Radial Migration from the Metallicity Gradient of Open Clusters and Outliers

Abstract

Radial migration is an important process in the evolution of the Galactic disk. The metallicity gradient of open clusters and its outliers provide an effective way to examine this process. In this work, we compile metallicity, age, and kinematic parameters for 225 open clusters and carry out a quantitative analysis of radial migration via the calculated migration distances. Based on clusters with age <0.5 Gyr, we obtain a present-day metallicity gradient of −0.074 ± 0.007 dex kpc−1. Three sequences are distributed along this gradient, and clusters in the upper, middle, and lower groups are found to be old outward migrators, in situ clusters, and inward migrators, respectively. The migration distance increases with age, but the time when it is most effective is probably less than 3 Gyr. The metallicity gradient breaks at guiding center radius R g ∼ 11.5 kpc, which is caused by the lack of young open clusters in the outer disk and the presence of old outward migrators in the upper sequence. This shows that this boundary is related to the different effects of radial migration between the inner and outer disks. We also found many special open clusters in and near the outer disk with R > 11 kpc and a steeper metallicity gradient from the inner disk of R g < 7 kpc, which tells of a complicated evolution history of the Galactic disk caused by different effects of stellar radial migration.