The Galactic Thick Disk Stellar Abundances

Abstract

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 investigating the formation history of the Galactic thick disk. We have obtained high-resolution, high signal-to-noise spectra of 10 thick disk stars with the HIRES spectrograph on the 10 m Keck I telescope. Our analysis confirms previous studies of O and Mg in the thick disk stars, which reported enhancements in excess of the thin disk population. Furthermore, the observations 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 exception of V and Co, the thick disk abundance patterns match or tend toward the values observed for halo stars with [Fe/H] ≈ -1. This suggests that the thick disk stars had a chemical enrichment history similar to the metal-rich halo stars. With the possible exception of Si, the thick disk abundance patterns are in excellent agreement with the chemical abundances observed in the metal-poor bulge stars, suggesting the two populations formed from the same gas reservoir at a common epoch. The principal results of our analysis are as follows. (1) All 10 stars exhibit enhanced α/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 iron-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 heavy elements Ba and Y are consistent with solar abundances, but Eu is significantly enhanced. If the trends of decreasing O, Si, and Ca with increasing [Fe/H] are explained by the onset of Type Ia SN, then the thick disk stars formed over the course of 1 Gyr. We argue that this formation time-scale would rule out most dissipational collapse scenarios for the formation of the thick disk. Models which consider the heating of an initial thin disk—either through gradual heating mechanisms or a sudden merger event—are favored. These observations provide new tests of theories of nucleosynthesis in the early universe. In particular, the enhancements of Sc, V, Co, and Zn may imply overproduction during an enhanced α-rich freeze out fueled by neutrino-driven winds. Meanwhile, the conflicting trends for Mg, Ti, Ca, Si, and O pose a difficult challenge to our current understanding of nucleosynthesis in Type Ia and Type II SN. The Ba/Eu ratios favor r-process dominated enrichment for the heavy elements, consistent with the ages (tage > 10 Gyr) expected for these stars. Finally, we discuss the impact of the thick disk abundances on interpretations of the abundance patterns of the damped Lyα systems. The observations 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 nucleosynthetic processes.