Do binaries in clusters form in the same way as in the field ? Richard

We examine the dynamical destruction of binary systems in star clusters of different densities. We find that at high densities (10 − 10 M⊙ pc ) almost all binaries with separations > 10 AU are destroyed after a few crossing times. At low densities (O(102)M⊙ pc ) many binaries with separations > 10 AU are destroyed, and no binaries with separations > 10 AU survive after a few crossing times. Therefore the binary separations in clusters can be used as a tracer of the dynamical age and past density of a cluster. We argue that the central region of the Orion Nebula Cluster was ∼ 100 times denser in the past with a half-mass radius of only 0.1 – 0.2 pc as (a) it is expanding, (b) it has very few binaries with separations > 10 AU, and (c) it is well-mixed and therefore dynamically old. We also examine the origin of the field binary population. Binaries with separations < 10 AU are not significantly modified in any cluster, therefore at these separations the field reflects the sum of all star formation. Binaries with separations in the range 10 – 10 AU are progressively more and more heavily affected by dynamical disruption in increasingly dense clusters. If most star formation is clustered, these binaries must be over-produced relative to the field. Finally, no binary with a separation > 10 AU can survive in any cluster and so must be produced by isolated star formation, but only if all isolated star formation produces extremely wide binaries.