A kinematic study of open clusters: implications for their origin

The Galactic population of open clusters provides an insight into star formation in the Galaxy. The open cluster catalogue by Dias et al. is a rich source of data, including kinematic information. This large sample made it possible to carry out a systematic analysis of 481 open cluster orbits, using parameters based on orbit eccentricity and separation from the Galactic plane. These two parameters may be indicative of origin, and we find them to be correlated. We also find them to be correlated with metallicity, another parameter suggested elsewhere to be a marker for origin in that high values of any of these two parameters generally indicate a low metallicity ([Fe/H] solar < -0.2 dex). The resulting analysis points to four open clusters in the catalogue being of extragalactic origin by impact of high-velocity cloud on the disc: Berkeley21, 32, 99 and Melotte66, with a possible further four due to this origin (NGC 2158, 2420, 7789 and IC1311). A further three may be due to Galactic globular cluster impact on the disc, i.e. of internal Galactic origin (NGC 6791, 1817 and 7044).