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Chemical Abundances of the Milky Way Thick Disk and Stellar Halo I.: Implications of [alpha/Fe] for Star Formation Histories in Their Progenitors

We present the abundance analysis of 97 nearby metal-poor (-3.3<[Fe/H]<-0.5) stars having kinematics characteristics of the Milky Way (MW) thick disk, inner, and outer stellar halos. The high-resolution, high-signal-to-noise optical spectra for the sample stars have been obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of Fe, Mg, Si, Ca and Ti have been derived using a one-dimensional LTE abundance analysis code with Kurucz NEWODF model atmospheres. By assigning membership of the sample stars to the thick disk, inner or outer halo components based on their orbital parameters, we examine abundance ratios as a function of [Fe/H] and kinematics for the three subsamples in wide metallicity and orbital parameter ranges. We show that, in the metallicity range of -1.5<[Fe/H]<= -0.5, the thick disk stars show constantly high mean [Mg/Fe] and [Si/Fe] ratios with small scatter. In contrast, the inner, and the outer halo stars show lower mean values of these abundance ratios with larger scatter. The [Mg/Fe], [Si/Fe] and [Ca/Fe] for the inner and the outer halo stars also show weak decreasing trends with [Fe/H] in the range [Fe/H]$>-2$. These results favor the scenarios that the MW thick disk formed through rapid chemical enrichment primarily through Type II supernovae of massive stars, while the stellar halo has formed at least in part via accretion of progenitor stellar systems having been chemically enriched with different timescales.