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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
| Ruthenium and hafnium abundances in giant and dwarf barium stars
Aims.We present abundances for Ru and Hf, compare them to abundances ofother heavy elements, and discuss the problems found in determining Ruand Hf abundances with laboratory gf-values in the spectra of bariumstars. Methods: We determined Ru and Hf abundances in a sample of giantand dwarf barium stars, by the spectral synthesis of two Ru I(λ4080.574 and λ4757.856) and two Hf II (λ4080.437and λ4093.155) transitions. The stellar spectra were observedwith FEROS/ESO, and the stellar atmospheric parameters lie in the range4300 < T_eff/K < 6500, -1.2 < [Fe/H] ≤ 0 and 1.4 ≤ log g< 4.6. Results: The Hf II λ4080 and the Ru I λ4758observed transitions result in a unreasonably high solar abundance,given certain known uncertainties, when fitted with laboratorygf-values. For these two transitions we determined empirical gf-valuesby fitting the observed line profiles of the spectra of the Sun andArcturus. For the sample stars, this procedure resulted in a goodagreement of Ru and Hf abundances given by the two available lines. Theresulting Ru and Hf abundances were compared to those of Y, Nd, Sm andEu. In the solar system Ru, Sm and Eu are dominated by the r-process andHf, Nd and Y by the s-process, and all of these elements are enhanced inbarium stars since they lie inside the s-process path. Ru abundancesshow large scatter when compared to other heavy elements, whereas Hfabundances show less scatter and closely follow the abundances of Sm andNd, in good agreement with theoretical expectations. We also suggest apossible, unexpected, correlation of Ru and Sm abundances. The observedbehaviour in abundances is probably due to variations in the13C pocket efficiency in AGB stars, and, though masked byhigh uncertainties, hint at a more complex scenario than proposed bytheory.Based on spectroscopic observations collected at the European SouthernObservatory (ESO), within the Observatório Nacional ON/ESO andON/IAG agreements, under FAPESP project No. 1998/10138-8. Tables 3-5 areonly available in electronic form at http://www.aanda.org
| 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.
| 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.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| 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/.
| 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
| 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
| 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.
| The nature of the F STR 4077-A stars. II - Frequency, kinematics, metallicity, binarity and rotational velocities
Fundamental characteristics of the F str 4077-A stars are investigated.Their frequency in the solar neighborhood is estimated at about 1.4percent of all main-sequence F stars. The space velocities areconsistent with Population I membership, with the exception of oneobject (in a sample of 17) whose velocity is above 100 km/s. The binaryfrequency is higher than normal, and the rotational velocities arenormal for the cooler stars but rather low for the hotter ones. Overallmetallicity, deduced from photometric and CORAVEL data, suggests adivision of these stars into two groups: metal-rich objects (generallythe hotter ones), which may resemble the Am or Ap stars, and slightlymetal-weak objects (generally the cooler ones), which may be linked withthe Ba II giants or the CH subgiants.
| The nature of the F STR lambda 4077 stars
Geneva photometry of seven stars classified F str 4077-A stars byBidelman (1981) is presented. Published uvby-beta measurements of fiveadditional stars of this type are used, as well to look for a possiblelink with the classical Am or Ap stars. It is shown that these starsexhibit a whole range of metallicities, the latter being correlated witheffective temperature. The existing proper motions point to a PopulationI membership, although one star seems to have a rather high velocity. Itis suggested that this new type of peculiarity may be interpreted in theframework of the diffusion theory and that at least some F str 4077-Astars may represent an intermediate stage between Am and Lambda Bootisstars, although they are generally cooler.
| Objective-prism discoveries in the declination zone 0 deg to -20 deg
An inspection of 130 10-deg-prism plates taken for the University ofMichigan Southern Spectral Survey has yielded 154 new peculiar orotherwise interesting stars. The regions surveyed are mainly at fairlyhigh galactic latitudes; this fills a gap in previous work of this type.About three-quarters of the objects will eventually be reclassified byHouk in a systematic work on all of the Henry Draper Catalogue stars,but many of these are of sufficient interest to justify earlypublication.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Гидра |
| Прямое восхождение: | 10h40m57.71s |
| Склонение: | -12°11'44.2" |
| Видимая звёздная величина: | 8.567 |
| Расстояние: | 127.877 парсек |
| Собственное движение RA: | -31.8 |
| Собственное движение Dec: | 70.8 |
| B-T magnitude: | 9.17 |
| V-T magnitude: | 8.617 |
Каталоги и обозначения:
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