2017ApJ...850...25L

We select giant stars from LAMOST data release 3 (hereafter DR3) based on their spectral properties and atmospheric parameters in order to detect the structure and kinematic properties of the Galactic thick disk. The spatial motions of our sample stars are calculated. We obtain 2035 thick-disk giant stars by using a kinematic criterion. We confirm the existence of the metal-weak thick disk. The most metal-deficient star in our sample has [Fe/H]=-2.34. We derive the radial and vertical metallicity gradients, which are +0.035 ± 0.010 and -0.164 ± 0.010 dex kpc^–1respectively. Then we estimate the scale length and scale height of the thick disk using the Jeans equation, and the results are h_R=3.0±0.1 kpc and h_Z=0.9±0.1 kpc. The scale length of the thick disk is approximately equal to that of the thin disk from several previous works. Finally, we calculate the orbital parameters of our sample stars, and discuss the formation scenario of the thick disk. Our result for the distribution of stellar orbital eccentricity excludes the accretion scenario. We conclude that the thick disk stars are mainly born inside the Milky Way.