HST OBSERVATIONS OF THE CORE OF GLOBULAR-CLUSTER NGC 6397

The core of the nearby and very concentrated globular cluster NGC 6397 has been imaged through the f/96-F140W, f/48-F140W, f/96-F210M, and f /48-F220W ultraviolet filters of the Faint Object Camera on board the Hubble Space Telescope down to an ultraviolet limiting magnitude of ap proximately 19. The most interesting objects in the field of view are six very bright, centrally concentrated, blue stragglers observed for the first time in the UV. Using these and other data from ground-based observations, we have been able to deduce from a comparison with Kuru cz's atmosphere models, temperatures of approximately 10 000 K and mas ses of approximately 1.6 M. for the four brightest ones, which is rema rkably close to twice the turn-off mass of NGC 6397. This finding supp orts the idea that two-star mechanisms (collisions, mergers) are at th e origin of the blue stragglers in the core of NGC 6397. Since the cen tral density is very high, collisions between main sequence stars are frequent, therefore providing the best formation mechanism. We have co mputed the number of such collisions in the core of NGC 6397 and found it to be of the same order as the number of bright blue stragglers ob served there. Thanks to the HST resolution, we have also been able to resolve one object, previously classified as a yellow straggler, into a blend of a blue straggler and three redder stars.