תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
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.
| 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.
| 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.
| An analysis of the optical spectra of the post-asymptotic giant branch stars LSIV -12 111 and HD 341617
High spectral resolution and signal-to-noise observations of theabsorption and emission line spectra in two post-asymptotic-giant-branch(PAGB) stellar candidates, LSIV -12 111 and HD 314617 are discussed. Theabsorption line spectra have been analysed using non-LTE modelatmosphere techniques to determine stellar atmospheric parameters andchemical compositions, both in absolute terms and relative to a standardstar, HD 13841. The atmospheric parameters differ from previousestimates based on LTE model atmospheres, probably due to non-LTEeffects. In turn these imply stellar masses that are generally largerthan have been previously estimated. Both PAGB candidates have relativeuniform underabundances of metals with mean values of -0.35 dex for LSIV-12 111 and -0.50 dex for HD 314617.Furthermore, their abundance patterns are remarkably similar to thatobserved for optically bright, F-type PAGBs. From the emission spectra,the plasma parameters and angular size of the circumstellar material areconstrained, and these are consistent with previous estimates and with aPAGB evolutionary stage.
| 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).
| A stellar relic from the early Milky Way
The chemical composition of the most metal-deficient stars largelyreflects the composition of the gas from which they formed. These oldstars provide crucial clues to the star formation history and thesynthesis of chemical elements in the early Universe. They are the localrelics of epochs otherwise observable only at very high redshifts; iftotally metal-free (`population III') stars could be found, this wouldallow the direct study of the pristine gas from the Big Bang. Earliersearches for such stars found none with an iron abundance less than1/10,000 that of the Sun, leading to the suggestion that low-mass starscould form from clouds above a critical iron abundance. Here we reportthe discovery of a low-mass star with an iron abundance as low as1/200,000 of the solar value. This discovery suggests that populationIII stars could still exist-that is, that the first generation of starsalso contained long-lived low-mass objects. The previous failure to findthem may be an observational selection effect.
| HD 331319: A Post-AGB F Supergiant with He I Lines
Not Available
| Discovery of an Edge-on Dust Disk around the [WC10] Central Star CPD -56°8032
We present Hubble Space Telescope ultraviolet and optical SpaceTelescope Imaging Spectrograph spectroscopy of the [WCL] planetarynebula central star CPD -56°8032, obtained during its latestlight-curve minimum. The UV spectrum shows the central star's continuumlight distribution to be split into two bright peaks separated by 0.10".We interpret this finding as due to an edge-on disk or torus structurethat obscures direct light from the star, which is seen primarily viaits light scattered from the disk's rims or lobes. CPD -56°8032 isan archetype of dual dust chemistry [WCL] planetary nebulae, whichexhibit strong infrared emission features from both carbon-rich andoxygen-rich materials, and for which the presence of a disk harboringthe O-rich grains had been suggested. Our direct observation of anedge-on occulting dust structure around CPD -56°8032 provides strongsupport for such a model and for binary interactions being responsiblefor the correlation between the dual dust chemistry phenomenon inplanetary nebulae and the presence of a hydrogen-deficient [WCL]Wolf-Rayet central star.
| RU Cen and SX Cen: Two strongly depleted RV Tauri stars in binary systems. The RV Tauri photometric b phenomenon and binarity
We present a chemical abundance analysis on the basis of highsignal-to-noise and high-resolution (lambda /Delta lambda ~ 48 000)optical spectra of two RV Tauri stars RU Cen and SX Cen. With an [Fe/H]= -1.9 and a [Zn/Fe] = +0.9 for RU Cen and a [Fe/H] = -1.1 and a [Zn/Fe]= +0.6 for SX Cen, both stars of spectroscopic class B display strongdepletion of refractory elements in their photospheres. Our CORALIEradial velocity measurements prove the stars to be members of binarysystems and a detailed construction of the spectral energy distributionindicate the presence of a large amount of hot circumstellar dust.Moreover, the orbital period of SX Cen of around 600 days is similar tothe published period of mean magnitude variation in the light curve (RVTauri phomometric class b phenomenon). All these observations indicatethe presence of a stable circumbinary disk in the objects and strengthenthe model that this is a necessary condition for the depletion processto take place. Based on observations collected at the European SouthernObservatory in Chile (64.L-0117(A), 67.D-0054(A)), on the 1.2 m SwissEuler telescope on La Silla and on the 70 cm Swiss photometric telescopealso on La Silla.
| Nanodiamonds around HD 97048 and Elias 1
We present an analysis of ISO-SWS observations of the Herbig Ae/Be starsHD 97048 and Elias 1. Besides the well-known family of IR emission bandsat 3.3, 6.2, ``7.7'', 8.6 and 11.2 mu m these objects show strong,peculiar emission features at 3.43 and 3.53 mu m. The latter twofeatures show pronounced substructure which is very similar in the twosources. Comparison of the spectra of HD 97048 and Elias 1 withlaboratory spectra of H-terminated diamond surfaces show excellent andvery convincing agreement in peak position and spectral detail (Guilloiset al. \cite{Gui99}). The position of the 3.53 mu m band indicates atemperature of ~ 1000 K. An analysis of the radiative energy budgetmakes us conclude that the diamond carrier of the 3.53 mu m feature hastypical sizes of 1-10 nm for HD 97048. A fit of the 3.53 mu m featurewith a theoretical, calculated profile indicates that the emittingdiamonds in HD 97048 see a FUV flux of 5.8 x 10-3[W/cm2]. The derived diamond mass, 1.5 x 10-10Msun, is only a tiny fraction of the total circumstellar dustmass and corresponds to only about 1 parts per billion relative tohydrogen. We discuss the origin of the diamond around these Herbig Ae/Bestars and conclude that most likely they are formed in situ. Theimplications for the nanodiamonds discovered in meteorites are alsodiscussed.
| 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.
| Detection of the unusual 3.5 mu m feature in the Herbig Be star MWC 297
We present spectroscopic observations of MWC 297 with medium spectralresolution in the 2.1-4.1 mu m region, that show the unusual emissionband at 3.53 mu m as well as gaseous emission lines of the Brackett,Pfund and Humphrey series of hydrogen. A unique aspect of the measured3.53 mu m emission band is the fact that, within our detection limit, itis not accompanied by the 3.3 and 3.4 mu m emission band. We suggestthat the 3.53 mu m feature could be emitted by highly dehydrogenizedcarbonaceous dust particles that were processed under the influence ofthe strong radiation field of the central star.
| Atmospheric abundances in post-AGB candidates of intermediate temperature
Detailed atmospheric abundances have been calculated for a sample of -Gsupergiant stars with IR fluxes and/or high galactic latitudes. HD172481 and HD 158616 show clear indications of being post-AGB stars thathave experienced third dredge-up. HD 158616 is carbon-rich while theabundance pattern of HD 172481 and its large Li enhancement givessupport to the hot bottom burning scenario that explains paucity ofcarbon-rich stars among AGB stars. HD 172324 is very likely a hotpost-AGB star that shows a strong carbon deficiency. HD 725, HD 218753and HD 331319 also appear to be evolved objects between the red giantand the AGB. HD 9167, HD 173638 with a few exceptions, reflect solarabundances and no signs of post red giant evolution. They are mostlikely young massive disk supergiants. Further analysis ofproto-Planetary Nebula HDE 341617 reveals that He lines show signs ofvelocity stratification. The emission lines have weakened considerablysince 1993. The envelope expands at 19 km s-1 relative to thestar. Atmospheric abundances, evolutionary tracks and isochrones areused to estimate masses and ages of all stars in the sample. Based onobservations obtained at the Haute-Provence Observatory, France. Tables11 and 12 are only available in electronic form athttp://www.edpsciences.org
| Binary "Post-AGB" Stars
Not Available
| Chemical Composition and Evolution of Post-AGB Stars
Not Available
| Chemical composition of HD 179821 (IRAS 19114+0002)
A LTE analysis of medium resolution spectra of {HD 179821} has beenmade. Derived atmospheric parameters of {HD 179821} are T_eff=5660 K,log g=-1.0 and [Fe/H]=-0.5. The position of the star in the H-R diagram,its high radial velocity (Vr=100 km s-1), its farinfrared excess similar to PNe and its chemical composition suggest that{HD 179821} is a low mass post-AGB carbon-poor supergiant and not apopulation I massive red supergiant. The underabundance of carbon and ofthe s-process element zirconium shows that HD 179821 has not gonethrough the third dredge-up. We emphasize the similarity of the chemicalpattern of the post-AGB star HD 179821 and the C-poor halo PN DDDM-1.Based on observations obtained at the Observatoire de Haute-Provence,France
| The RV\ Tauri phenomenon and binarity
We present accurate radial velocity measurements on the pulsatingextremely iron-deficient post-AGB object HD 52961 and the RV Tauri starEN TrA (HD 131356) proving them to be binaries. Our long-termphotometric monitoring campaign shows that the RV Tauri photometricclass ``b'' phenomenon in HD 52961 is due to variable circumstellarextinction during orbital motion. By comparing carefully theobservational characteristics of RV Tauri stars and the class ofextremely iron-deficient post-AGB objects we conclude that binarity is awidespread phenomenon in the RV Tauri class of objects. The observedchemical depletion patterns, weak circumstellar CO emission, peculiarspectral energy distribution and the difference in photospheric class ofthe RV Tauri objects can all be naturally explained by assuming that thecircumstellar material is not freely expanding, but trapped in thebinary system. Based on observations collected at the European SouthernObservatory (proposals codes 51.7-0052; 51.7-0053; 52.7-0048; 58.E-0462;59.E-0432; 61.E-0426); with the Swiss telescopes at ESO and OHP and theAPT telescope at Mt. Hopkins
| The ultraviolet variations of the post-AGB star HD 89353
Based on an analysis of all low resolution spectra of HD 89353 (HR 4049)obtained with the International Ultraviolet Explorer (IUE) over severalyears and uniformly reprocessed, we report on large ultravioletvariations for this star whose amplitude increases towards longwavelengths. Ultraviolet maximum and minimum occur respectively at phase0.52 and 0.92 of Waelkens et al. (1991) ephemeris which suggests thatultraviolet and optical variations occur in phase. The ultravioletenergy distribution of HD 89353 is very deficient compared to that ofthe standard A6Ib star, HD 80404, and compared to the predictions of anATLAS 9 model (Teff = 7500K, log g = 2.0 and [M/H]= -5.0)which bests fit the optical continuum. The ultraviolet flux deficiencyscales as <~mbda(-1) , a trend already noted earlier (Waelkens et al.1995). The most likely interpretation is that variable circumstellarextinction drives the ultraviolet and optical variations as proposed byWaelkens et al. (1991). Based on data from the International UltravioletExplorer
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Refining the Fundamental Parameters of the 3 Post-AGB Stars: HR 4049, HD 52961 and HD 133656 Using Improved Model Atmospheres
Not Available
| The binary RV Tauri star AC HER and evidence for a long-lived dust-disc
We present in this letter a homogeneous set of CORAVEL radial velocitymeasurements of the well studied RV Tauri star AC Her showing it to be abinary with a period of 1196 +/- 6 days. The photospheric abundances arededuced using high resolution, high signal-to-noise optical spectra andprove that AC Her is another RV Tauri star displaying depletion ofrefractory elements, strengthening the relation between binarity and thedepletion process. Our full ISO-SWS spectrum shows that the unusualbroad emission feature at 8-12 mu m, is due to a high crystallisationfraction of the circumstellar silicates. We review all observationalevidence that the circumstellar material is trapped in a long-lived discin the system similar to what is observed in the Red Rectangle nebula.based on observations collected at the European Southern Observatory inChile, at La Palma Spain, and with the Swiss telescope at OHP, France
| The spectral evolution of post-asymptotic giant branch stars
A parameter study of the spectral evolution of a typical post-AGB star,with particular emphasis on the evolution of the IR colours, ispresented. The models are based on the latest evolutionary tracks forhydrogen-burning post-AGB stars. For such tracks the evolutionary rateis very dependent on the assumed mass-loss rate as a function of time.We investigate this effect by modifying the mass-loss prescription. Thenewly calculated evolutionary rates and density distributions are usedto model the spectral evolution of a post-AGB star with thephotoionization code cloudy, including dust in the radiative transfer.Different assumptions for the dust properties and dust formation areconsidered. It is shown that by varying these parameters in a reasonableway, entirely different paths are followed in the IRAS colour-colourdiagram. First of all, the effects of the evolution of the central staron the expanding dust shell cannot be neglected. Also the dustproperties and the definition of the end of the AGB phase have animportant effect. The model tracks show that objects occupying the samelocation in the IRAS colour-colour diagram can have a differentevolutionary past, and therefore the position in the IRAS colour-colourdiagram alone cannot a priori give a unique determination of theevolutionary status of an object. An alternative colour-colour diagram,the K-[12] versus [12]-[25] diagram, is presented. The tracks in thisdiagram seem less affected by particulars of the grain emission. Thisdiagram may be a valuable additional tool for studying post-AGBevolution.
| The 73rd Name-List of Variable Stars
Not Available
| Supergiants with large IR excesses.
Not Available
| Circumstellar Molecular Spectra Towards Evolved Stars
In this paper we discuss the relevance, and possible scientific gains,which can be acquired from studying circumstellar molecular spectratoward evolved stars. Where can we expect circumstellar molecularspectra, why would we want to study these spectra, which molecules mightbe present, and what can we learn from these studies? We present anoverview of reported detections, and discuss some of the results.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | תאומים |
| התרוממות ימנית: | 07h03m39.63s |
| סירוב: | +10°46'13.1" |
| גודל גלוי: | 7.383 |
| מרחק: | 10000000 פארסק |
| תנועה נכונה: | -1.6 |
| תנועה נכונה: | -1.5 |
| B-T magnitude: | 7.833 |
| V-T magnitude: | 7.421 |
קטלוגים וכינוים:
|
INHABITED SKY
