Assigned ID: 489 ----- Attempting Submission 1 (Fri Mar 07 19:13:02 GMT 2008) ----- The Orion Nebula Cluster (ONC) is the best-ever studied rich system of very young stars and other objects, and has already provided crucial clues about protoplanetary disks and the very early stages of star formation, ranging from the most massive stars down to brown dwarfs. One of the last unsolved mysteries of the ONC is its dynamical status: it is unclear whether the ONC is bound or unbound. The key to this quest is a reliable and precise measurement of the internal velocity dispersion via proper motions. We propose to derive this dispersion with only a 15% uncertainty which then will enable: 1) to settle the issue on the dynamical stability of ONC; 2) to examine the radial dependence of the internal velocity dispersion; 3) to explore its dependency on stellar mass. This will provide the foundation upon which the next generation of dynamical models of systems, like the ONC, can be built. Our group has the world's best archival images of the ONC spanning more than 40 years. The combination of HST ACS/WFC and WFPC2 images with our superb and deep ground-based collection of plates and CCD images is the key condition to reach the required 0.1 mas/yr precision level - at least three times better than the current estimates for stars fainter than V=12. This appears to be the most rational way to extract the invaluable astrometric information from the HST Orion Treasury Program and other extensive HST datasets.

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