Constraining a multiple stellar origin of blue stragglers via speckle imaging

As single stellar evolution cannot explain the existence of blue stragglers (BSs), several formation scenarios involving binaries have been put forward, with mass transfer and direct collisions taking the lead. A third mechanism, operating on its own or enhancing the former, is the Kozai-Lidov mechanism, where the amplitude of the inner binary in a hierarchical triple system, changes via secular torque of its outer companion, eventually leading to a merger of the inner binary and the formation of a BS. While much of our knowledge of the binarity of BSs comes from multi-year radial velocity campaigns (e.g. WOCS), these approaches are necessarily insensitive to the most distant binaries, like those predicted by the Kozai-Lidov mechanism. These companions on wide orbits may be visible through direct high-resolution imaging. We observed 51 BSs in 10 open clusters with the speckle interferometers, Zorro and 'Alopeke at both Gemini telescopes, reaching a spatial resolution of about 25 mas, the diffraction limit at ~500nm. Our observations probe for main-sequence companions down to a mass close to 0.3 Msun, revealing that about 10% of BSs have these distant main-sequence companions. On-going spectroscopy will reveal whether the companions can be considered bound