On the Chemical and Structural Evolution of the Galactic Disk
  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 $\sim$ 18,500 disk stars taken from two
datasets, the Sloan Digital Sky Survey (SDSS) and the High-Accuracy Radial
velocity Planetary Searcher (HARPS). On these surveys we examine the
metallicity gradient, $\Delta$[Fe/H]/$\Delta R_{\rm g}$, along the
guiding-center radii, $R_{\rm g}$, of stars and its dependence on the
[$\alpha$/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 [$\alpha$/Fe]
ratios ([$\alpha$/Fe] $>$ 0.3 in SDSS, [$\alpha$/Fe] $>$ 0.2 in HARPS) are
found to show a positive $\Delta$[Fe/H]/$\Delta R_{\rm g}$, whereas the
thin-disk candidate stars characterized by lower [$\alpha$/Fe] ratios show a
negative one. Furthermore, we find that the relatively young thin-disk
population characterized by much lower [$\alpha$/Fe] ratios ([$\alpha$/Fe] $<$
0.2 in SDSS, [$\alpha$/Fe] $<$ 0.1 in HARPS) shows notably a flattening
$\Delta$[Fe/H]/$\Delta R_{\rm g}$ with decreasing [$\alpha$/Fe], in contrast to
the old one with higher [$\alpha$/Fe] ratios ([$\alpha$/Fe] $\sim$ 0.2 in SDSS,
[$\alpha$/Fe] $\sim$ 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.
