Are there two kinds of blue stragglers in globular clusters?

Abstract

Blue stragglers made from collisions may have excess He abundance outside the core compared to those made from binary mergers. This effect may be observable, particularly in ultraviolet colors. Most blue stragglers have been discovered in clusters sufficiently sparse that collisions are improbable, and there is direct evidence for binarity in several cases. However, the newly discovered blue straggler sequences in the cores of NGC 6397 and 47 Tuc show characteristics which may be more easily explained if they are formed from collisional mergers.