The Galactic thick disk stellar abundances

We present first results from a program to measure the chemical abundances of a large (N > 30) sample of thick disk stars with the principal goal of i nvestigating the formation history of the Galactic thick disk. We have obta ined high-resolution, high signal-to-noise spectra of 10 thick disk stars w ith the HIRES spectrograph on the 10 m Reck I telescope. Our analysis confi rms previous studies of O and Mg in the thick disk stars, which reported en hancements in excess of the thin disk population. Furthermore, the observat ions of Si, Ca, Ti, Mn, Co, V, Zn, Al, and Eu all argue that the thick disk population has a distinct chemical history from the thin disk. With the ex ception of V and Co, the thick disk abundance patterns match or tend toward the values observed for halo stars with [Fe/H] approximate to -1. This sug gests that the thick disk stars had a chemical enrichment history similar t o the metal-rich halo stars. With the possible exception of Si, the thick d isk abundance patterns are in excellent agreement with the chemical abundan ces observed in the metal-poor bulge stars, suggesting the two populations formed from the same gas reservoir at a common epoch. T he principal results of our analysis are as follows. (1) All 10 stars exhib it enhanced alpha /Fe ratios with O, Si, and Ca showing tentative trends of decreasing overabundances with increasing [Fe/H]. In contrast, the Mg and Ti enhancements are constant. (2) The light elements Na and Al are enhanced in these stars. (3) With the exception of Ni, Cr, and possibly Cu, the iro n-peak elements show significant departures from the solar abundances. The stars are deficient in Mn, but overabundant in V, Co, Sc, and Zn. (4) The h eavy elements Ba and Y are consistent with solar abundances, but Eu is sign ificantly enhanced. If the trends of decreasing O, Si, and Ca with increasi ng [Fe/H] are explained by the onset of Type Ia SN, then the thick disk sta rs formed over the course of greater than or similar to1 Gyr. We argue that this formation time-scale would rule out most dissipational collapse scena rios for the formation of the thick disk. Models which consider the heating of an initial thin disk-either through "gradual" heating mechanisms or a s udden merger event-are favored. These observations provide new tests of the ories of nucleosynthesis in the early universe. In particular, the enhancem ents of Sc, V, Co, and Zn may imply overproduction during an enhanced a-ric h freeze out fueled by neutrino-driven winds. Meanwhile, the conflicting tr ends for Mg, Ti, Ca, Si, and O pose a difficult challenge to our current un derstanding of nucleosynthesis in Type Ia and Type II SN. The Ba/Eu ratios favor r-process dominated enrichment for the heavy elements, consistent wit h the ages (t(age) > 10 Gyr) expected for these stars. Finally, we discuss the impact of the thick disk abundances on interpretati ons of the abundance patterns of the damped Ly alpha systems. The observati ons of mildly enhanced Zn/Fe imply an interpretation for the damped systems which includes a dust depletion pattern on top of a Type II SN enrichment pattern. We also argue that the S/Zn ratio is not a good indicator of nucle osynthetic processes.