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

We investigate the radial metallicity gradients and azimuthal metallicity distributions on the Galactocentric $X$–$Y$ plane using mono-temperature stellar populations selected from LAMOST MRS young stellar sample. The estimated radial metallicity gradient ranges from $-$0.015,dex/kpc to $-$0.07,dex/kpc, which decreases as effective temperature decreases (or stellar age increases) at $7500<T_{\mathrm{e}\mathrm{f}\mathrm{f}}<12500$,K ($\tau <$1.5 Gyr). The azimuthal metallicity excess (metallicity after subtracting radial metallicity gradient, $\mathrm{\Delta }$,[M/H]) distributions exhibit inhomogeneities with dispersions of 0.04,dex to 0.07,dex, which decrease as 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 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.