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J. A. Simmerer (University of Texas at Austin), T. C. Beers (Michigan State University), C. Senden (University of Texas at Austin), B. W. Carney (University of North Carolina at Chapel Hill)
Previous studies of likely thick-disk stars have indicated that a substantial pause exists between the time of formation of the thick disk and the thin disk, in part because thick-disk stars appear enhanced in their alpha-capture and r-process elements relative to thin-disk stars. Although the metallicity distributions of the two Galactic populations overlap, thick-disk stars appear to be much older than thin-disk stars and are kinematically distinct as well. Recent studies have also indicated that the thick disk likely includes stars with metallicities as low as [Fe/H] = -2.2, well into the regime normally thought of as dominated by pure halo-population stars. Explanations for the origin of this so-called metal-weak thick disk (MWTD) component invoke either a very long formation timescale (essentially beginning at the same time as the halo) or suggest that the formation is associated with capture/destruction of dwarf-like galaxies, from which low-metallicity stars have been subsumed.
In order to investigate these issues more thoroughly, we have obtained high-resolution, high-S/N spectra for a large sample of MWTD stars identified by Beers et al. (2002, AJ, 124, 931). These objects are kinematically associated with the thick disk, but possess iron abundances that are substantially lower than previously studied thick-disk stars. An analysis of selected alpha-capture, iron-peak, and neutron-capture abundances will confirm (a) whether these particular stars are as metal-poor as indicated by Beers et al. and (b) whether they are chemically more closely related to the halo or to the cannonical thick or thin disks. Stars representative of each population have been subjected to the same analysis for comparison purposes.
T.B. acknowledges support from NSF grants AST 00-98508 and AST 00-98549. C.S and J.S. acknowledge support from NSF grants AST 99-87162 and AST 03-07495. J.S.also thanks CTIO for financial support.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.