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J. Simmerer (Dept. of Astronomy, University of Texas Austin)
Studies of the existing stellar components of the Galaxy offer a fossil record of its formation history. Of the various populations (bulge, halo, thin disk,and thick disk), the thick disk in particular is still difficult to characterize. Although the metallicity distributions of the two Galactic populations overlap, thick disk stars appear to be much older than thin disk stars, as they are enhanced in alpha-capture and rapid process neutron-capture elements relative to thin disk stars. Recent studies have also indicated that the thick disk includes stars as metal-poor as [Fe/H]=-2.2, well into the ``pure halo'' metallicity regime. Explanations for the origin of this metal-weak thick disk (MWTD) invoke either a long formation time scale (essentially coeval with the halo) or suggest the capture of dwarf galaxies, from which low-metallicity stars have been subsumed. In order to help distinguish between formation scenarios, high-resolution, high-S/N spectra of a large sample of MWTD stars identified by Chiba & Beers (2000) and Beers et al. (2002) have been gathered and analyzed. These objects are kinematically associated with the thick disk, but are far more iron-poor than previously studied thick-disk stars. A detailed abundance analysis will confirm whether these stars are truly metal-poor, and whether they are chemically related to the halo, the canonical thick disk, the thin disk, or none of these.
This work has been supported in part by NSF grant AST 03-07495.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.