GALACTIC ORBITS OF THE OLD OPEN CLUSTERS NGC-188, NGC-2682, NGC-2420,NGC-752 AND NGC-2506

The orbits of five intermediate age and old open clusters, namely NGC 188, NGC2682 (M 67), NGC 2420, NGC 752 and NGC 2506 are computed adopt ing the new radial velocities by Friel & Janes (1993). For the first t hree clusters absolute proper motions are available from literature, w hereas for the latter two the absolute proper motions are obtained fro m the relative ones according to the procedure described in the text. The orbital motions of the clusters are calculated by means of the Miy amoto-Nagai like potential by Allen & Santillan (1991) which describes the Galaxy as a three component system, namely Bulge, Disk, and Halo. The orbits are followed backwards for a time interval equal to the ag e of each cluster, and the main parameters of each orbit are determine d. With the aid of these results, we address the question whether or n ot the original gradient in metallicity relative to the gas from which the clusters have been formed has been later destroyed by their motio n. In other words we check whether the gradient derived by the present day position of the clusters is a good indicator of the original grad ient. We find that while the radial gradient is very close to the orig inal one, the vertical gradient (if any existed at all) is wiped out b y the rapid oscillatory motions of the clusters across the galactic di sk.