MULTIMODAL DISTRIBUTIONS ALONG THE HORIZONTAL-BRANCH

We report on the Hubble Space Telescope (HST) Wide Field Planetary Cam era 2 (WFPC2) U, V, and far-ultraviolet observations of three galactic globular clusters (GGCs), NGC 5272 = M3, NGC 6205 = M13, and NGC 6093 = M80. Two of these clusters (namely, M13 and M80) have horizontal-br anch (HB) tails that extend to the helium-burning main sequence, with the hottest stars reaching theoretical effective temperatures above 35 ,000 K. In both dusters, groups of stars are found to be separated by narrow gaps along the blue HB sequence. These gaps appear at similar l ocations in the color-magnitude diagrams of the two dusters. While sto chastic effects may give rise to variations in the color distribution along the HB, the coincidence of gaps in different clusters effectivel y rules this out as the primary cause. The comparison among the cluste rs strongly suggests that there are separate physical processes operat ing during the earlier red giant phase of evolution to produce mass lo ss.