Absolute proper motions and galactic orbits of M5, M12 and M15 from Schmidt plates

Within the Potsdam Globular Cluster Programme we have obtained absolute proper motions of M5 (NGC 5904), M12 (NGC 6218) and M15 (NGC 7078) from measurements of Tautenburg, Palomar and UK Schmidt plates. The plates were scanned with the APM facility in Cambridge. The tangential motion of the globular clusters was obtained by a plate overlap solution using large numbers of background galaxies as reference objects. Before that an extensive error-removal technique was applied in order to exclude systematic positional plate-to-plate distortions of the different Schmidt plates and the known errors of the measuring machine. Combining our results with the known distances and radial velocities of the clusters, we obtained their space motions and Galactic orbits in a three-component Galactic potential. Whereas the presently observed Galactocentric distances of M12 and M15 are typical for their calculated orbits, we conclude from our results that M5 is apparently an outer halo cluster only briefly visiting the nearer regions.