Multimodal Horizontal Branches: Empirical Evidence and Possible Evolutionary Scenarios

We review the available empirical evidence for the presence of "gaps" and multimodal distributions among horizontal branch (HB) stars, along with some of the theoretical scenarios that have been proposed to explain these features. While gaps along the HB have become increasingly less prominent and frequent as more and better color-magnitude diagram data have been obtained for Galactic globular clusters, the evidence for multimodal HBs has instead become stronger. In addition, different HB modes have recently started to be traced down to multiple components that have been detected among subgiant branch and main sequence stars, thus suggesting that their origin lies in the complex physical processes that took place at the earliest stages in the history of massive stellar clusters.