Elemental Abundance Survey of The Galactic Thick Disk

We have performed an abundance analysis for Fand Gdwarfs of the Galactic thick disk component. A sample of 176 nearby (d ≤ 150 pc) thick disk candidate stars was chosen from the Hipparcos catalogue and subjected to a high-resolution spectroscopic analysis. Using accurate radial velocities combined with Hipparcos astrometry, kinematics (U, V, and W ) and Galactic orbital parameters were computed. We estimate the probability for a star to belong to the thin disk, the thick disk or the halo. With a probability P ≥ 70% taken as certain membership, we assigned 95 stars to the thick disk, 13 to the thin disk, and 20 to the halo. The remaining 48 stars in the sample cannot be assigned with reasonable certainty to one of the three components. Abundances of C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ba, Ce, Nd, and Eu have been obtained. The abundances for thick disk stars are compared with those for thin disk members from Reddy et al. (2003). The ratios of α-elements (O, Mg, Si, Ca and Ti) to iron for thick disk disk stars show a clear enhancement compared to thin disk members in the range −0.3 < [Fe/H] < −1.2. There are also other elements – Al, Sc, V, Co, and possibly Zn – which show enhanced ratios to iron in the thick disk relative to the thin disk. The abundances of Na, Cr, Mn, Ni, and Cu (relative to Fe) are very similar for thin and thick disk stars. The dispersion in abundance ratios [X/Fe] at given [Fe/H] for thick disk stars is consistent with the expected scatter due to measurement errors, suggesting a lack of ‘cosmic’ scatter. A few stars classified as members of the thick disk by our kinematic criteria show thin disk abundances. These stars, which appear older than most thin disk stars, are also, on average, younger than the thick disk population. They may have originated early in the thin disk history, and been subsequently scattered to hotter orbits by collisions. The thick disk may not include stars with [Fe/H] > −0.3. The observed compositions of the thin and thick disks seem to be consistent with models of galaxy formation by hierarchical clustering in a ΛCDM universe. In particular, the distinct abundance patterns observed in the thin and thick disks, and the chemical homogeneity of the thick disk at different galactocentric distances favor a scenario in which the majority of thick-disk stars were formed in situ, from gas rich merging blocks.