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 thickdisk stellar 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 [alpha/Fe] and [Eu/Fe] vs. [Fe/H] arequite different for the two populations. As has long been known, the thin diskO, Mg, Si, Ca, and Ti ratios are enhanced relative to iron at the lowestmetallicities, and decline toward solar values as [Fe/H] rises above -1.0. Forthe thick disk, the decline in [alpha/Fe] and [Eu/Fe] does not begin at [Fe/H]= -1.0, but at -0.4. Other elements share this behavior, including Sc, Co, andZn, suggesting that at least in the chemical enrichment history of the thickdisk, these elements were manufactured in similar-mass stars. Combining ourresults for the oldest and longest-lived stars with prior work, we find clearsigns for an independent origin for the Galactic thick disk. (Abridged)Comment: 48 pages and 20 figures, accepted for publication in the Astronomical Journa