The origin of the distribution of binary star periods

Pre-main-sequence and main-sequence binary systems are observed to have per iods, P, ranging from 1 day to 10(10) days and eccentricities ranging from 0 to 1. We pose the problem of whether stellar-dynamical interactions in ve ry young and compact star clusters will broaden an initially narrow period distribution to the observed width. N-body computations of extremely compac t clusters containing 100 and 1000 stars initially in equilibrium and in co ld collapse are performed. In all cases, the assumed initial period distrib ution is uniform in the narrow range 4.5 less than or equal to log P less t han or equal to5.5 (P in days), which straddles the maximum in the observed period distribution of late-type Galactic field dwarf systems. None of the models lead to the necessary broadening of the period distribution, despit e our adopted extreme conditions that favor binary-binary interactions. Ste llar-dynamical interactions in embedded clusters thus cannot, under any cir cumstances, widen the period distribution sufficiently. The wide range of o rbital periods of very young and old binary systems is therefore a result o f cloud fragmentation and immediate subsequent magnetohydrodynamic processe s operating within the multiple protostellar system.