Deep HST Imaging In 47 Tuc And NGC 6397: Stellar Dynamics On The Outskirts Of NGC 6397

Multi-epoch observations with ACS on HST provide a unique and comprehensive probe of stellar dynamics within NGC 6397. We are able to confront analytic, Monte Carlo and nbody models of the globular cluster with the observed stellar proper motions . The observations probe well along the main sequence from 0.5 to below 0.1 solar masses as well as white dwarfs younger than one gigayear. The observed field lies beyond the half-light radius where standard models of globular cluster dynamics (e.g. based on a lowered Maxwellian velocity distribution) make very robust predictions for the stellar proper motions as a function of mass. The observations agree with the models for the main sequence stars; however, the proper motions of the young white dwarfs are somewhat puzzling, and we examine some possible explanations.