps of IRAS16594-4656 and IRAS 17150-3224 show thatboth PPNe have little separation between blue, centre and red-shiftedemission, and also that the CO(J=2-1) emission is of a similar size tothe telescope beam. Fractional abundances of all detected molecules(except CO) are calculated using the results of CO line modelling and asimple photodissociation model. For those species not detected, upperlimits are derived. Comparisons between these fractional abundances andthose of other PPNe show that IRAS 16594-4656 andIRAS 17150-3224 are quite under-abundant whencompared to molecule-rich sources like CRL618,CRL2688 and OH231.8+4.2. As areason for this deficit, the difference in circumstellar envelope/torusdensity between the molecule-rich sources and the molecule-poor sourcesis proposed, and supported by a chemical model which follows thetransition of a circumstellar envelope through the AGB phase and intothe PPN phase of evolution. The model includes the effects of UVradiation, cosmic rays and also X-rays. Finally, the post-AGB ages ofthese two objects (200-400 yr) are estimated using CN/HCN and HCN/COratios and both ages are found to be in agreement with previous figurescited in the literature, IRAS 17150-3224 being theyounger of the two PPNe.
| Kinematics of the envelope of the post-AGB star V510 PupNucleus of a future planetary nebula
We have carried out a detailed identification of lines in the opticalspectrum of the post-AGB star V510 Pup associated with the infraredsource IRAS 08005-2356 based on observations with high spectralresolution. Absorption lines of the ions FeII, TiII, CrII, and YII arepresent at wavelengths from 4549 to 8546 Å. The absorption by YIIand other s-process elements is anomalously strong, and the absorptionis also strong in the circumstellar C2 Swan bands. The profiles of mostof the lines (of hydrogen and metals) display P Cygniabsorption-emission profiles. All the absorption lines are shiftedtoward the blue, suggesting an outflow of stellar material. Theexpansion velocity of the envelope derived from the Swan bands arisingthere is V exp =42 km/s. The highest wind velocity determined from theabsorption wings of the FeII(42) P Cygni profiles reaches 240 km/s.Based on the star’s kinematic characteristics and the amount ofinterstellar absorption, it is at a distance of d≈3-4 kpc, whichcorresponds to an absolute magnitude of M v≈-6m.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| The nature of Hen 3-1312: A post-AGB star in a binary system
This work reports the low- and high-resolution spectroscopic diagnosticdiagrams, radial velocity, stellar parameters and abundance analysis ofthe planetary nebula Hen 3-1312. The low- and high-resolution spectrareveal that Hen 3-1312 is in fact a very-low-excitation object, in abinary system with a supergiant as a cool central star. The analysis ofthe high-resolution spectrum shows the cool stellar component to have aneffective temperature of Teff=6500±100 K and a surfacegravity of log g=0.8±0.2 corresponding to a spectral type ofF(6-7)I. These parameters result in an estimated primary luminosity of4100 Lȯ, implying a distance of 4400 pc which is inagreement with previous determinations. The abundance analysis revealsHen 3-1312 to be a metal-poor object having [Fe/H]=-1.1. The meanabundances of carbon, nitrogen and oxygen are found to be solar, howeverthe α-elements (Mg, Si and Ca) are underabundant relative to theSun. The abundance profile of Hen 3-1312 is analyzed and compared withother classes of stars with similar atmospheric parameters.Based on observations made with the 1.52 m telescope at the EuropeanSouthern Observatory (La Silla, Chile) under the agreement with theCNPq-Observatório Nacional (Brazil).Table \ref{tab4} is only available in electronic form athttp://www.edpsciences.org
| A Revised Calibration of the MV-W(O I 7774) Relationship using Hipparcos Data: Its Application to Cepheids and Evolved Stars
A new calibration of the MV-W(O I 7774) relationship hasbeen calculated using better reddening and distance estimates for asample of 27 calibrator stars of spectral types A to G, based onaccurate parallaxes and proper motions from the Hipparcos and Tychocatalogues. The present calibration predicts absolute magnitude withaccuracies of +/-0.38mag for a sample covering a large range ofMV, from -9.5 to +0.35 mag. The color term included in aprevious paper has been dropped since its inclusion does not lead to anysignificant improvement in the calibration. The variation of the O I7774 feature in the classical cepheid SS Sct has been studied. Wecalculated a phase-dependent correction to random phase OI featurestrengths in Cepheids, such that it predicts mean absolute magnitudesusing the above calibration. After applying such a correction, we couldincrease the list of calibrators to 58 by adding MV and O Itriplet strength data for 31 classical Cepheids. The standard error ofthe calibration using the composite sample was comparable to thatobtained from the primary 27 calibrators, showing that it is possible tocalculate mean Cepheid luminosities from random phase observations ofthe O I 7774 feature. We use our derived calibrations to estimateMV for a set of evolved objects to be able to locate theirpositions in the HR diagram.
| Spectrophotometric Signature of Circumstellar Matter around 89 Her
The bright supergiant and suspected binary star 89 Herculis is studiedwith help of infrared and optical spectroscopy. The high-resolutionsodium D profiles suggest multiple velocity structure of circumstellarclouds. Astrophysical parameters are derived by fitting model spectra inthe infrared region. Our results are in agreement with the recentlyemerged view of the system based on radio and infrared observations. Wealso discuss photometric and radial velocity variations concluding thatpulsations are barely detectable in velocity data. This behaviour islikely to be caused by the spectral line profile distorsions due tocircumstellar envelope.
| Post-Agb Stars
In this contribution, a review is presented on the ample data obtainedon post-AGB stars, both on the central stars and their circumstellarmaterial. The fast evolutionary phase is characterized by a rapid changein the properties of the objects, but the variety is so large that thereis yet no clear consensus on how the detailed studies of individualobjects are linked together by evolutionary channels. The absence ofstrong molecular veiling in the photospheres of the central stars,together with a spread in intrinsic metallicity make post-AGB stars veryuseful in constraining AGB chemical evolutionary models. We discuss thesurprisingly wide variety of chemical signatures observed. The onset inthe creation process of the panoply of structures and shapes observed inplanetary nebulae occurs during the short post-AGB evolution, but thephysical nature of the processes involved is still badly understood. Inthe rapidly growing field of circumstellar mineralogy, post-AGB starshave their story to tell and also the molecular envelope changessignificantly due to dilution and hardening of the stellar radiation.The real-time evolution of some objects suffering a late thermal flashis reviewed and their possible link to other hydrogen-deficient objectsis discussed. Any review on stellar evolution has a section on binariesand this contribution is no exception because binaries make up asignificant fraction of the post-AGB stars known to date.
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| High-resolution spectroscopy of QY Sge: an obscured RV Tauri variable?
The first high-resolution optical spectra of QY Sge are presented anddiscussed. Menzies & Whitelock, on the basis of photometry andlow-resolution spectra, suggested that this G0I supergiant was obscuredby dust and seen only by scattered light from a circumstellar reflectionnebula. The new spectra confirm and extend this picture. Photosphericlines are unusually broad indicating scattering of photons from dust inthe stellar wind. The presence of very broad, NaD emission lines isconfirmed. Sharp emission lines from low levels of abundant neutralmetal atoms are reported for the first time. An abundance analysis ofphotospheric lines shows that the stellar atmosphere is of approximatelysolar composition but with highly condensable (e.g. Sc and Ti) elementsdepleted by factors of 5-10.
| Detection of Zinc in the Very Metal-Poor Post-AGB Star HR 4049
We report on the detection of two Zn I lines at4722.15Å and 4810.53Å in the high-quality spectrum of thevery metal-poor post-AGB star HR4049, which was obtained with the HighDispersion Spectrograph attached to the Subaru Telescope. The strengthsof these lines indicate an appreciable underabundance of Zn by ~ -1.3dex relative to the Sun. The fact that this volatile element, similarlyto others belonging to the same group (e.g., C, N, O, S), does notconform to the extreme depletion ( > 4 dex) of refractory metals(e.g., Fe), strongly suggests that grain formation has something to dowith the origin of the chemical peculiarity. This (not extremely butsignificantly) subsolar value of [Zn/H] is quantitatively discussed inconnection with those of other volatile species, especially with respectto S. We also detected a new Fe II line at5159.03Å along with the already known Fe II4923.93Å line; based on these two lines the Fe abundance of HR4049is determined to be ~ 2.8 ([Fe/H] ~ -4.7).
| Polarimetric Studies of Stars with an Infrared Emission Excess
The results of polarimetric and IR (IRAS) observations of 24 B-A-F starsare given. Intrinsic polarization of the light from 11 of the 24 starsis observed. The degree of polarization for the other 13 stars is withinthe measurement errors. Two-color diagrams are also constructed. From acomparison of the degree of polarization with the color index on thetwo-color diagrams it is seen that 8 of these 13 stars probably are ofthe Vega type, while 5 are stars with gasdust shells and/ordiskshells. It is shown that 6 of the aforementioned 11 starswith intrinsic polarization evidently are stars with gasdustshells and/or diskshells, while 5 of them (also including No. 24)are of the Vega type. It is also shown that the IR emission from 10 ofthe stars corresponds to a power-law distribution F . This fact may beexplained both by freefree transitions of electrons and bythermal emission from dust grains in circumstellar gasdust shells(disks).
| The structure and dynamics of the molecular envelope of M 2-56
M 2-56 is a protoplanetary nebula (PPN) in which strong shocks aretaking place, therefore, useful to study the post-AGB wind interaction.It is well known that molecular observations allow studying the massdistribution of PPNe, even in those regions that have been recentlyshocked. We present high-resolution maps of the emission of12CO J = 2-1 and J = 1-0 in M 2-56. Such maps show a bipolar,molecular nebula that extends ~28 arcsec along the symmetry axis. Thenebula is composed of two contiguous, incomplete shells located alongthe symmetry axis, which has an inclination of ~17o withrespect to the plane of the sky. Those empty lobes intersect in thecenter of the nebula, where there is a small and dense ringperpendicular to the axis. This central ring expands radially at about 8km s-1 and seems to be the remnant of the circumstellarenvelope of the AGB star, that has not been accelerated by theinteraction with the fast post-AGB jets. The radius of the central ringis of ~4*E16 cm, for a distance of 2.1 kpc (deduced from ananalysis of the main properties of the object). At ~4*E17 cmfrom the nebular center, the tips of the lobes reach axial expansionvelocities of ~200 km s-1. We have developed a model for thespatio-kinematical distribution and the excitation conditions of themolecular gas in M 2-56. From the best fitting of the observations withthe predictions of the model for both lines, we have estimated thephysical conditions of the molecular nebula. It is found that thedensity varies from 5*E3 to 0.6*E3 cm-3from the nebular center to the lobe tips, and that the part of the lobesthat has not been detected is probably composed of photodissociated gas,due to the effect of interstellar photons on low-density regions. Therotational temperature is estimated to be approximately constant, ~13-16K. For the assumed geometry, a velocity field composed by a dominantradial component plus an axial contribution has been deduced. Theemission of both lines is found to be optically thin, and thereforeprobes the whole molecular gas, which has a mass of ~0.05 ms.The ``scalar'' momentum and the kinetic energy of the different regionsof the molecular nebula have been calculated, finding that the highmomentum won by the gas in the post-AGB phase cannot have been suppliedby the radiation pressure mechanism. Although the central star of M 2-56is not very hot yet (~20 000 K), this PPN has a large kinematical age,between 1000 yr and 1700 yr, in comparison with other PPNe that havehotter central stars. M 2-56 may not be a typical PPN, but anintermediate object between the known low-mass post-AGB nebulae and thestandard PPNe. Based on observations carried out with the IRAM Plateaude Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG(Germany) and IGN (Spain).
| Crystalline silicate dust around evolved stars. III. A correlations study of crystalline silicate features
We have carried out a quantitative trend analysis of the crystallinesilicates observed in the ISO spectra of a sample of 14 stars withdifferent evolutionary backgrounds. We have modeled the spectra using asimple dust radiative transfer model and have correlated the resultswith other known parameters. We confirm the abundance difference of thecrystalline silicates in disk and in outflow sources, as found byMolster et al. (\cite{Molster1999a}). We found some evidence that theenstatite over forsterite abundance ratio differs, it is slightly higherin the outflow sources with respect to the disk sources. It is clearthat more data is required to fully test this hypothesis. We show thatthe 69.0 micron feature, attributed to forsterite, may be a verysuitable temperature indicator. We found that the enstatite is moreabundant than forsterite in almost all sources. The temperature of theenstatite grains is about equal to that of the forsterite grains in thedisk sources but slightly lower in the outflow sources. Crystallinesilicates are on average colder than amorphous silicates. This may bedue to the difference in Fe content of both materials. Finally we findan indication that the ratio of ortho to clino enstatite, which is about1:1 in disk sources, shifts towards ortho enstatite in the highluminosity (outflow) sources. Based on observations with ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
| Crystalline silicate dust around evolved stars. II. The crystalline silicate complexes
This is the second paper in a series of three in which we present anexhaustive inventory of the solid state emission bands observed in asample of 17 oxygen-rich dust shells surrounding evolved stars. The datawere taken with the Short and Long Wavelength Spectrographs on board ofthe Infrared Space Observatory (ISO) and cover the 2 to 200 mu mwavelength range. Apart from the broad 10 and 18 mu m bands that can beattributed to amorphous silicates, at least 49 narrow bands are foundwhose position and width indicate they can be attributed to crystallinesilicates. Most of these emission bands are concentrated in well definedspectral regions (called complexes). We define 7 of these complexes; the10, 18, 23, 28, 33, 40 and 60 micron complex. We derive averageproperties of the individual bands. Almost all of these bands were notknown before ISO. Comparison with laboratory data suggests that botholivines (Mg2xFe(2-2x)SiO4) andpyroxenes (MgxFe(1-x)SiO3) are present,with x close to 1, i.e. the minerals are very Mg-rich and Fe-poor. Thiscomposition is similar to that seen in disks surrounding young stars andin the solar system comet Hale-Bopp. A significant fraction of theemission bands cannot be identified with either olivines or pyroxenes.Possible other materials that may be the carriers of these unidentifiedbands are briefly discussed. There is a natural division into objectsthat show a disk-like geometry (strong crystalline silicate bands), andobjects whose dust shell is characteristic of an outflow (weakcrystalline silicate bands). In particular, stars with the 33.5 mu molivine band stronger than about 25 percent over continuum areinvariably disk sources. Likewise, the 60 mu m region is dominated bycrystalline silicates in the disk sources, while it is dominated bycrystalline H2O ice in the outflow sources. We show that thedisk and outflow sources have significant differences in the shape ofthe emission bands. This difference must be related to the compositionor grain shapes of the dust particles. The incredible richness of thecrystalline silicate spectra observed by ISO allows detailed studies ofthe mineralogy of these dust shells, and is the origin and history ofthe dust. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA.
| Crystalline silicate dust around evolved stars. I. The sample stars
This is the first paper in a series of three where we present the firstcomprehensive inventory of solid state emission bands observed in asample of 17 oxygen-rich circumstellar dust shells surrounding evolvedstars. The data were taken with the Short and Long WavelengthSpectrographs on board of the Infrared Space Observatory (ISO) and coverthe 2.4 to 195 mu m wavelength range. The spectra show the presence ofbroad 10 and 18 mu m bands that can be attributed to amorphoussilicates. In addition, at least 49 narrow bands are found whoseposition and width indicate they can be attributed to crystallinesilicates. Almost all of these bands were not known before ISO. Theincredible richness of the crystalline silicate spectra observed by ISOallows detailed studies of the mineralogy of these dust shells, and is atelltale about the origin and evolution of the dust. We have measuredthe peak positions, widths and strengths of the individual, continuumsubtracted bands. Based on these measurements, we were able to order thespectra in sequence of decreasing crystalline silicate band strength. Wefound that the strength of the emission bands correlates with thegeometry of the circumstellar shell, as derived from direct imaging orinferred from the shape of the spectral energy distribution. Thisnaturally divides the sample into objects that show a disk-like geometry(strong crystalline silicate bands), and objects whose dust shell ischaracteristic of an outflow (weak crystalline silicate bands). Allstars with the 33.6 mu m forsterite band stronger than 20 percent overcontinuum are disk sources. We define spectral regions (calledcomplexes) where a concentration of emission bands is evident, at 10,18, 23, 28, 33, 40 and 60 mu m. We derive average shapes for thesecomplexes and compare these to the individual band shapes of theprogramme stars. In an Appendix, we provide detailed comments on themeasured band positions and strengths of individual sources. Based onobservations with ISO, an ESA project with instruments funded by ESAMember States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Tables 4 to 20 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/382/184
| Binary (Post) AGB evolution
In this contribution the observational characteristics of F to G typepost-AGB stars, showing both O-rich and C-rich circumstellar chemistry,are reviewed. It turns out that binarity and the presence of a stablecircumbinary dusty disc are fundamental properties of these objects. Thepossible common origin of the mixed chemistry observed in these systemsand in IR-bright [WC] stars is discussed.
| Mass, linear momentum and kinetic energy of bipolar flows in protoplanetary nebulae
We have studied the CO emission from protoplanetary nebulae (PPNe). Oursample is composed of 37 objects and includes, we think, all wellidentified PPNe detected in CO, together with the two yellow hypergiantsemitting in CO and one young PN. We present a summary of the existing COdata, including accurate new observations of the 12CO and13CO J=1-0 and J=2-1 lines in 16 objects. We identify in thenebulae a slowly expanding shell (represented in the spectra by acentral core) and a fast outflow (corresponding to the line wings), thatin the well studied PPNe is known to be bipolar. Excluding poor data, weend up with a sample of 32 sources (including the 16 observed by us);fast flows are detected in 28 of these nebulae, being absent in only 4.We present a method to estimate from these data the mass, ``scalar''momentum and kinetic energy of the different components of the molecularoutflows. We argue that the uncertainties of our method can hardly leadto significant overestimates of these parameters, althoughunderestimates may be present in not well studied objects. The totalnebular mass is often as high as ~1 Msun, and the mass-lossrate, that (presumably during the last stages of the AGB phase)originated the nebula, had typical values ~10-4Msun yr-1. The momentum corresponding to this massejection process in most studied nebulae is accurately coincident withthe maximum momentum that radiation pressure, acting through absorptionby dust grains, is able to supply (under expected conditions). Weestimate that this high-efficiency process lasts about 1000-10 000 yr,after which the star has ejected a good fraction of its mass and the AGBphase ends. On the other hand, the fast molecular outflows, that haveprobably been accelerated by shock interaction with axial post-AGB jets,carry a significant fraction of the nebular mass, with a very highmomentum (in most cases between 1037 and 1040 g cms-1) and very high kinetic energy (usually between1044 and 1047 erg). In general, yellow hypergiantsand post-AGB objects with low initial mass show nebular masses andmomenta that are, respectively, higher and lower than these values. Wecompare the momenta of the fast outflows with those that can be suppliedby radiation pressure, taking into account the expected shortacceleration times and some effects that can increase the momentumtransfer. We find that in about 80% of PPNe, the fast molecular flowshave too high momenta to be powered by radiation pressure. In some casesthe momentum of the outflow is ~1000 larger than that carried byradiation pressure; such high factors are difficult to explain evenunder exceptional conditions. Wind interaction is the basic phenomenonin the PN shaping from the former AGB envelopes; we conclude that thisinteraction systematically takes place along a dominant direction andthat this process is not powered by radiation pressure. Due to the lackof theoretical studies, the possible momentum source remains a matter ofspeculation.
| Low-excitation atomic gas around evolved stars. II. ISO observations of O-rich nebulae
We have observed atomic fine-structure lines in the far-infrared (FIR)from 12 oxygen-rich evolved stars. The sample is composed of mostlyproto-planetary nebulae (PPNe) and some planetary nebulae (PNe) andasymptotic giant branch (AGB) stars. ISO LWS and SWS observations of [OI], [C II], [N II], [Si I], [Si II], [S I], [Fe I], and [Fe II] lineswere obtained. Taking into account also the sample presented by Fong etal. (Paper I) of carbon-rich evolved stars, we find that PPNe emit inthese low-excitation atomic transitions only when the central star ishotter than ~10 000 K. This result suggests that such linespredominantly arise from photodissociation regions (PDRs), and not fromshocked regions. The line widths determined from our Fabry-Perot dataalso suggest that the FIR lines arise from relatively quiescent PDR gas,as opposed to shocked gas. Our results are in reasonable agreement withpredictions from PDR emission models, allowing the estimation of thedensity of the emitting layers by comparison with the model results. Onthe other hand, the comparison with predictions of the emission fromJ-type and C-type shocked regions suggests that detected lines do notcome from shocks. The [C II] line flux has been used to measure the massof the low-excitation atomic component in PPNe, since this transitionhas been found to be a useful model-independent probe to estimate thetotal mass of these PDRs. The derivation of the mass formula andassumptions made are also discussed. Based on observations with ISO, anESA project with instruments funded by ESA Member States (especially thePI countries: France, Germany, The Netherlands and the United Kingdom)and with the participation of ISAS and NASA.
| Low-excitation atomic gas around evolved stars. I. ISO observations of C-rich nebulae
We present ISO LWS and SWS spectra of far-infrared (FIR) atomic finestructure lines in 12 carbon-rich evolved stars including asymptoticgiant branch (AGB) stars, proto-planetary nebulae (PPNe) and planetarynebulae (PNe). The spectra include grating and Fabry-Perot measurementsof the line emission of [O I], [C Ii], [Si I], [Si Ii], [S I], [Fe I],[Fe Ii], [Ne Ii] and [N Ii]. Only 5 out of our 12 object sample havebeen detected in at least one of these FIR lines. When we include the 12oxygen-rich evolved stars from Castro-Carrizo et al.(\cite{CastroCarrizo00}, Paper II), we find that atomic line emission isobserved only in those sources in which the central star's T_eff >=10 000 K. Above this cutoff, the number of detectable lines and theintensity of the line emission increase as T_eff increases. These trendssuggest that the atomic lines originate from photodissociation regions(PDRs). In general, the kinematics of the atomic gas, derived from linefits to the Fabry-Perot data, are comparable to the molecular expansionvelocities. These kinematics are expected for atomic cooling linesassociated with circumstellar PDRs. AFGL 618, however, appearsexceptional with dual velocity components: a narrow component (<20 kms-1) that may be associated with a PDR, and a broad component(~66 km s-1) that may be produced in post-shocked,accelerated gas. A new PDR code which properly treats enhanced carbonabundances was used to model the observations of our carbon-richobjects. The predicted line intensities agree reasonably well with theobservations. Shock models, however, do not compare well with theobserved line intensities. PDR mass estimates ranging from ~0.01-0.2M_sun were derived from the [C Ii] 158 mu m line emission. The atomicgas constitutes only a small fraction of the total mass for youngplanetary nebulae, but its importance grows significantly as the nebulaeevolve. Our overall analysis shows that photodissociation, and notshocks, dominates the evolution of the circumstellar envelope bytransforming the initially molecular asymptotic giant branch envelopesinto the atomic gas found in proto-planetary and planetary nebulae.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with the participation of ISASand NASA.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Herkules |
| Rektaszcenzió: | 17h55m25.20s |
| Deklináció: | +26°03'00.0" |
| Vizuális fényesség: | 5.46 |
| Távolság: | 980.392 parszek |
| RA sajátmozgás: | 5.1 |
| Dec sajátmozgás: | 4.9 |
| B-T magnitude: | 5.891 |
| V-T magnitude: | 5.505 |
Katalógusok és elnevezések:
|
INHABITED SKY
