DYNAMIC EVOLUTION EFFECTS ON THE HOT STELLAR POPULATIONS IN GLOBULAR-CLUSTERS

We present results of a study of far ultraviolet (FUV) properties of Galactic globular clusters. Using the spatially resolved spectra measured with the IUE satellite, we find indications of color gradients in two clusters with the post-core-collapse (PCC) morphology, NGC 6752 and NGC 7099, but not in the case of NGC 6093, a cluster with the classical King-model-type morphology. The PCC cluster profiles appear to be systematically sharper at the shorter wavelengths. Several additional cases have been identified in the literature which follow the same pattern. These FUV color gradients may be caused by the presence of a highly concentrated population of hot objects, such as the extreme BHB stars, blue stragglers, etc. This result extends to FUV regime the trends (color and population gradients) seen in the ground-based data in the visible regime. We also find that PCC or highly concentrated, small core clusters have bluer HB morphologies and bluer FUV colors, and the bluest FUV colors at a given metallicity. These trends indicate that dynamical evolution of clusters played some role in determining the net abundance and the spatial distribution of their hot stellar populations. Possible physical mechanisms responsible for the gradients remain unclear, but may involve stellar mergers, tidal encounters, and/or formation of binaries and their effects on the post-main-sequence phases of stellar evolution.