  We investigate the formation history of the stellar disk component in the
Milky Way (MW) based on our new chemical evolution model. Our model considers
several fundamental baryonic processes, including gas infall, re-accretion of
outflowing gas, and radial migration of disk stars. Each of these baryonic
processes in the disk evolution is characterized by model parameters, which are
determined by fitting to various observational data of the stellar disk in the
MW, including the radial dependence of the metallicity distribution function
(MDF) of the disk stars, which has recently been derived in the APOGEE survey.
We succeeded to obtain the best set of model parameters, which well reproduces
the observed radial dependences of the mean, standard deviation, skewness, and
kurtosis of the MDFs for the disk stars. We analyze the basic properties of our
model results in detail to get new insights into the important baryonic
processes in the formation history of the MW. One of the remarkable findings is
that outflowing gas, containing much heavy elements, preferentially re-accretes
onto the outer disk parts, and this recycling process of metal-enriched gas is
a key ingredient to reproduce the observed narrower MDFs at larger radii.
Moreover, important implications for the radial dependence of gas infall and
the influence of radial migration on the MDFs are also inferred from our model
calculation. Thus, the MDF of disk stars is a useful clue for studying the
formation history of the MW.
