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 <sup>2

1</sup> 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 ( ), high S/N spectra have been obtained for 13 halo stars and 16 disk stars with  K, and overlapping metallicities in the range . Equivalent widths of weak Fe I and Fe II lines are used to determine differential values of 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, , the maximum distance from the Galactic center, and , the maximum distance from the Galactic plane.

A group of 8 halo stars have significantly lower [ /Fe] values than disk stars of the same metallicity ( 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 [ /Fe] and [Na/Fe] are found for the stars with the largest values of and . 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 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

SIMBAD Objects

Contents

• 1. Introduction
• 2. Selection of stars
• 3. Observations and data reduction
• 4. Stellar parameters and chemical abundances
  • 4.1. Model atmospheres and line calculations
  • 4.2. Stellar atmospheric parameters
  • 4.3. Abundance ratios and their errors
• 5. Kinematics and orbits of the stars
  • 5.1. Kinematic data
  • 5.2. Orbit calculations
• 6. Discussion
  • 6.1. The -elements O, Mg, Si, Ca and Ti
  • 6.2. Na, Cr, Ni, Y and Ba
  • 6.3. Correlations with orbital parameters
  • 6.4. HD 106038
• 7. Conclusions
• Acknowledgements
• Appendix A
  • Appendix A
  • References

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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