Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
A holistic approach to carbon-enhanced metal-poor stars
Context. Carbon-enhanced metal-poor (CEMP) stars are known to haveproperties that reflect the nucleosynthesis of the first low- andintermediate-mass stars, because most have been polluted by anow-extinct AGB star. Aims: By considering abundances in thevarious CEMP subclasses separately, we try to derive parameters (such asmetallicity, mass, temperature, and neutron source) characterising AGBnucleosynthesis from the specific signatures imprinted on theabundances, and separate them from the impact of thermohaline mixing,first dredge-up, and dilution associated with the mass transfer from thecompanion. Methods: To place CEMP stars in a broader context, wecollect abundances for about 180 stars of various metallicities (fromsolar to [Fe/H] =-4), luminosity classes (dwarfs and giants), andabundance patterns (e.g. C-rich and poor, Ba-rich and poor), from bothour own sample and the literature. Results: We first show thatthere are CEMP stars that share the properties of CEMP-s stars andCEMP-no stars (which we refer to as CEMP-low-s stars). We also show thatthere is a strong correlation between Ba and C abundances in the s-onlyCEMP stars. This represents a strong detection of the operation of the13C neutron source in low-mass AGB stars. For the CEMP-rsstars (seemingly enriched with elements from both the s- andr-processes), the correlation of the N abundances with abundances ofheavy elements from the 2nd and 3rd s-process peaks bears instead thesignature of the 22Ne neutron source. Since CEMP-rs starsalso exhibit O and Mg enhancements, we conclude that extremely hotconditions prevailed during the thermal pulses of the contaminating AGBstars. We also note that abundances are not affected by the evolution ofthe CEMP-rs star itself (especially by the first dredge-up). Thisimplies that mixing must have occurred while the star was on the mainsequence, and that a large amount of matter must have been accreted soas to trigger thermohaline mixing. Finally, we argue that most CEMP-nostars (with neutron-capture element abundances comparable to non-CEMPstars) are likely the extremely metal-poor counterparts of CEMPneutron-capture-rich stars. We also show that the C enhancement inCEMP-no stars declines with metallicity at extremely low metallicity([Fe/H] < -3.2). This trend is not predicted by any of the currentAGB models.Tables 1-4 are only available in electronic form at http://www.aanda.org
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is 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/501/941
| Galactic distributions and statistics of the HD stars in the michigan spectral catalogue.
Not Available
| The Origin of Carbon Enhancement and the Initial Mass Function of Extremely Metal-poor Stars in the Galactic Halo
It is known that the carbon-enhanced, extremely metal-poor (CEMP) starsconstitute a substantial proportion of the extremely metal-poor (EMP)stars of the Galactic halo, and a by far larger proportion than CH starsamong Population II stars. We investigate their origin by taking intoaccount an additional evolutionary path to the surface carbonenrichment, triggered by hydrogen engulfment by the helium flashconvection, in EMP stars with [Fe/H]<~-2.5. This process is distinctfrom the third dredge-up operating in more metal-rich stars and in EMPstars. In binary systems of EMP stars, the secondary stars become CEMPstars through mass transfer from the low- and intermediate- mass primarystars that have developed the surface carbon enhancement. Our binaryscenario can predict the variations in the abundances not only forcarbon but also for nitrogen and s-process elements and can reasonablyexplain the observed properties such as the stellar distributions of thecarbon abundances, the binary periods, and the evolutionary stages.Furthermore, from the observed frequencies of CEMP stars with andwithout s-process element enhancement, we demonstrate that the initialmass function of EMP stars needed gives the mean mass~10Msolar under the reasonable assumptions for the distributionsof orbital separations and mass ratios of the binary components. Thisalso indicates that the currently observed EMP stars were exclusivelyborn as the secondary members of binaries, making up ~10% of EMP binarysystems, with mass~108 Msolar in total; inaddition to CEMP stars with white dwarf companions, a significantfraction of them have experienced supernova explosions of theircompanions. We discuss the implications of the present results for theformation of the Galactic halo.
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Analysis of 26 barium stars. II. Contributions of s-, r-, and p-processes in the production of heavy elements
Context: .Barium stars show enhanced abundances for the slow neutroncapture (s-process) heavy elements, so they are suitable objects forstudying s-process elements. Aims: .The aim of this work is toquantify the contributions of the s-, r-, and p-processes for the totalabundance of heavy elements from abundances derived for a sample of 26barium stars. The abundance ratios between these processes and neutronexposures were studied. Methods: .The abundances of the samplestars were compared to those of normal stars, thus identifying thefraction relative to the main component of the s-process. Results:.The fittings of the σ N curves (neutron-capture cross-sectiontimes abundance, plotted against atomic mass number) for the samplestars suggest that the material from the companion asymptotic giantbranch star had approximately the solar isotopic composition as concernsfractions of abundances relative to the s-process main component. Theabundance ratios of heavy elements, hs, ls, and s and the computedneutron exposure are similar to those of post-AGB stars. For some samplestars, an exponential neutron exposure fits the observed data well,whereas a single neutron exposure provides a better fit for others. Conclusions: .The comparison of barium and AGB stars supports thehypothesis of binarity for the barium star formation. Abundances ofr-elements that are part of the s-process path in barium stars areusually higher than those in normal stars, so barium stars also seemedto be enriched in r-elements, although to a lower degree thans-elements. No dependence on luminosity classes was found in theabundance-ratio behaviour among the dwarfs and giants of the sample ofbarium stars.
| Analysis of 26 barium stars. I. Abundances
Context: .We present a detailed analysis of 26 barium stars, includingdwarf barium stars, providing their atmospheric parameters (T_eff, logg, [Fe/H], v_t), and elemental abundances. Aims: .We aim atderiving gravities and luminosity classes of the sample stars, inparticular to confirm the existence of dwarf barium stars. Accurateabundances of chemical elements were derived. We present the abundanceratios between nucleosynthetic processes, by using Eu and Ba asrepresentatives of the r- and s-processes. Methods:.High-resolution spectra were obtained with the FEROS spectrograph atthe ESO-1.52 m Telescope, along with photometric data with Fotrap at theZeiss telescope at the LNA. The atmospheric parameters were derived inan iterative way, with temperatures obtained from colour-temperaturecalibrations. The abundances were derived using spectrum synthesis forLi, Na, Al, α-, iron-peak, s-, and r-element atomic lines, and forC and N molecular lines. Results: .Atmospheric parameters in therange 4300 < T_eff < 6500, -1.2 < [Fe/H] < 0.0, and 1.4≤ log g < 4.6 were derived, confirming that our sample containsgiants, subgiants, and dwarfs. The abundance results obtained for Li,Al, Na, α-, and iron-peak elements for the sample stars show thatthey are compatible with the values found in the literature for normaldisk stars in the same range of metallicities. Enhancements of C, N, andheavy elements relative to Fe, that characterise barium stars, werederived and showed that [X/Ba] vs. [Ba/H] and [X/Ba] vs. [Fe/H] presentdifferent behaviour as compared to [X/Eu] vs. [Eu/H] and [X/Eu] vs.[Fe/H], reflecting the different nucleosynthetic sites for the s- andr-processes.
| Estimation of Carbon Abundances in Metal-Poor Stars. I. Application to the Strong G-Band Stars of Beers, Preston, and Shectman
We develop and test a method for the estimation of metallicities([Fe/H]) and carbon abundance ratios ([C/Fe]) for carbon-enhancedmetal-poor (CEMP) stars based on the application of artificial neuralnetworks, regressions, and synthesis models to medium-resolution (1-2Å) spectra and J-K colors. We calibrate this method by comparisonwith metallicities and carbon abundance determinations for 118 starswith available high-resolution analyses reported in the recentliterature. The neural network and regression approaches make use of apreviously defined set of line-strength indices quantifying the strengthof the Ca II K line and the CH G band, in conjunction with J-K colorsfrom the Two Micron All Sky Survey Point Source Catalog. The use ofnear-IR colors, as opposed to broadband B-V colors, is required becauseof the potentially large affect of strong molecular carbon bands onbluer color indices. We also explore the practicality of obtainingestimates of carbon abundances for metal-poor stars from the spectralinformation alone, i.e., without the additional information provided byphotometry, as many future samples of CEMP stars may lack such data. Wefind that although photometric information is required for theestimation of [Fe/H], it provides little improvement in our derivedestimates of [C/Fe], and hence, estimates of carbon-to-iron ratios basedsolely on line indices appear sufficiently accurate for most purposes.Although we find that the spectral synthesis approach yields the mostaccurate estimates of [C/Fe], in particular for the stars with thestrongest molecular bands, it is only marginally better than is obtainedfrom the line index approaches. Using these methods we are able toreproduce the previously measured [Fe/H] and [C/Fe] determinations withan accuracy of ~0.25 dex for stars in the metallicity interval-5.5<=[Fe/H]<=-1.0 and with 0.2<=(J-K)0<=0.8. Athigher metallicity, the Ca II K line begins to saturate, especially forthe cool stars in our program, and hence, this approach is not useful insome cases. As a first application, we estimate the abundances of [Fe/H]and [C/Fe] for the 56 stars identified as possibly carbon-rich, relativeto stars of similar metal abundance, in the sample of ``strong G-band''stars discussed by Beers, Preston, and Shectman.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Carbon-rich giants in the HR diagram and their luminosity function
The luminosity function (LF) of nearly 300 Galactic carbon giants isderived. Adding BaII giants and various related objects, about 370objects are located in the RGB and AGB portions of the theoretical HRdiagram. As intermediate steps, (1) bolometric corrections arecalibrated against selected intrinsic color indices; (2) the diagram ofphotometric coefficients 1/2 vs. astrometric trueparallaxes varpi are interpreted in terms of ranges of photosphericradii for every photometric group; (3) coefficients CR andCL for bias-free evaluation of mean photospheric radii andmean luminosities are computed. The LF of Galactic carbon giantsexhibits two maxima corresponding to the HC-stars of the thick disk andto the CV-stars of the old thin disk respectively. It is discussed andcompared to those of carbon stars in the Magellanic Clouds and Galacticbulge. The HC-part is similar to the LF of the Galactic bulge,reinforcing the idea that the Bulge and the thick disk are part of thesame dynamical component. The CV-part looks similar to the LF of theLarge Magellanic Cloud (LMC), but the former is wider due to thesubstantial errors on HIPPARCOS parallaxes. The obtained meanluminosities increase with increasing radii and decreasing effectivetemperatures, along the HC-CV sequence of photometric groups, except forHC0, the earliest one. This trend illustrates the RGB- and AGB-tracks oflow- and intermediate-mass stars for a range in metallicities. From acomparison with theoretical tracks in the HR diagram, the initial massesMi range from about 0.8 to 4.0 Msun for carbongiants, with possibly larger masses for a few extreme objects. A largerange of metallicities is likely, from metal-poor HC-stars classified asCH stars on the grounds of their spectra (a spheroidal component), tonear-solar compositions of many CV-stars. Technetium-rich carbon giantsare brighter than the lower limit Mbol =~ -3.6+/- 0.4 andcentered at =~-4.7+0.6-0.9 at about =~(2935+/-200) K or CV3-CV4 in our classification. Much like the resultsof Van Eck et al. (\cite{vaneck98}) for S stars, this confirms theTDU-model of those TP-AGB stars. This is not the case of the HC-stars inthe thick disk, with >~ 3400 K and>~ -3.4. The faint HC1 and HC2-stars( =~ -1.1+0.7-1.0) arefound slightly brighter than the BaII giants ( =~-0.3+/-1.3) on average. Most RCB variables and HdC stars range fromMbol =~ -1 to -4 against -0.2 to -2.4 for those of the threepopulation II Cepheids in the sample. The former stars show the largestluminosities ( <~ -4 at the highest effectivetemperatures (6500-7500 K), close to the Mbol =~ -5 value forthe hot LMC RCB-stars (W Men and HV 5637). A full discussion of theresults is postponed to a companion paper on pulsation modes andpulsation masses of carbon-rich long period variables (LPVs; Paper IV,present issue). This research has made use of the Simbad databaseoperated at CDS, Strasbourg, France. Partially based on data from theESA HIPPARCOS astrometry satellite. Table 2 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/967
| Catalogue of [Fe/H] determinations for FGK stars: 2001 edition
The catalogue presented here is a compilation of published atmosphericparameters (Teff, log g, [Fe/H]) obtained from highresolution, high signal-to-noise spectroscopic observations. This newedition has changed compared to the five previous versions. It is nowrestricted to intermediate and low mass stars (F, G and K stars). Itcontains 6354 determinations of (Teff, log g, [Fe/H]) for3356 stars, including 909 stars in 79 stellar systems. The literature iscomplete between January 1980 and December 2000 and includes 378references. The catalogue is made up of two tables, one for field starsand one for stars in galactic associations, open and globular clustersand external galaxies. The catalogue is distributed through the CDSdatabase. Access to the catalogue with cross-identification to othersets of data is also possible with VizieR (Ochsenbein et al.\cite{och00}). The catalogue (Tables 1 and 2) is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/373/159 and VizieRhttp://vizier.u-strasbg.fr/.
| Kinematics of Hipparcos Visual Binaries. I. Stars with Orbital Solutions
A sample consisting of 570 binary systems is compiled from severalsources of visual binary stars with well-known orbital elements.High-precision trigonometric parallaxes (mean relative error about 5%)and proper motions (mean relative error about 3%) are extracted from theHipparcos Catalogue or from the reprocessed Hipparcos data. However, 13%of the sample stars lack radial velocity measurements. Computed galacticvelocity components and other kinematic parameters are used to dividethe sample stars into kinematic age groups. The majority (89%) of thesample stars, with known radial velocities, are the thin disk stars,9.5% binaries have thick disk kinematics and only 1.4% are halo stars.85% of thin disk binaries are young or medium age stars and almost 15%are old thin disk stars. There is an urgent need to increase the numberof the identified halo binary stars with known orbits and substantiallyimprove the situation with their radial velocity data.
| Re-processing the Hipparcos Transit Data and Intermediate Astrometric Data of spectroscopic binaries. I. Ba, CH and Tc-poor S stars
Only 235 entries were processed as astrometric binaries with orbits inthe Hipparcos and Tycho Catalogue (\cite{Hipparcos}). However, theIntermediate Astrometric Data (IAD) and Transit Data (TD) made availableby ESA make it possible to re-process the stars that turned out to bespectroscopic binaries after the completion of the Catalogue. This paperillustrates how TD and IAD may be used in conjunction with the orbitalparameters of spectroscopic binaries to derive astrometric parameters.The five astrometric and four orbital parameters (not already known fromthe spectroscopic orbit) are derived by minimizing an objective function(chi 2) with an algorithm of global optimization. This codehas been applied to 81 systems for which spectroscopic orbits becameavailable recently and that belong to various families ofchemically-peculiar red giants (namely, dwarf barium stars, strong andmild barium stars, CH stars, and Tc-poor S stars). Among these 81systems, 23 yield reliable astrometric orbits. These 23 systems make itpossible to evaluate on real data the so-called ``cosmic error''described by Wielen et al. (1997), namely the fact that an unrecognizedorbital motion introduces a systematic error on the proper motion.Comparison of the proper motion from the Hipparcos catalogue with thatre-derived in the present work indicates that the former are indeed faroff the present value for binaries with periods in the range 3 to ~ 8years. Hipparcos parallaxes of unrecognized spectroscopic binaries turnout to be reliable, except for systems with periods close to 1 year, asexpected. Finally, we show that, even when a complete orbital revolutionwas observed by Hipparcos, the inclination is unfortunately seldomprecise. Based on observations from the Hipparcos astrometric satelliteoperated by the European Space Agency (ESA 1997).
| Barium stars, galactic populations and evolution.
In this paper HIPPARCOS astrometric and kinematical data together withradial velocities from other sources are used to calibrate bothluminosity and kinematics parameters of Ba stars and to classify them.We confirm the results of our previous paper (where we used data fromthe HIPPARCOS Input Catalogue), and show that Ba stars are aninhomogeneous group. Five distinct classes have been found i.e. somehalo stars and four groups belonging to disk population: roughlysuper-giants, two groups of giants (one on the giant branch, the otherat the clump location) and dwarfs, with a few subgiants mixed with them.The confirmed or suspected duplicity, the variability and the range ofknown orbital periods found in each group give coherent resultssupporting the scenario for Ba stars that are not too highly massivebinary stars in any evolutionary stages but that all were previouslyenriched with Ba from a more evolved companion. The presence in thesample of a certain number of ``false'' Ba stars is confirmed. Theestimates of age and mass are compatible with models for stars with astrong Ba anomaly. The mild Ba stars with an estimated mass higher than3Msun_ may be either stars Ba enriched by themselves or``true'' Ba stars, which imposes new constraints on models.
| 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
| The barium stars in the Hertzsprung-Russel diagram.
We present absolute magnitudes for a sample of 52 barium stars observedby the HIPPARCOS satellite, and their location in the HR diagram. Ourplot (Fig. 1) is restricted to stars with parallax accuracies betterthan 22%. The luminosity classes range from Ib supergiants down to Vdwarfs on the main sequence, as expected from spectral classificaiton.Discrepancies are however notes. No gap is observed in the regionextending from the main sequence to the giant branch, excatly as shownby Perryman et al. (1995A&A...304...69P) for normal stars. This isalso true for class II bright giants. A clump is however obvious atG8-K0 IIIb and M_V_~0. 85 which correspond to the one noted as (B-V)~1.0and M_Hp_~1.0 by Perryman et al. It appears that barium stars on themain sequence are earlier than G4, upward evolution being noticeable forlater types. They are also distributed in the subgiant zone followingthe locus of normal stars, i.e. increasing brightness for later types. Afew stars in our sample are also classified as CH stars: four of themare definitely main sequence class V-dwarfs, one is a class IVb faintsubgiant while two possible CH-stars are class III-giants. These resultsare consistent with the currently-admitted model of surface pollution ofa normal star through mass transfer in a binary system whose primary hasbecome a white dwarf (WD). HIPPARCOS data show perturbations of theastrometric solution which can be attributed to proved (or possible)binarity for 21 stars our of 121, and 8 of them were already quoted inthe CCDM catalogue (not necessarily with a WD component). This lowproportion can be explained by the 5-11 magnitudes differences predictedbetween the two components and/or low angular separation with periodsclose to one year.
| Absolute magnitudes and kinematics of barium stars.
The absolute magnitude of barium stars has been obtained fromkinematical data using a new algorithm based on the maximum-likelihoodprinciple. The method allows to separate a sample into groupscharacterized by different mean absolute magnitudes, kinematics andz-scale heights. It also takes into account, simultaneously, thecensorship in the sample and the errors on the observables. The methodhas been applied to a sample of 318 barium stars. Four groups have beendetected. Three of them show a kinematical behaviour corresponding todisk population stars. The fourth group contains stars with halokinematics. The luminosities of the disk population groups spread alarge range. The intrinsically brightest one (M_v_=-1.5mag,σ_M_=0.5mag) seems to be an inhomogeneous group containing bariumbinaries as well as AGB single stars. The most numerous group (about 150stars) has a mean absolute magnitude corresponding to stars in the redgiant branch (M_v_=0.9mag, σ_M_=0.8mag). The third group containsbarium dwarfs, the obtained mean absolute magnitude is characteristic ofstars on the main sequence or on the subgiant branch (M_v_=3.3mag,σ_M_=0.5mag). The obtained mean luminosities as well as thekinematical results are compatible with an evolutionary link betweenbarium dwarfs and classical barium giants. The highly luminous group isnot linked with these last two groups. More high-resolutionspectroscopic data will be necessary in order to better discriminatebetween barium and non-barium stars.
| A New Version of the Catalog of CH and Related Stars (CH95 Catalog)
A new version of the catalog of CH and related stars contains 244 fieldstars and 17 globular cluster stars. Here a list of these stars withtheir coordinates, their positions in the HR diagram and somestatistical diagrams is presented. The catalog will soon be available inthe printed and computerized versions.
| Lithium in dwarf and subgiant Ba stars
Not Available
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| The nature of the F STR λ4077 stars. V. Spectroscopic data
We report first high-resolution spectroscopic observations of 17 starsclassified F str λ4077 by Bidelman. They are compared todifferent types of stars, which have been observed at the same time,including 3 CH subgiants, 3 barium stars, 5 λ Bootis stars, Ap/Amstars, and normal main-sequence stars. The detailed equivalent widthsand abundances are given for all measured spectral lines, as well as themean abundance per species for each object. These data and theirsignificance are described in detail and discussed in an articlepublished in the Main Journal.
| The nature of the F STR lambda 4077 stars. 3: Spectroscopy of the barium dwarfs and other CP stars
The abundances of C, O, Al, Ca, iron-peak and s-process elements havebeen derived from high-resolution spectra for a sample of starsclassified as F str lambda 4077 by Bidelman. Among the 20 starsmentioned by Bidelman, we have discovered 8 barium dwarfs (or CHsubgiants, according to Bond's terminology), while a 9th star, HD182274, was already known as a CH subgiant. In addition, we haveanalyzed three barium stars taken from the list of Lu et al. (1983)which are probably dwarfs rather than giants, and three CH subgiants.The other 11 F str lambda 4077 stars resemble either the delta Delphinistars, since their iron abundance is enhanced while Ca is normal, or areprobably spectrum composites. A few Am, Ap, lambda Bootis and normalstars have been analyzed for comparison. In particular, we have includedthree lambda Boo candidates, selected from their photometric properties,and their iron deficiency is confirmed. The spectroscopic, photometricand statistical evidences concerning the Ba dwarfs, support the ideathat these stars may be the main sequence counterparts, and possibly theprogenitors of the Ba giants. The C/O ratio varies in these stars fromnormal values to a maximum of 1.5, but mostly within 0.6 and 1.2. Someof these objects may therefore be considered, in this sense, as carbonstars. On the other hand, the abundances of carbon and s-processelements relative to iron are inversely correlated with metallicity, andmay even exceed significantly those of typical, solar-metallicity carbonstars. Metal-deficient C stars must therefore have (C/Fe) greater thanor approximately equal to 1 and (s/Fe) greater than or approximatelyequal to 1.5 as soon as (Fe/H) less than or approximately equal to -1.The neutron exposure is shown to increase when the metallicitydecreases, which is compatible with the C-13 (alpha, n) O-16 neutronsource, but not with the Ne-22 (alpha, n) Mg-25 one. The evolutionarystate (within the main sequence) of the Ba dwarfs, is rediscussed inrelation with their photometric and spectroscopic surface gravity, butit remains unclear.
| Subgiant CH stars. II - Chemical compositions and the evolutionary connection with barium stars
Chemical abundances in 15 subgiant CH stars, five barium stars, and fourobjects classified as metal-deficient barium stars are determinedthrough model-atmosphere analyses of high-resolution photographicechelle spectrograms. The stars in each of these groups show enhancedsurface abundances of carbon and the very heavy elements produced bys-process neutron-capture reactions. The atmospheric parameters ofsubgiant CH stars are found to be typical of F- and G-type main-sequencestars and subgiants. Barium stars are more luminous G- and K-typegiants. Both classes of objects constitute about 1 percent of theirrespective populations, and exhibit a high incidence (about 100 percent)of the membership in wide spectroscopic binaries. Subgiant CH starsoccur on the main sequence, but only in the T(eff) range 6000-6800 K. Itis argued that subgiant CH stars are created when an AGB star in a widebinary contaminates its main-sequence companion with carbon ands-processed material.
| Taxonomy of barium stars
Spectral classification, barium intensity, radial velocity, luminosity,and kinematical properties are determined for 389 barium stars byanalyzing image-tube spectra and photometric observation data. Diskkinematics for the stars are based on whether they are Ba weak or Bastrong. Weak barium stars in general have smaller velocity dispersions,brighter apparent magnitude, and lower luminosity than strong bariumstars. These characteristics are confirmed by solving for meanspectroscopic distances, z-scale height distances, and reduced propermotions.
| Astrometric and astrophysical discontinuities between the galactic old disk and halo stellar populations
Intermediate band, RI, and DDO photometry of the weak-lined stars in thefirst three volumes of the Michigan catalogs of spectral type arediscussed on the basis of luminosity and heavy element abundance. Theinterface between the old disk (Fe/H greater than -0.8 dex) and halo(Fe/H less than -1.2 dex) populations represents discontinuities in boththe stellar motions and the stellar physics. The CN strengths of bothevolved and unevolved halo stars decrease with decreasing temperature,in a mirror image of the increase with decreasing temperature for thedisk objects. The result for the halo giants has been attributed to deepmixing in the stellar atmospheres but the similar result for unevolveddwarfs indicates a difference in formation rather than in evolutionaryprocess of the two populations.
| Catalogue of CH and metal-deficient barium stars
Not Available
| Kinematic and spatial distributions of barium stars - Are the barium stars and AM stars related?
The possibility of an evolutionary link between Am stars and bariumstars is considered, and an examination of previous data suggests thatbarium star precursors are main-sequence stars of intermediate mass, aremost likely A and/or F dwarfs, and are intermediate-mass binaries withclose to intermediate orbital separations. The possible role of masstransfer in the later development of Am systems is explored. Masstransfer and loss from systems with a range of masses and orbitalseparations may explain such statistical peculiarities of barium starsas the large dispersion in absolute magnitude, the large range ofelemental abundances from star to star, and the small number of starswith large peculiar velocities.
| Large and Kinematically Unbiased Samples of G- and K-Type Stars. III. Evolved Young Disk Stars in the Bright Star Sample
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1989PASP..101...54E&db_key=AST
| Weak-lined stars in the Michigan spectral survey. I - A-, F-, and G-type stars
Data from intermediate-band and H-beta photometric observations of 89early-type stars classified as weak-lined by Bidelman and MacConnell(1973), Houk and Cowley (1975), or Houk (1978 and 1982) are presented intables and graphs and analyzed. The group is found to comprise 55 diskstars and 34 halo stars (Fe/H less than -0.65), including 21 near themain-sequence turnoff, seven red horizontal-branch stars, one bluehorizontal-branch star, one suprahorizontal-branch object, and four bluestragglers; five of the newly identified halo stars are shown to haveradial velocities in excess of 100 km/s. Special consideration is givento the 17 early-type members of Kapetyn's star group (Eggen, 1977), andthe mean luminosity and Fe/H ratio of the four very-short-periodCepheids of this group are determined as 0.75 mag and -1.7.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Kentaur |
| Rektaszcenzió: | 14h09m35.80s |
| Deklináció: | -44°22'01.2" |
| Vizuális fényesség: | 9.213 |
| RA sajátmozgás: | 37 |
| Dec sajátmozgás: | -29.2 |
| B-T magnitude: | 9.917 |
| V-T magnitude: | 9.272 |
Katalógusok és elnevezések:
|
