Chemical composition of halo and disk stars with overlapping metallicities.

Details Chemical composition of halo and disk stars with overlapping metallicities. Chemical composition of halo and disk stars with overlapping metallicities.

Oct 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...326..751n&link_type=abstract

Astronomy and Astrophysics, v.326, p.751-762

Astronomy and Astrophysics

Astrophysics

239

Stars: Abundances, Population Ii, Galaxy: Abundances, Galaxy: Evolution, Galaxy: Halo

Scientific paper

High resolution (R=60000), high S/N spectra have been obtained for 13 halo stars and 16 disk stars with 5400<~T_eff_<~6500K, 4.0<~logg<~4.6 and overlapping metallicities in the range -1.3<~[Fe/H]<~-0.5. Equivalent widths of weak FeI and FeII lines are used to determine differential values of T_eff_ and logg. 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 0.07dex. Kinematical data have been collected and used to calculate the stellar orbital parameters, R_max_, the maximum distance from the Galactic center, and z_max_, the maximum distance from the Galactic plane. A group of 8 halo stars have significantly lower [α/Fe] values than disk stars of the same metallicity (α= 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 [α/Fe] and [Na/Fe] are found for the stars with the largest values of R_max_ and z_max_. 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 [Fe/H]=~-1.0 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.

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