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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.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| On metal-deficient barium stars and their link with yellow symbiotic stars
This paper addresses the question of why metal-deficient barium starsare not yellow symbiotic stars (YSyS). Samples of (suspected)metal-deficient barium (mdBa) stars and YSyS have been collected fromthe literature, and their properties reviewed. It appears in particularthat the barium nature of the suspected mdBa stars needs to beascertained by detailed abundance analyses. Abundances are thereforederived for two of them, HD 139409 and HD 148897, which reveal that HD148897 should not be considered a barium star. HD 139409 is a mildbarium star, with overabundances observed only for elements belonging tothe first s-process peak (Y and Zr). It is only moderately metal-poor([Fe/H] = -0.4). The evidence for binarity among mdBa stars is thenreviewed, using three different methods: (i) radial-velocity variations(from CORAVEL observations), (ii) Hipparcos astrometric data, and (iii)a method based on the comparison between the Hipparcos and Tycho-2proper motions. An orbit is obtained for HIP 55852, whereas evidence forthe (so far unknown) binary nature of HIP 34795, HIP 76605, HIP 97874and HIP 107478 is presented. No conclusion regarding the binary natureof HIP 11595, HIP 25161 could be reached. Two stars with no evidence forbinarity whatsoever (HIP 58596 and BD +3°2688) are candidateslow-metallicity thermally-pulsing asymptotic giant branch stars, asinferred from their large luminosities. The reason why mdBa stars arenot YSyS is suggested to lie in their different orbital perioddistributions: mdBa stars have on average longer orbital periods thanYSyS, and hence their companion accretes matter at a lower rate, for agiven mass loss rate of the giant star. The definite validation of thisexplanation should nevertheless await the determination of the orbitalperiods for the many mdBa stars still lacking periods, in order to makethe comparison more significant.
| 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.
| 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.
| Light element abundances in barium stars
The paper obtains abundances of the elements Li, C, N, O, and Al, aswell as the C-12:C-13 and Mg-24:Mg-25:Mg-26 isotopic ratios for 11barium stars with the aim of deriving constraints on the nucleosynthesisevent responsible for the chemical peculiarities observed in theenvelope of these stars. Constraints on the composition of the matterthat has polluted the barium star envelope can also be derived from theC-12:C-13 and C-12:O-16 ratios. For most barium stars, these ratios arecompatible with pollution by matter from cool carbon stars. It issuggested that, if the accretion of matter from a cool carbon starcompanion occurs when the barium star is still on the main sequence, thelow C-12:C-13 ratios can be reconciled with the carbon overabundance byconsidering the effect on the first dredge-up of the prior enrichment ofthe envelope by C-12 from the accreted matter.
| Photometry of F-K type bright giants and supergiants. I - Intermediate band and H-Beta observations
Over 1500 observations of 560 bright giants and supergiants of types F-Kare presented and compared to the observations by Gray and Olsen (1991).The present results include intermediate-band which is slightlydifferent from the Stromgren data by Gray and Olsen due to a differentwidth for the v filter. A systematic difference in m(1) - M(1) withdecreasing temperature is noted in the two H-Beta data sets, and thecorrelations are defined.
| 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.
| 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.
| The reddening, metal abundance, and luminosity of high-luminosity G-type stars
The abundance parameters of the sample of 100 G-type bright giants andsupergiants whose photometry is presently discussed indicate Fe/H valuesof between about + 0.6 dex and the solar value, using a preliminarycalibration of the photometric indices. The few long period Cepheidsthat are not heavily blanketed F-type stars give reddening values thatare in agreement with current results from other methods of analysis.Supergiants show no clear gradients in the galactic distribution ofabundances, and those nearest the sun range from an Fe/H of about + 0.3dex, for members of the Pleiades Group, to near solar abundance. A smallsample of Large Magellanic Cloud supergiants shows nearly the same rangein metal abundance as the galactic stars, and gives a modulus for thisCloud of 18.3 + or - 0.20 mag.
| A catalog of spectral classification and photometry of barium stars
Many other Ba II stars have been found, since the enhancement of theline of singly ionized barium (4554 A) in late-type, high-luminositystars was discovered by Bidelman and Keenan (1951). The majority ofstars so identified are listed in a study conducted by MacConnell et al.(1972). MacConnell et al. identified 150 'certain' barium stars and anadditional 90 'marginal' barium stars from inspection of objective-prismplates of the Michigan Spectral Survey of the southern sky. Since themajority of known Ba II stars were discovered with objective-prismplates, they have lacked high-quality spectral classifications. It hasbeen attempted to obtain these data along with broad- andintermediate-band photometry, in order to study the properties of thissubgroup of stars in greater detail than has heretofore been possible.Except for the stars recently identified by Bidelman (1981), the list ofspectroscopic and photometric data in Table 1 includes virtually allrecognized barium stars. The stars identified by Bidelman are listed inTable 2.
| DDO Observations of Southern Stars
Not Available
| Spectroscopic radial velocity and photometric observations of barium stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977MNRAS.181..391C&db_key=AST
| Photometry of possible barium stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975PASP...87..111E&db_key=AST
| The absolute magnitudes of the barium stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972AJ.....77..384M&db_key=AST
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