High-α-Metal-Rich stars in the LAMOST-MRS survey and its connection with the galactic bulge

Abstract

We report the detection of a large sample of high-α-metal-rich stars on the low giant branch with 2.6 < log g < 3.3 dex in the LAMOST-MRS survey. This special group corresponds to an intermediate-age population of 5 - 9 Gyr based on the [Fe/H]-[C/N] diagram and age-[C/N] calibration. A comparison group is selected to have a solar α ratio at super metallicity, which is young and has a narrow age range around 3 Gyr. Both groups have thin-disk like kinematics but the former shows slightly large velocity dispersions. The special group shows a larger extension in a vertical distance toward 1.2 kpc, a second peak at smaller Galactic radius and a larger fraction of super metal rich stars with [Fe/H] > 0.2 than the comparison group. These properties strongly indicate its connection with the outer bar/bulge region at R = 3 - 5 kpc. A tentative interpretation of this special group is that its stars were formed in the X-shaped bar/bulge region, close to its corotation radius, where radial migration is the most intense, and brings them to present locations at 9 kpc and beyond. Low eccentricities and slightly outward radial excursions of its stars are consistent with this scenario. Its kinematics (cold) and chemistry ([α/Fe] ~ 0.1) further support the formation of the instability-driven X-shaped bar/bulge from the thin disk.