The Binary Populations of Eight Globular Clusters in the Outer Halo of
  the Milky Way
  We analyse color-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 anti-correlation
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% 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
normalised to the core binary fraction the radial density profiles follow a
common trend. It has a maximum in the center and declines by a factor of two at
a distance of about two core radii from the cluster center. After dropping to
its minimum at a radial distance of $\sim$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.
