Chemical Abundances of the Outer Halo Stars in the Milky Way
  We present chemical abundances of 57 metal-poor stars that are likely
constituents of the outer stellar halo in the Milky Way. Almost all of the
sample stars have an orbit reaching a maximum vertical distance (Z_max) of >5
kpc above and below the Galactic plane. High-resolution, high signal-to-noise
spectra for the sample stars obtained with Subaru/HDS are used to derive
chemical abundances of Na, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y and Ba with an LTE
abundance analysis code. The resulting abundance data are combined with those
presented in literature that mostly targeted at smaller Z_max stars, and both
data are used to investigate any systematic trends in detailed abundance
patterns depending on their kinematics. It is shown that, in the metallicity
range of -2<[Fe/H]<-1, the [Mg/Fe] ratios for the stars with Z_max>5 kpc are
systematically lower (~0.1 dex) than those with smaller Z_max. This result of
the lower [alpha/Fe] for the assumed outer halo stars is consistent with
previous studies that found a signature of lower [alpha/Fe] ratios for stars
with extreme kinematics. A distribution of the [Mg/Fe] ratios for the outer
halo stars partly overlaps with that for stars belonging to the Milky Way dwarf
satellites in the metallicity interval of -2<[Fe/H]<-1 and spans a range
intermediate between the distributions for the inner halo stars and the stars
belonging to the satellites. Our results confirm inhomogeneous nature of
chemical abundances within the Milky Way stellar halo depending on kinematic
properties of constituent stars as suggested by earlier studies. Possible
implications for the formation of the Milky Way halo and its relevance to the
suggested dual nature of the halo are discussed.
