Spatial metallicity variations of mono-temperature stellar populations revealed by early-type stars in LAMOST

Abstract

We investigate the radial metallicity gradients and azimuthal metallicity distributions on the Galactocentric X–Y plane using mono-temperature stellar populations selected from the LAMOST-MRS young stellar sample. The estimated radial metallicity gradient ranges from −0.015 dex/kpc to −0.07 dex/kpc, which decreases as the effective temperature decreases (or when the stellar age increases) at 7500 < Teff < 12 500 K (τ < 1.5 Gyr). The azimuthal metallicity excess (the metallicity after subtracting the radial metallicity gradient, Δ [M/H]) distributions exhibit inhomogeneities with dispersions of 0.04 dex to 0.07 dex, which decrease as the effective temperature decreases. We also identify five potential metal-poor substructures with large metallicity excess dispersions. The metallicity excess distributions of these five metal-poor substructures suggest that they contain a larger fraction of metal-poor stars compared to other control samples. These metal-poor substructures may be associated with high-velocity clouds that infall into the Galactic disk from the Galactic halo, which are not quickly well mixed with the pre-existing interstellar medium (ISM) of the Galactic disk. As a result, these high-velocity clouds produce some metal-poor stars and the observed metal-poor substructures. The variations of metallicity inhomogeneities with different stellar populations indicate that high-velocity clouds are not well mixed with the pre-existing Galactic disk ISM within 0.3 Gyr.