The relations between the metallicity and orbital parameters of Galactic disk stars

Abstract

The iron abundance gradient in the Galactic disk provides fundamental constraints on the chemical evolution of this important component of the Galaxy. However, there are still some arguments about the shape of the iron abundance gradient across the Galactic stellar disk. To provide quantitative constraints on these trends, we constructed an unbiased stellar sample from the common objects of two large observations. The sample consists of 4004 disk stars in the solar neighborhood with the range of metallicity [Fe/H] from ~−1.0 to ~0.5, including 3855 thin disk stars and 146 thick disk stars. The range of their mean Galactocentric distances R m is from ~4 to ~11 kpc, and the range of their maximum vertical distance from the Galactic Plane Z max is from 0 to ~3.5 kpc. The relations of the stellar metallicity [Fe/H] with the orbital eccentricity e, and [Fe/H] with R m , as well as [Fe/H] with Z max are investigated in detail. The results showed that: (1) The orbital eccentricity of disk stars decreases with increasing metallicity, while this trend becomes flat for stars with [Fe/H]>−0.2.(2) For thin disk, there is a clear radial iron gradient of −0.051±0.005 dex/kpc. However, this metallicity gradient is discontinuous once we split the sample into the inner (R m <8 kpc) and outer disk (R m ≥8 kpc): a slope of about 0 dex/kpc is present in the inner disk and a slope of −0.12 dex/kpc is in the outer disk. For the thick disk, there is no trend of [Fe/H] with R m . (3) The vertical gradient of iron abundance is −0.146 dex/kpc and −0.251 dex/kpc for the thick and thin disks, respectively. Moreover, the vertical abundance gradient shows a steeper trend with the evolution of the Galaxy. Our results confirm the collapse mechanism associated with the formation and evolution of the Galactic disk.