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Astron. Astrophys. 326, 751-762 (1997)

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Chemical composition of halo and disk stars
with overlapping metallicities * **

P.E. Nissen 1 and W.J. Schuster 2

1 Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark
2 Observatorio Astronómico Nacional, UNAM, Apartado Postal 877, C.P. 22800 Ensenada, B.C., México

Received 7 February 1997 / Accepted 30 April 1997

Abstract

High resolution ( [FORMULA] ), high S/N spectra have been obtained for 13 halo stars and 16 disk stars with [FORMULA]  K, [FORMULA] and overlapping metallicities in the range [FORMULA]. Equivalent widths of weak Fe I and Fe II lines are used to determine differential values of [FORMULA] and log g. Relative abundances of O, Na, Mg, Si, Ca, Ti, Cr, Fe, Ni, Y and Ba are determined to a precision ranging from 0.02 to 0.07 dex. Kinematical data have been collected and used to calculate the stellar orbital parameters, [FORMULA], the maximum distance from the Galactic center, and [FORMULA], the maximum distance from the Galactic plane.

A group of 8 halo stars have significantly lower [ [FORMULA] /Fe] values than disk stars of the same metallicity ( [FORMULA] O, Mg, Si, Ca or Ti). These stars are also underabundant in Na and Ni and maybe in Cr. [Na/Fe] ranges from -0.4 to +0.1, and [Ni/Fe] is surprisingly well correlated with [Na/Fe]. The smallest values of [ [FORMULA] /Fe] and [Na/Fe] are found for the stars with the largest values of [FORMULA] and [FORMULA]. This may indicate that the anomalous halo stars have been accreted from dwarf galaxies with a chemical evolution history different from that of the inner halo and the disk. In any case the data show that abundance ratios in stars are not universal functions of [Fe/H] and that the chemical evolution of the Galaxy at [FORMULA] is more complicated than assumed in many models.

One halo star, HD 106038, is found to be strongly overabundant in Si, Ni, Y and Ba relative to Fe.

Key words: stars: abundances; Population IIGalaxy: abundancesGalaxy: evolutionGalaxy: halo

* Tables A1-A5 are only available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
** Based on observations carried out at the European Southern Observatory, La Silla, Chile

Send offprint requests to: P.E. Nissen

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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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