Multimodal Distributions along the Horizontal Branch 1.pdfVIP

Multimodal Distributions along the Horizontal Branch 1 Francesco R. Ferraro2, Barbara Paltrinieri2, Flavio Fusi Pecci2,3 Ben Dorman4,5 and Robert T. Rood4 ABSTRACT We report on HST/WFPC2 U, V and far-ultraviolet observations of two Galactic Globular Clusters (GGCs), NGC 6205 = M13 and NGC 6093 = M80. Both of these clus- ters have horizontal-branch (HB) tails that extend to the helium-burning main sequence, with the hottest stars reaching theoretical effective temperatures above 35,000 K. In both clusters, groups of stars are found to be separated by narrow gaps along the blue HB sequence. These gaps appear at similar locations in the color-magnitude diagrams of the two clusters. While stochastic effects may give rise to variations in the color distribution along the HB, the coincidence of gaps in different clusters effectively rules this out as the primary cause. The comparison among the clusters strongly suggests that there are separate physical processes operating during the earlier red-giant phase of evolution to produce mass loss. Subject headings: globular clusters: individual (M80, M13, M3)—stars: horizontal- branch— ultraviolet: stars—stars: evolution 1Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555 2Osservatorio Astronomico di Bologna, via Zamboni 33, I-40126 Bologna, ITALY 3Stazione Astronomica di Cagliari, 09012 Capoterra, ITALY 4Astronomy Dept, University of Virginia, P.O.Box 3818, Charlottesville, VA 22903-0818 5Laboratory for Astronomy Solar Physics, Code 681, NASA/GSFC, Greenbelt MD 20771 1 1. Introduction A growing number of Galactic Globular Clusters (GGCs) have been found to show discontinuous stel- lar distributions along the horizontal-branch (HB). Some of them are clearly bimodal with a HB clump of red stars and a tail of blue, hot stars populating the blue side of the RR Lyrae strip (NGC 2808—Ferraro et al. 1990; NG