Title: Chemical abundance analysis of the Galactic outer halo stars with Subaru/HDS
Authors: M. Ishigaki, M. Chiba, W. Aoki, L. Zhang

Abstract:
We present the result of our work on chemical abundances of ~60 metal-poor ([Fe/H]<-1)
stars that are likely constituents of the Milky Way outer halo. The main goal of our work
is to make constraints on how the Milky Way halo formed through hierarchical assembly
of smaller progenitor systems. For this purpose, we examined diversity in chemical
abundances of metal-poor stars in the Milky Way halo depending on their kinematics.
The candidates for outer halo stars were conventionally selected based on their calculated
orbital parameters; e.g. a maximum distance that the orbit reaches above and below the
Galactic plane (Zmax).
The high-resolution (R~35000-55000) and high signal-to-noise spectra of these stars were
obtained with Subaru/HDS and analyzed in a homogeneous manner to derive chemical
abundances of several key elements (alpha, Fe-peak, neutron capture elements).
We report the preliminary results that a systematic difference in abundance ratios ([X/Fe])
between the assumed inner and outer halo populations is significant for Mg in the metallicity
range of -2<[Fe/H]<-1. For the same metallicity range, modest degree of depressions in the
[X/Fe] ratios for Si, Ca and Zn are also seen in our data. Implications for the formation
history of the Galaxy from the present results will be discussed in the poster.