  We investigate the role of radial migration history of stars in chemical
evolution of a disk galaxy, in particular in understanding the origin of their
bimodal distribution on the [alpha/Fe]-[Fe/H] plane. For this purpose, we
examine the three different models with no, continuous, and discontinuous
radial migration, respectively. We find that for the model with radial
migration, the [alpha/Fe] ratios of stars in outer disk regions decrease more
rapidly with time than the model without radial migration, because the
associated net transfer of intermediate and old disk stars from inner to outer
disk regions increases the rate of Type Ia relative to that of Type II
supernovae in the latter regions. Moreover, in the model assuming rapid and
discontinuous radial migration, its effect on the stellar abundances at larger
radii is significant enough to provide the large difference in the evolution of
stars on the [alpha/Fe]-[Fe/H] plane between inner and outer disk regions. As a
result we obtain the bimodal distribution of disk stars on the
[alpha/Fe]-[Fe/H] plane as observed in the Galactic stellar disk, thereby
implying that the event of discontinuous radial migration may play a key role
in reproducing the observed bimodality of stars on this abundance-ratio
diagram. We discuss possible mechanisms causing such discontinuous radial
migration in the early evolution of the Galactic disk, including the event of
minor merging of a relatively massive satellite onto the stellar disk.
