Do binaries in clusters form in the same way as in the field?
Abstract
We examine the dynamical destruction of binary systems in star clusters of different densities. We find that at high densities (104 - 105 Msun pc-3) almost all binaries with separations > 103 AU are destroyed after a few crossing times. At low densities (order(102) Msun pc-3) many binaries with separations > 103 AU are destroyed, and no binaries with separations > 104 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 around 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 > 103 AU, and (c) it is well-mixed and therefore dynamically old. We also examine the origin of the field binary population. Binaries with separations < 102 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 102 - 104 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 > 104 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.