2020ApJ...896...14H

We present a kinematic analysis of a sample of 23,908 G- and K-type dwarfs in the Galactic disk. Based on the α-abundance ratio, [α/Fe], we separated our sample into low-α thin-disk and high-α thick-disk stars. We find a V_φ 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_R, V_φ, and V_Z components with [Fe/H]. The metal-poor (MP; [Fe/H] ←0.3) thin-disk stars with low-V_φ velocities have high eccentricities (e) and small perigalacticon distances (r_p), while the high-V_φ MP thin-disk stars possess low e and large r_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.