On the blue straggler population of the globular cluster M55

We have identified a large sample of 74 blue straggler stars (BSS) in the central region of the sparse, metal-poor globular cluster M55 (NGC 6809). Most of them form a relatively narrow, well-defined sequence extending from 0.6 mag below the main-sequence turnoff (MSTO) to about 2 mag brighter than the MSTO. Within the one core radius field that we observed, the BSS are more centrally concentrated than the MSTO and subgiant stars in the same magnitude range. About 15% of the BSS appear to form a secondary sequence ~ 0.75 mag above the main blue straggler sequence. Artificial star tests and the fact that these stars are strongly concentrated towards the cluster center suggest that at least some of them are binary BSS. Given the location and the small width of the blue straggler sequence we argue that the BSS in M55 are born with helium-enriched cores, but not envelopes. We conclude that the observed blue straggler sequence is the equivalent of a core helium-enriched main sequence (MS) where the BSS spend most of their life. Our observations are consistent with the assumption that the majority of the M55 blue stragglers are unmixed binary/collisional mergers or mass-transfer remnants.