The Surprising External Upturn of the Blue Straggler Radial Distribution in M55

By combining high-resolution HST and wide-field ground-based observations in ultraviolet and optical bands, we study the blue straggler star (BSS) population of the low-density galactic globular cluster M55 (NGC 6809) over its entire radial extent. The BSS projected radial distribution is found to be bimodal, with a central peak, a broad minimum at intermediate radii, and an upturn at large radii. Similar bimodal distributions have been found in other globular clusters (M3, 47 Tucanae, NGC 6752, and M5), but the external upturn in M55 is the largest found to date. This might indicate a large fraction of primordial binaries in the outer regions of M55, which seems somehow in contrast with the relatively low (~10%) binary fraction recently measured in the core of this cluster.