The binary populations of eight globular clusters in the outer halo of the Milky Way

Abstract

We analyse colour–magnitude diagrams of eight globular clusters (GCs) in the outer Galactic halo. Images were taken with the Wide Field Channel of the Advanced Camera for Survey and the Ultraviolet and Visual Channel of the Wide Field Camera 3 on board of the Hubble Space Telescope. We have determined the fraction of binary stars along the main sequence and combined results with those of a recent paper where some of us have performed a similar analysis on 59 Galactic GCs. In total, binaries have been now studied homogeneously in 67 GCs. We studied the radial and luminosity distributions of the binary systems, the distribution of their mass ratios and investigated univariate relations with several parameters of the host GCs. We confirm the anticorrelation between the binary fraction and the luminosity of the host cluster, and find that low-luminosity clusters can host a large population in excess of ∼40 per cent in the cluster core. However, our results do not support a significant correlation with the cluster age as suggested in the literature. In most GCs, binaries are more centrally concentrated than single stars. If the fraction of binaries is normalized to the core binary fraction the radial density profiles follow a common trend. It has a maximum in the centre and declines by a factor of 2 at a distance of about two core radii from the cluster centre. After dropping to its minimum at a radial distance of ∼5 core radii it stays approximately constant at larger radii. We also find that the mass ratio and the distribution of binaries as a function of the mass of the primary star is almost flat.