High-{\alpha}-Metal-Rich stars in the LAMOST-MRS survey and its connection with the galactic bulge

TL;DR

This study identifies a distinct population of high-alpha, metal-rich stars in the LAMOST-MRS survey, linking them to the galactic bulge and suggesting their formation in the bar/bulge region through radial migration.

Contribution

It reveals a new population of high-alpha, metal-rich stars and proposes their origin in the galactic bar/bulge region via radial migration, connecting chemical, kinematic, and spatial properties.

Findings

High-alpha-metal-rich stars are linked to the galactic bulge.

These stars likely formed in the X-shaped bar/bulge region.

Their properties support formation from thin disk via radial migration.

Abstract

We report the detection of a large sample of high-$\alpha$-metal-rich stars on the low giant branch with $2.6<logg<3.3$ 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 solar $\alpha$ 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 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 ($[\alpha/Fe]$ $\sim 0.1$) further support the formation of the instability-driven X-shaped bar/bulge from the thin disk.