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Symposium 241

Abstract details

Decomposition of the Galactic Disk: Abundances and Kinematics
Reddy, B.E., Lambert, D.L., Allende Prieto, C

I will discusss our results from abundance survey of Galactic disk:thin and thick disk. We have performed an abundance analysis for a few hundred stars of F- and G- dwarfs belonging to respective components. A sample stars was chosen from the $Hipparcos$ catalogue. Using accurate radial velocities combined with $Hipparcos$ astrometry, kinematics $(U, V,$ and $~W)$ and Galactic orbital parameters were computed. We estimated the probability for a star to belong to one of these components. With a probability $P \geq 70$\% taken as certain membership. 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 ratios of $\alpha$-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 $\Lambda$CDM universe.

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