A Comparison of the Chemical Evolutionary Histories of the Galactic Thin Disk and Thick Disk Stellar Populations

We have studied 23 long-lived G dwarfs that belong to the thin disk and thick disk stellar populations. The stellar data and analyses are identical, reducing the chances for systematic errors in the comparisons of the chemical abundance patterns in the two populations. Abundances have been derived for 24 elements: O, Na, Mg, Al, Si, Ca, Ti, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, and Eu. We find that the behavior of [α/Fe] and [Eu/Fe] versus [Fe/H] are quite different for the two populations. As has long been known, the thin disk O, Mg, Si, Ca, and Ti ratios are enhanced relative to iron at the lowest metallicities and decline toward solar values as [Fe/H] rises above -1.0. For the thick disk, the decline in [α/Fe] and [Eu/Fe] does not begin at [Fe/H] = -1.0, but at -0.4. Other elements share this same behavior, including Sc, Co, and Zn, suggesting that at least in the chemical enrichment history of the thick disk, these elements were manufactured in similar-mass stars. The heavy s-process elements Ba, La, Ce, and Nd are overabundant in the thin disk stars relative to the thick disk stars. On the other hand, the constancy of the [Ba/Y] ratio suggests that only one s-process site was manufacturing these elements or, possibly, that the r-process was responsible for the bulk of the nucleosynthesis of these elements. We combine our results with other studies (Edvardsson et al., Prochaska et al., Bensby et al., and Reddy et al.), who had already found very similar trends, in order to further explore the origin of the thick disk. The signs for an independent (parent galaxy) evolution of the thick disk are clear, in terms of the different metallicities at which the [α/Fe] ratios begin to decline, as well as the "step function" behavior of some elements, including [Eu/Y], [Ba/Fe], and possibly [Cu/Fe], at [Fe/H] ≈ -0.2.