(Proposal ID) S10A-048

(PI) Ishigaki, Miho

(Proposal Title)
Deciphering the formation history of the metal-weak thick disk and halos

(Abstract)
The metal-weak thick disk (MWTD) is one of the oldest structural components of 
the Milky Way, which consists of metal-poor stars but intriguingly having disk-
like kinematics. Kinematics and abundance data on the MWTD stars provide fossil
 records on the yet controversial formation mechanism of the thick disk. 
However, the information for the detailed chemical abundance of this component 
is yet quite limited. Here, we propose a high-resolution spectroscopy for > 30 
candidate MWTD stars with HDS. Candidate stars are selected based on the most 
likely range where the MWTD occupies, i.e., intermediate metallicity -1.7 <
[Fe/H]< -1, rapid rotation150 < V_phi < 250 km s^-1, maximum height of the 
orbit of Z_max < 1 kpc. Chemical abundances of key elements derived from the 
observed spectra allow us to characterize the MWTD in terms of detailed 
chemical abundance pattern, thereby providing important insights on the star 
formation history of this component which is expected to be either distinct or 
continuous from the inner halo. Additionally, we obtain spectra for the 
candidate inner halo stars selected by kinematics as a control sample for the 
MWTD sample and also the outer halo sample previously obtained at the same 
metallicity range where all of these components coexist.