Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
A non-LTE abundance analysis of the post-AGB star ROA5701
An analysis of high-resolution Anglo-Australian Telescope(AAT)/University College London Échelle Spectrograph(UCLÉS) optical spectra for the ultraviolet (UV)-bright starROA5701 in the globular cluster ω Cen (NGC5139) is performed,using non-local thermodynamic equilibrium (non-LTE) model atmospheres toestimate stellar atmospheric parameters and chemical composition.Abundances are derived for C, N, O, Mg, Si and S, and compared withthose found previously by Moehler et al. We find a general metalunderabundance relative to young B-type stars, consistent with theaverage metallicity of the cluster. Our results indicate that ROA5701has not undergone a gas-dust separation scenario as previouslysuggested. However, its abundance pattern does imply that ROA5701 hasevolved off the asymptotic giant branch (AGB) prior to the onset of thethird dredge-up.
| 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.
| Detection of spectral variability of the optical component of the IR source IRAS 20508+2011
Our high-resolution spectral observations have revealed variability ofthe optical spectrum of the cool star identified with the IR source IRAS20508+2011. We measured the equivalent widths of numerous absorptionlines of neutral atoms and ions at wavelengths 4300 7930 Å, alongwith the corresponding radial velocities. Over the four years of ourobservations, the radial velocity derived from photospheric absorptionlines varied in the interval V rȯ = 15 30 km/s. In the same period,the Hα profile varied from being an intense bell-shaped emissionline with a small amount of core absorption to displaying two-peakedemission with a central absorption feature below the continuum level. Atall but one epoch, the positions of the metallic photospheric lines weresystematically shifted relative to the Hα emission: ΔV r = Vr(met) - V r(Hα, emis) ≈ -23 km/s. The Na D doublet displayed acomplex profile with broad (half-width ≈ 120 km/s) emission andphotospheric absorption, as well as an interstellar component. We usedmodel atmospheres to determine the physical parameters and chemicalcomposition of the star’s atmosphere: T eff = 4800 K, log g = 1.5,ξt = 4.0 km/s. The metallicity of the star differs little from thesolar value: [Fe/H]ȯ = -0.36. We detected overabundances of oxygen[O/Fe]ȯ = +1.79 (with the ratio [C/O] ≈ -0.9), andα-process elements, as well as a deficit of heavy metals. Theentire set of the star’s parameters suggests that the opticalcomponent of IRAS 20508+2011 is an “O-rich” AGB star withluminosity M v ≈ -3m that is close to its evolutionary transition tothe post-AGB stage.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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).
| The Cepheids of Population II and Related Stars
The Type II Cepheids include most intrinsic variables with periodsbetween 1 and about 50 days, except for the classical Cepheids and theshortest semiregular variables of type M. The Type II Cepheids may bedivided in groups by period, such that the stars with periods beween 1and 5 days (BL Her class), 10-20 days (W Vir class), and greater than 20days (RV Tau class) have differing evolutionary histories. The chemicalcomposition of Type II Cepheids reflects the material they were madefrom as modified by their internal nuclear evolution and mixing.Finally, RV Tau stars are affected by mass loss by dust and speciesattached to the dust. The populations to which the various classes ofType II Cepheids are assigned constitute important clues to the originand evolution of the halo of our Galaxy and the dwarf spheroidal systemsfrom which at least part of the halo seems to have been accreted.
| 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.
| BVRIJHK photometry of post-AGB candidates
BVRIJHK photometric observations are presented for 27 post-AGBcandidates. Almost all objects show a double peaked SED curve in theoptical to far-infrared wavelengths. Seventeen objects were classifiedas post-AGB stars on the basis of their spectral type, location in theIRAS color-color diagram and SED. The physical parameters of theobserved post-AGB stars, the inner radius of the detached shell, themass of the shell and the distance were derived using the simple dustshell model. We compared our observational sequence of post-AGB objectsto the theoretical evolutionary sequence (Schönberner\cite{schonberner1983}; Blöcker \cite{blocker}) in the stellartemperatures versus age diagram. We found that two post-AGB stars, IRAS05040+4820 and 08187-1905, have low stellar temperature with a largedynamical age of the dust shell. They appear to provide the firstobservational evidence that some low-mass stars bypass the planetarynebulae stage because of their slow increase in stellar temperature.Based on observations obtained at Kiso Observatory, Nagano, Japan.Table2 is also available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/385/884
| HD 331319: A Post-AGB F Supergiant with He I Lines
Not Available
| K 413, a star near the AGB in the globular cluster M 12
CCD spectra obtained with the echelle spectrometer of the 6-metertelescope were used to determine, by the model atmospheres method, thefundamental parameters ( Teff = 4800 K, log g = 0.7) anddetailed chemical abundances for the star K 413, a member of theglobular cluster M 12. The resulting value, [Fe/H] = -1.38, is the firstmetallicity determination for M 12 using high-resolution spectra. Themain characteristic feature of the star's atmospheric chemical abundancepattern is a large oxygen excess, [O/Fe] ~ +2. The s-process elementsare probably slightly depleted compared to metallicity: [X/Fe] = -0.04for yttrium and zirconium, [Ba/Fe] = -0.12 for barium. Abundances of theheavier elements: La, Ce, Nd, and Pr, do not differ, relative to iron,from the solar ones: [heavy/Fe] = 0.0. The europium excess, [Eu/Fe] =+0.48, is typical of members of low-metallicity globular clusters. Thespectrum of K 413 shows, for the Hα line, an variable absorptionand emission profile. From its high luminosity and chemical abundanceanomalies, we can suppose that K 413 is in an evolutionary stage afterthe AGB. In the spectrum, we find absorption details that can beidentified with diffuse interstellar bands displaced by 16 kms-1 to longer wavelengths relative to the star's velocity.
| 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
| The peculiar post-AGB supergiant IRAS 04296+3429: optical spectroscopy and its spectral energy distribution
The optical spectrum of the infrared source IRAS 04296+3429 (opticalcounterpart - G0 Ia star, V = 14.2) was obtained with the echellespectrometer PFES at the prime focus of the 6 m telescope. We discoveremission bands (0,0) and (0,1) of the Swan system of the C_2 molecule inthe optical spectrum of IRAS 04296+3429. Comparison with the spectrum ofthe Hale-Bopp comet leads us to propose that in both cases the samemechanism (resonance fluorescence) is responsible for the emission inthe C_2 molecular bands. Several strong absorption features whosepositions coincide with known diffuse interstellar bands (DIBs) arerevealed in the spectrum of IRAS 04296+3429. The infrared spectrum ofIRAS 04296+3429 shows the famous 21 μm feature (Kwok et al. 1989),but this object has not been observed by KAO (Omont et al. 1995).However, like IRAS 05113+1347, IRAS 05341+0852 and IRAS 22223+4327 (Kwoket al. 1995, Szczerba et al. 1996), our detailed modelling of itsspectral energy distribution suggested that this source also should showthe 30 μm band. In fact, ISO discovered a broad, relatively strongfeature around 30 μm for IRAS 04296+3429 (Szczerba et al. 1999). Thesurface chemical composition of the source IRAS 04296+3429 ismetal-deficient (the averaged value of the abundances of the iron groupelements Ti, V, Cr and Fe relative to the solar values is [M/H]_{sun =-0.9}) and has been considerably altered during the evolution: carbon,nitrogen and s-process elements are overabundant relative to themetallicity. The totality of physical and chemical parameters derivedfor IRAS 04296+3429 confirms a relation between presence of the featureat 21 μm in the spectrum of a carbon rich star and an excess of thes-process elements.
| 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
| Lithium and r-Process Abundances in the Population II Cepheid M5 V42
We report the results of an abundance analysis of the 25.7 dayPopulation II Cepheid V42 in the globular cluster M5. V42 is probably apost-asymptotic giant branch (AGB) star, in its final crossing of theH-R diagram. We find [Fe/H] = -1.22, in excellent agreement with asimilar analysis of this star by Gonzalez & Lambert, and of redgiants in M5 by Sneden et al. The analysis is based on six spectraobtained during three different phases of the star's pulsational cycle.Oxygen is depleted, and sodium is enhanced, while the ``alpha'' elementsMg, Si, Ca, and Ti are enhanced by about 0.3 dex. Most interesting,lithium is detected, with log ɛ(Li) ~ +1.8, suggesting theproduction of lithium and its rapid transport to the surface. Zinc andsilicon abundances are normal with respect to iron and calcium,suggesting that depletion onto dust grains during the AGB stage has notaffected the star's abundances. The neutron-capture elements show ratiosconsistent with a nearly pure r-process origin. This r-process dominanceis probably unrelated to the production of lithium, since the cluster'sred giant branch stars show a similar pattern. Nonetheless, it appearsthat some low-mass, metal-poor stars do not solely destroy lithium inthe course of their normal evolution.
| 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
| 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.
| Chemical composition of optically bright post-AGB stars.
We present a detailed LTE chemical analysis of 10 optically brightF-type post-AGB objects on the basis of the analysis of high-resolutionoptical spectra and compare the results with similar objects discussedin the literature. The iron content is low on average, and so confirmsthe old and hence low-mass nature of the supergiants, with a noticableexception of HD 95767. We emphasize the fact that the chemical patternsobserved are very diverse : several different classes can bedistinguished. Only a minor fraction of the objects are conform tostandard post third dredge-up theory. Only in HD 187885 (Van Winckel etal., 1996A&A...306L..37V), HD 56126 (Klochkova, 1995MNRAS.272..710K)and HD 158616 (this paper) is there conclusive chemical evidence thatthey occur in a post-AGB evolutionary phase : a high total CNOabundance, for HD 187885 a supersolar He content and-above all-a largeoverabundance of s-process elements. The other objects, together withother well studied high galactic latitude F-supergiants, display nos-process enhancement but even depletion in some cases. The high Nabondance and the mildly enhanced total CNO abundance indicate that theatmospheres of these objects contain a mixture of CNO-cycled materialand He-burning products. For some sources, however, this enhancement ofthe total CNO abundance is barely significant. HD 107369, the onlyobject in our sample with neither Hα emission nor observed IRexcess, displays also unique chemical patterns among our sample stars (aC deficiency coupled with a moderate Fe depletion of [Fe/H]=-1.1). Thisstar is the only object in our sample showing similar chemical patternsto the metal poor B stars at high galactic latitude (Conlon et al.,1993, in ASP Conf. Ser., Vol. 45, p. 146). Our chemical analysis doestherefore not point to an evolutionary connection between the dustyhigh-latitude supergiants and the metal-poor B stars, but rathersuggests that the latter evolve from stars such as HD 107369.
| Supergiants with large IR excesses.
Not Available
| New and misclassified Planetary Nebulae
Altogether 86 objects discovered in the period 1991-1994 have beenaccepted by the author as new PN (Table 1). In Appendices A and B tothis table the lists of possible pre-PN as well as of possible post-PN,respectively, are given. Table 2 presents 6 misclassified objects whichwe suggest removing from CGPN.
| The chemical composition of the protoplanetary nebula candidate HD 179821
A high-resolution and signal-to-noise ratio spectrum has been obtainedfor the protoplanetary nebula candidate HD 179821=IRAS 19114+0002 usingthe Academy of Sciences Astrophysical Observatory 6-m telescope and CCDdetector. The spectroscopic indicators provide the atmosphericparameters T_eff=6800K and logg=1.3 (cgs), approximately equal to thosefor normal F5 supergiants. A standard local thermodynamic equilibrium(LTE) analysis by the model atmospheres method using these parametersshows that most of the iron-peak elements are at most only slightlydeficient ([X]~=-0.10 dex) relative to the normal red giant epsilonVir,however, Sc and Ti are overabundant (~0.5 dex). The alpha-processelements are solar or slightly deficient, except Na, which isoverabundant. Carbon and elements heavier than Ni are enhanced by +0.7dex on average compared with the standard star. The heliocentric radialvelocity is found to be +88.4 km s^-1. The detached circumstellar shell,its radial velocity, and the likely overabundance of carbon ands-process elements suggest that HD 179821 is probably an evolvedlow-mass star in the evolutionary stage between the asymptotic giantbranch (AGB) and planetary nebulae (PNe).
| HD 105262: a high latitude metal-poor post-AGB. A supergiant with large proper motion.
The large proper motion post-AGB supergiant stars are very rare. HD105262 is a high galactic latitude (+72deg) A-type star with largeproper motion (0.057arcsec/year) and high c1-index. Earlier, it wasclassified as a Field Horizontal Branch (FHB) star. Recently, Abt (1993)suggested that it may be a star similar to HR 4049. From an analysis ofthe spectra the chemical composition of HD 105262 has been determined.The elemental abundances are found to be, [Fe/H]=-2.2, [C/H]=-0.1,[N/H]=-0.2, [O/H]=-0.5, [α/H]=-1.6. The C/O=1.2 shows that HD105262 is a carbon rich star. The chemical composition of HD 105262indicates that it has gone through the AGB nucleosynthesis. The chemicalcomposition, absolute magnitude, high galactic latitude and kinematicsindicate that HD 105262 is a halo metal-poor post-AGB A supergiant. Itis not a FHB star.
| Light and colour curves of the variable UU Herculis.
Not Available
| Classification of Population II Stars in the Vilnius Photometric System. I. Methods
The methods used for classification of Population II stars in theVilnius photometric system are described. An extensive set of standardswith known astrophysical parameters compiled from the literature sourcesis given. These standard stars are classified in the Vilnius photometricsystem using the methods described. The accuracy of classification isevaluated by a comparison of the astrophysical parameters derived fromthe Vilnius photometric system with those estimated from spectroscopicstudies as well as from photometric data in other systems. For dwarfsand subdwarfs, we find a satisfactory agreement between our reddeningsand those estimated in the uvbyscriptstyle beta system. The standarddeviation of [Fe/H] deter mined in the Vilnius system is about 0.2 dex.The absolute magnitude for dwarfs and subdwarfs is estimated with anaccuracy of scriptstyle <=0.5 mag.
| The extremely iron-deficient "Post-AGB" stars and binaries.
We present radial velocity measurements of the extremely iron-deficient"post-AGB" stars HR 4049, HD 44179 and HD 52961. The photometric binaryperiod of HR 4049 is confirmed and refined to 429 days, HD 44179 isfound to be a binary with a period of 298 days and a large eccentricity,and also for HD 52961 orbital motion is observed. We have thusconclusive evidence that all the known extremely Fe deficient "post-AGB"stars (BD+39 4926, HR 4049, HD 44179, HD 52961 and HD 46703) arebinaries with periods in the order of one to a few years. It is clearthen that binarity, with a narrow range of orbital periods, is a keyaspect for the understanding of the chemical separation and reaccretionprocess which causes the peculiar abundance patterns. A masstransferring process in which the evolution of the system was affectedwithout destroying it must have taken place in the recent past. With theobserved orbital parameters, it is more likely that the stars representa particular stage in binary evolution rather than being typicalpost-AGB stars.
| Evidence for the early onset of aspherical structure in the planetary nebula formation process: Spectropolarimetry of post-AGB stars
We present optical spectropolarimetry of 31 post-asymptotic giant branch(post-AGB) stars, objects that are believed to represent the firstphases of the transition from the AGB to the planetary nebula stage. 24of these objects are found to be intrinsically polarized. We group theseobjects into four classes based on their observed polarizationproperties, rho (lambda) and theta (lambda), and discuss possibleexplainations for the observed behaviors. Type 1 objects display highlevels of polarization and large position angle rotations. Type 2objects lack large position angle rotations, but have polarizations toolarge to be attributed to foreground interstellar material. Type 3objects show position angle rotations and polarization changes acrossTiO absorption features; and finally, Type 4 are objects in which theobserved polarization can be entirely attributed to interstellareffects. The currently popular paradigm of planetary nebula morphologyof a dense torus plus bipolar lobes can explain the rho and thetabehavior of the Type 1 and Type 2 objects. However, a large number ofthe objects exhibit time-variable rho and theta. This implies that theirmorphologies may not be stable, but rather evolving or transientstructures. Regardless of any specific model for the morphology, ourmain result is that aspherical structure appears very early in thetransition from the AGB to the planetary nebula stage. We compare thepolarization properties of the post-AGB stars to the morphologicalcharacteristics of evolved planetary nebulae. Our results indicate thatthe nebular morphology may originate at an early evolutionary stage. Wedo not observe any correlation between chemistry (O-rich vs C-rich) andpolarization class in the AGB stars. We examine the formation ofaspherical planetary nebulae in the context of binary star evolution. Ifbipolar geometry is a consequence of binary star evolution, the numberof highly polarized post-AGB stars in our sample requires a high binarystar frequency.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Cochero |
| Ascensión Recta: | 06h37m52.43s |
| Declinación: | +53°31'02.0" |
| Magnitud Aparente: | 9.053 |
| Movimiento Propio en Ascensión Recta: | 0.8 |
| Movimiento Propio en Declinación: | -1 |
| B-T magnitude: | 9.638 |
| V-T magnitude: | 9.102 |
Catálogos y designaciones:
|
