BIMODIAL DISTRIBUTION OF GALACTIC DISK STARS ON THE [α/Fe]–[Fe/H] PLANE AS POSSIBLE EVIDENCE OF DISCONTINUOUS RADIAL MIGRATION HISTORY

Abstract

ABSTRACT 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 [α/Fe]–[Fe/H] plane. For this purpose, we examine three different models with no, continuous, and discontinuous radial migration (DRM). We find that for the model with radial migration, the [α/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 SNe II in the latter regions. Moreover, in the model assuming rapid and DRM, its effect on the stellar abundances at larger radii is significant enough to provide the large difference in the evolution of stars on the [α/Fe]–[Fe/H] plane between inner and outer disk regions. As a result, we obtain the bimodal distribution of disk stars on the [α/Fe]–[Fe/H] plane as observed in the Galactic stellar disk, thereby implying that the event of DRM may play a key role in reproducing the observed bimodality of stars on this abundance-ratio diagram. We discuss possible mechanisms causing such DRM in the early evolution of the Galactic disk, including the event of minor merging of a relatively massive satellite onto the stellar disk.