1996A&A...306..747M

The tidal radii of 21 globular clusters are studied using positions and spatial velocities based on recent compilations as well as new measurements of their proper motions. The calculation of the perigalactic position of each cluster gives the possibility to determine their tidal radii using the classical formula of King (1962). Previous papers have claimed that the present tidal radius is approximately the same as the radius at the perigalactic position. We show here that this hypothesis is not verified and that there is in fact a discrepancy between theoretical and observed tidal radii. We also show that this difference depends on the relative orbital position of the cluster, leading to an orbital phase dependence. For this phase, which reflects complex relaxation processes in the core cluster, we give an empirical linear equation. However, no evident relation has been found between relaxation time and the difference between classical theory and observations. The present work shows that a rigorous estimation of the tidal radii of the globular clusters requires not only good kinematical data for the clusters themselves, but also a good understanding of the internal relaxation processes.