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Published online by Cambridge University Press: 02 December 2009
We present the results of our recent abundance survey of the Galactic thick disk. We selected from the Hipparcos catalog 176 sample stars satisfying the following criteria: they are nearby (d ≤ 150 pc) subgiants and dwarfs, of spectral types F and G, and with thick-disk kinematics (VLSR ≤ −40 kms−1, │WLSR│ ≤ 30 kms−1). Assuming that the velocity distribution of each stellar population is Gaussian, we assigned stars with a probability P ≤ 70% to one of the three components. This resulted in 95 thick-disk stars, 17 thin-disk stars, and 24 halo stars. The remaining 40 objects cannot be unambiguously assigned to one of the three components.
We derived abundances for 23 elements from C to Eu. The thick-disk abundance patterns are compared with earlier results from the thin-disk survey of Reddy et al. (2003). The levels of α-elements (O, Mg, Si, Ca, and Ti), thought to be produced dominantly in Type-ii supernovae, are enhanced in thick-disk stars relative to the values found for thin-disk members in the range −0.3 > [Fe/H] > −1.2. The scatter in the abundance ratios [X/Fe] at a given [Fe/H] for thick-disk stars is consistent with the predicted dispersion due to measurement errors, as is the case for the thin disk, suggesting a lack of “cosmic” scatter. The observed compositions seem consistent with a model of galaxy formation by mergers in a ∧ CDM universe.
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