2019ApJ...880...36Y

We determined the chemical and kinematic properties of the Galactic thin and thick disk using a sample of 307,246 A/F/G/K-type giant stars from the LAMOST spectroscopic survey and Gaia DR2 survey. Our study found that the thick disk globally exhibits no metallicity radial gradient, but the inner disk (R <= 8 kpc) and the outer disk (R > 8 kpc) have different gradients when they are studied separately. The thin disk also shows two different metallicity radial gradients for the inner disk and the outer disk, and has a steep metallicity vertical gradient of d[Fe/H]/d| z| =-0.12±0.0007 dex kpc^–1, which becomes flat when it is measured at increasing radial distance. The metallicity radial gradient on the other hand becomes weaker with increasing vertical distance. Adopting a Galaxy potential model, we derived the orbital eccentricity of sample stars and found a downtrend of average eccentricity with increasing metallicity for the thick disk. The variation of the rotation velocity with the metallicity shows a positive gradient for the thick disk stars and a negative one for the thin disk stars. Comparisons of our observed results with models of disk formation suggest that radial migration could have influenced the chemical evolution of the thin disk. The formation of the thick disk could be affected by more than one process: the accretion model could play an indispensable role, while other formation mechanisms, such as the radial-migration or disk-heating model could also have a contribution.