The Rich Lack Close Neighbours: The Dependence of Blue-Straggler
  Fraction on Metallicity
  Blue straggler stars (BSS) have been identified in star clusters and in field
populations in our own Milky Way galaxy and in its satellite galaxies. They
manifest as stars bluer and more luminous than the dominant old population, and
usually have a spatial distribution that follows the old population. Their
progenitors are likely to have been close binaries. We investigate trends of
the BSS population in dwarf spheroidal galaxies (dSph) and in the bulge of the
Milky Way and find an anti-correlation between the relative frequency of BSS
and the metallicity of the parent population. The rate of occurrence of BSS in
the metal-poor dwarf galaxies is approximately twice that found in the
solar-metallicity bulge population. This trend of decreasing relative
population of BSS with increasing metallicity mirrors that found for the
close-binary fraction in the field population of the Milky Way. We argue that
the dominant mode of BSS formation in low-density environments is likely to be
mass transfer in close-binary systems. It then follows that the similarity
between the trends for BSS in the dSph and field stars in our Galaxy supports
the proposal that the small-scale fragmentation during star formation is driven
by the same dominant physical process, despite the diversity in environments,
plausibly gravitational instability of proto-stellar discs.
