Binary stars in young clusters: models versus observations of the Trapezium Cluster

The frequency of low-mass pre-main sequence binary systems is significantly lower in the Trapezium Cluster than in Taurus-Auriga. We investigate if th is difference can be explained through stellar encounters in dense clusters . To this effect, a range of possible models of the well observed Trapezium Cluster are calculated using Aarseth's direct N-body code, which treats bi naries accurately. The results are confronted with observational constraint s. The range of models include clusters in virial equilibrium, expanding cl usters as a result of instantaneous mass loss, as well as collapsing cluste rs. In all cases the primordial binary proportion is larger than 50 per cen t, with initial period distributions as observed in Taurus-Auriga and the G alactic field. It is found that the expanding model, with an initial binary population as in the Galactic field, is most consistent with the observati onal constraints. This raises the possibility that the primordial group of OB stars may have expelled the cluster gas roughly 50 000 yr ago. The clust er's bulk expansion rate is thus a key observable that needs to be determin ed. The other models demonstrate that the rapidly decreasing binary proport ion, its radial dependence and the form of the period distribution, togethe r with structural and kinematical data, are very useful diagnostics on the present and past dynamical state of a young cluster. In particular, kinemat ical cooling from the disruption of wide binaries is seen for the first tim e. (C) 1999 Elsevier Science B.V. All rights reserved.