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  • Print publication year: 2008
  • Online publication date: December 2009

8 - An abundance survey of the Galactic thick disk

Summary

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.