We study the detailed properties of the radial metallicity gradient in the stellar disk of our Galaxy to constrain its chemical and structural evolution. For this purpose we select and analyze ~18,500 disk stars taken from two data sets, the Sloan Digital Sky Survey (SDSS) and the High-Accuracy Radial velocity Planetary Searcher (HARPS). On these surveys we examine the metallicity gradient, Δ[Fe/H]/ΔR g, along the guiding center radii, R g, of stars and its dependence on the [α/Fe] ratios to infer the original metallicity distribution of the gas disk from which those stars formed and its time evolution. In both sample sources, the thick-disk candidate stars characterized by high [α/Fe] ratios ([α/Fe] 0.3 in SDSS, [α/Fe] 0.2 in HARPS) are found to show a positive Δ[Fe/H]/ΔR g, whereas the thin-disk candidate stars characterized by lower [α/Fe] ratios show a negative one. Furthermore, we find that the relatively young thin-disk population characterized by much lower [α/Fe] ratios ([α/Fe] 0.2 in SDSS, [α/Fe] 0.1 in HARPS) notably shows a flattening Δ[Fe/H]/ΔR g with decreasing [α/Fe], in contrast to the old one with higher [α/Fe] ratios ([α/Fe] ~ 0.2 in SDSS, [α/Fe] ~ 0.1 in HARPS). The possible implication for early disk evolution is discussed in the context of galaxy formation accompanying the rapid infall of primordial gas on the inner disk region, which can generate a positive metallicity gradient, and the subsequent chemical evolution of the disk, which results in a flattening effect of a metallicity gradient at later epochs.
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