GLOBULAR-CLUSTER ORBITS BASED ON HIPPARCOS PROPER MOTIONS

We present and analyse space motions and orbits for a sample of 15 gal actic globular clusters. The absolute proper motions of these clusters have been determined with respect to reference stars of the new Hippa rcos system. Orbital integrations in two model potentials for the Gala xy are considered. The sample shows a mean rotation near 40 km s(-1) i n the sense of rotation of the galactic disk. Six clusters are however found to be in retrograde motion. Velocity dispersions are around 104 km s(-1) in the direction of rotation, near 116 km s(-1) in latitudin al direction and near 127 km s(-1) in radial direction. The orbits of the clusters preferentially have small axial angular momenta and high eccentricities, the median of the orbital eccentricities being 0.62. F rom the spatial extent of the orbits we conclude that the Galaxy must have a massive halo with a radius of at least 30 kpc. The space densit y distribution of our sample of clusters as derived from averaging ove r their orbits resembles the observed space density distribution of th e total globular cluster system, except for distances less than 4 kpc from the galactic center. The largest apogalactic distances in the sam ple reach out to 65 kpc. The orbits provide evidence that the more met al-rich clusters are concentrated towards the galactic center. The clu sters with significant retrograde motion have metal abundances between -1.5 and -2.0 and hence appear to be relatively homogeneous in chemic al composition. The small subgroup of `young halo' clusters within our sample is orbiting with a net retrograde rotation of -9 km s(-1). A g eneral relation between orbital eccentricity and metal-abundance does not show up in the sample. The observed radii of the clusters are foun d to be in a well-defined relation to the tidal limits imposed by orbi tal motion in the galactic field. It is shown that the cluster radii a re however not uniquely determined by the perigalactic distances, but involve at least also the geometry of the orbit. (C) 1997 Elsevier Sci ence B.V.