RADIAL-VELOCITIES, PROPER MOTIONS AND THE DETERMINATION OF THE ORBITAL STRUCTURE OF A SPHERICAL STAR CLUSTER

In how far does the presence of both radial velocities and proper moti ons constrain the orbital structure and the dynamical distance of a sp herical star cluster? To investigate this question, we create artifici al stellar systems of various sizes and orbital structure, and use a Q uadratic Programming technique for the modeling. This technique turns out to be a very fast and dependable method. We find that, if the obse rvational errors are small (25% or less), 500 data points are no luxur y in order to reach reliable conclusions. Larger errors will lead to a n overestimation of the mass and will tend to isotropise the orbital s tructure. By comparing the line of sight velocity dispersions with the proper motion dispersions we find that an accuracy of 10-15% on the d ynamical distance is obtainable for good quality data.