A pas de trois birth for wide binary stars

Abstract

The widest binary star systems pose a challenge to theory: true stellar twins could not form so far apart. Simulations suggest these twins are in fact triplets, two of which masquerade as one star and cast out the third. See Letter p.221 Very wide binary star systems—widely separated pairs of gravitationally bound stars—are relatively common in the Milky Way, but they challenge current theories of star formation. The problem is that their separation can exceed the typical size of the collapsing cloud of dust and gas from which the stars form. Recent observations have shown that very wide binaries are frequently members of triple systems and that close binaries often have a distant third companion. Bo Reipurth and Seppo Mikkola report N-body simulations of the dynamical evolution of newborn triple systems that demonstrate that although the triple systems are compact when they are born, they can develop extreme hierarchical architectures on timescales of millions of years as one component is dynamically scattered into a very distant orbit. The energy of ejection comes from shrinking the orbits of the other two stars, often making them look like a single star. Such loosely bound triple systems will therefore appear as very wide binaries.