4. Cluster orbits
A factor in determining the expected amount of interstellar matter injected into a GC is the time since it last passed through the Galactic plane. Knowledge of the details of the orbit (velocity, distance from the Galactic plane and from the Galactic centre versus time) also allows the effects of external mechanisms which depend on a cluster's Galactic position and velocity to be determined.
Until recently, our knowledge of cluster orbits was limited to the information of its present position in the Galaxy and the radial velocity of the cluster. This enabled only the minimum time since the last Galactic plane crossing to be determined, as was done, for example by Tayler & Wood (1975). However, work in the last few years have measured the absolute proper motion of a number of clusters. Together with a mass model of the Galaxy, a radial velocity and proper motion enable the full orbit parameters to be determined for a cluster (see e.g. Dauphole et al. 1996, in press).
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998