Insights into the Formation and Evolution History of the Galactic Disk
  System
  We present a kinematic analysis of a sample of 23,908 G- and K-type dwarfs in
the Galactic disk. Based on the $\alpha$-abundance ratio, [$\alpha$/Fe], we
separated our sample into low-$\alpha$ thin-disk and high-$\alpha$ thick-disk
stars. We find a $V_{\rm \phi}$ gradient of $-$28.2 km s$^{-1}$ dex$^{-1}$ over
[Fe/H] for the thin disk, and an almost flat trend of the velocity dispersions
of $V_{\rm R}$, $V_{\rm \phi}$, and $V_{\rm Z}$ components with [Fe/H]. The
metal-poor (MP; [Fe/H] $<$ $-$0.3) thin-disk stars with low-$V_{\rm \phi}$
velocities have high eccentricities ($e$) and small perigalacticon distances
($r_{\rm p}$), while the high-$V_{\rm \phi}$ MP thin-disk stars possess low $e$
and large $r_{\rm p}$. Interestingly, half of the super metal-rich ([Fe/H] $>$
$+$0.1) stars in the thin disk exhibit low-$e$, solar-like orbits. Accounting
for the inhomogeneous metallicity distribution of the thin-disk stars with
various kinematics requires radial migration by churning $-$ it apparently
strongly influences the current structure of the thin disk; we cannot rule out
the importance of blurring for the high-$e$ stars. We derive a rotation
velocity gradient of $+$36.9 km s$^{-1}$ dex$^{-1}$ for the thick disk, and
decreasing trends of velocity dispersions with increasing [Fe/H]. The
thick-disk population also has a broad distribution of eccentricity, and the
number of high-$e$ stars increases with decreasing [Fe/H]. These kinematic
behaviors could be the result of a violent mechanism, such as a gas-rich merger
or the presence of giant turbulent clumps, early in the history of its
formation. Dynamical heating by minor mergers and radial migration may also
play roles in forming the current thick-disk structure.
