Search for the metal-weak thick disk from the LAMOST DR5
Abstract Based on the data release of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope survey (LAMOST DR5) and the Gaia Early Data Release 3 (Gaia EDR3), we construct a sample containing 46,109 giant (log g 3.5 dex) stars with heliocentric distance d 4 kpc, and the sample is further divided into two groups of the inner (RGC < 8.34 kpc) and outer region (RGC > 8.34 kpc). The LZ distributions of our program stars in the panels with different [Fe/H] and [α/Fe] suggest that the thick-disk consists of two distinct components with different chemical compositions and kinematic properties. For the inner region, the metal-weak thick disk (MWTD) contributes significantly when [α/Fe] > +0.2 dex and [Fe/H] < −0.8 dex, while the canonical thick-disk (TD) dominates when [Fe/H]> −0.8 dex. However, MWTD clear appears only when [α/Fe] > +0.2 dex and [Fe/H] < −1.2 dex for the outer region, and its proportion is lower than that of the inner region within the same metallicity. Similar result can be obtained from the Vφ distribution.