Stellar Population Gradients in Post-Core Globular Clusters

The nature of color and population gradients in globular clusters is one of the major outstanding puzzles in modern globular cluster reseach. Clusters with central cusps (collapsed cores) become bluer towards their centers, while no clear gradients are seen in clusters with King-model morphology. The effect involves at least a few percent of the total visible light. The color gradients seem to be caused by the demise of red giants and/or subgiants, and possibly an increased number of faint blue objects. These effects represent a strong evidence that dynamical evolution of star clusters can physically modify their stellar populations. These phenomena are not yet understood, but a population of centrally concentrated binaries is most likely responsible for them. The underlying physical cause of these effects may be also related to the origin of millisecond pulsars and low mass x-ray binaries in globular clusters. Star counts in the UV near the centers of highly concentrated clusters with the HST can probe the regions where the gradients should be the strongest and which are very difficult to study from the ground. In addition to extending the counts of HB and RGB stars into the central regions we expect to find a new population of faint blue objects near the centers of these clusters, for which tantalizing hints have been seen in the best-seeing ground-based data, and in the UV colors measured by the IUE and ANS satellites.