The surprising external upturn of the Blue Straggler radial distribution in M55

Abstract

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, 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 (\sim 10%) binary fraction recently measured in the core of this cluster.

in press on ApJ

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• Blue Straggler Populations of Seven Open Clusters with Gaia DR2
• The binary mass transfer origin of the red blue straggler sequence in M30
• Probing the role of dynamical friction in shaping the BSS radial distribution. I - Semi-analytical models and preliminary N-body simulations
• Empirical measurement of the dynamical ages of three globular clusters and some considerations on the use of the dynamical clock
• The Blue Straggler population in the globular cluster M53 (NGC5024): a combined HST, LBT, CFHT study
• Formation and Evolution of Blue Stragglers in 47 Tucanae
• Blue Straggler Stars in Globular Clusters: a powerful tool to probe the internal dynamical evolution of stellar systems
• A possible solution to the Milky Way's binary-deficient retrograde stellar population. Evidence that $\omega$ Centauri has formed in an extreme starburst
• The temperature distribution of horizontal branch stars: methods and first results
• Rotational velocities of Blue Straggler Stars in the Globular Cluster M55
• Blue Stragglers in Globular Clusters: Observations, Statistics and Physics
• Study of dynamical status of the globular cluster NGC 1851 using Ultraviolet Imaging Telescope