The rich lack close neighbours: the dependence of blue-straggler fraction on metallicity

Abstract

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 anticorrelation 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 MilkyWay. 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.