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CD-62°1346: an extreme halo or hypervelocity CH star?
Context. High-velocity halo stars provide important information aboutthe properties of the extreme Galactic halo. The study of unbound andbound Population II stars permits us to better estimate the mass of thehalo. Aims: We carried out a detailed spectroscopic and kinematicstudy and have significantly refined the distance and the evolutionarystate of the star. Methods: Its atmospheric parameters, chemicalabundances and kinematical properties were determined usinghigh-resolution optical spectroscopy and employing thelocal-thermodynamic-equilibrium model atmospheres of Kurucz and thespectral analysis code moog. Results: We found thatCD-62°1346 is a metal-poor ([Fe/H] = -1.6) evolved giant star withTeff = 5300 K and log g = 1.7. The star exhibits high carbonand s-element abundances typical of CH stars. It is also a lead star.Our kinematic analysis of its 3D space motions shows that this star hasa highly eccentric (e = 0.91) retrograde orbit with an apogalacticdistance of ~100 kpc, exceeding by a factor of two the distance of theMagellanic Clouds. The star travels with very high velocity relative tothe Galactocentric reference frame (VGRF = 570 kms-1). Conclusions: CD-62°1346 is an evolved giantstar and not a subgiant star, as was considered earlier. Whether it isbound or unbound to the Galaxy depends on the assumed mass and on theadopted Galactic potential. We also show that the star HD 5223 isanother example of a high-velocity CH star that exceeds the Galacticescape velocity. Possible origins of these two high-velocity stars arebriefly discussed. CD-62°1346 and HD 5223 are the first red giantstars to join the restricted group of hypervelocity stars.Based on observations made with the 2.2 m telescope at the EuropeanSouthern Observatory (La Silla, Chile).Tables 1 and 3 are available inelectronic 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.
| Nucleosynthesis and Mixing on the Asymptotic Giant Branch. III. Predicted and Observed s-Process Abundances
We present the results of s-process nucleosynthesis calculations forasymptotic giant branch (AGB) stars of different metallicities anddifferent initial stellar masses (1.5 and 3 Msolar), and wepresent comparisons of them with observational constraints fromhigh-resolution spectroscopy of evolved stars over a wide metallicityrange. The computations were based on previously published stellarevolutionary models that account for the third dredge-up phenomenonoccurring late on the AGB. Neutron production is driven by the13C(α,n)16O reaction during the interpulseperiods in a tiny layer in radiative equilibrium at the top of the He-and C-rich shell. The neutron source 13C is manufacturedlocally by proton captures on the abundant 12C; a few protonsare assumed to penetrate from the convective envelope into the radiativelayer at any third dredge-up episode, when a chemical discontinuity isestablished between the convective envelope and the He- and C-richzones. A weaker neutron release is also guaranteed by the marginalactivation of the reaction 22Ne(α,n)25Mgduring the convective thermal pulses. Owing to the lack of a consistentmodel for 13C formation, the abundance of 13Cburnt per cycle is allowed to vary as a free parameter over a wideinterval (a factor of 50). The s-enriched material is subsequently mixedwith the envelope by the third dredge-up, and the envelope compositionis computed after each thermal pulse. We follow the changes in thephotospheric abundance of the Ba-peak elements (heavy s [hs]) and thatof the Zr-peak ones (light s [ls]), whose logarithmic ratio [hs/ls] hasoften been adopted as an indicator of the s-process efficiency (e.g., ofthe neutron exposure). Our model predictions for this parameter show acomplex trend versus metallicity. Especially noteworthy is theprediction that the flow along the s-path at low metallicities drainsthe Zr and Ba peaks and builds an excess at the doubly magic208Pb, which is at the termination of the s-path. We thendiscuss the effects on the models of variations in the crucialparameters of the 13C pocket, finding that they are notcritical for interpreting the results. The theoretical predictions arecompared with published abundances of s-elements for AGB giants ofclasses MS, S, SC, post-AGB supergiants, and for various classes ofbinary stars, which supposedly derive their composition by mass transferfrom an AGB companion. This is done for objects belonging both to theGalactic disk and to the halo. The observations in general confirm thecomplex dependence of neutron captures on metallicity. They suggest thata moderate spread exists in the abundance of 13C that isburnt in different stars. Although additional observations are needed,it seems that a good understanding has been achieved of s-processoperation in AGB stars. Finally, the detailed abundance distributionincluding the light elements (CNO) of a few s-enriched stars atdifferent metallicities are examined and satisfactorily reproduced bymodel envelope compositions.
| 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.
| The CH Stars. III. Heavy Element Abundances
Abstract image available at:http://adsabs.harvard.edu/abs/1992AJ....104.1997V
| The CH stars. I - Carbon isotope ratios
Using the 1-0 C2 Swan band at 4737 A and the CN red system 2-0 band near8000 A, the ratio of stable carbon isotopes, C-12/C-13, has beendetermined for eight CH giants. For a majority of the sample stars, thecarbon isotope ratio is about 3 near the equilibrium value of the CNcycle, though there seems to be a second population of CH stars withhigh carbon isotope ratios. This range of ratios is the same as foundfor the Population II giants and globular cluster giant stars. Since theabundance anomalies which typify CH giants are believed to haveoriginated by the transfer of mass from a now extinct AGB companion, theCH giant's atmosphere should be enhanced in triple alpha products fromthe AGB star's interior. The low carbon isotope ratios imply that thematerial transferred from the now unseen companion has been mixed intothe CN burning region of the CH star or constitutes a minor fraction ofthe envelop mass of the CH star, thus giving isotope ratios typical ofstars on their first ascent of the giant branch.
| Catalogue of CH and metal-deficient barium stars
Not Available
| CH and metal-dificient barium stars and their color excesses
Not Available
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