  Observations indicate that in young stellar clusters the binary fraction for
massive stars is higher than for solar mass stars. For the Orion Nebula Cluster
(ONC) there is a binary frequency of ~ 50% for solar-mass stars compared to
70-100% for the massive O- and B-stars. We explore the reasons for this
discrepancy and come up with two possible answers: a) a primordially higher
binarity of massive stars could be inherent to the star formation process or b)
the primordial binary rate might be the same for solar-mass and massive stars,
but the higher capture cross section of the massive stars possibly leads to the
formation of additional massive binaries in the early cluster development. Here
we investigate the likelihood of the latter using the ONC as an example. N-body
simulations are performed to track the capture events in an ONC-like cluster.
We find that whereas low-mass stars rarely form bound systems through capture,
the dynamics of the massive stars - especially in the first 0.5 Myrs - is
dominated by a rapid succession of ``transient binary or multiple systems''. In
observations the transient nature of these systems would not be apparent, so
that they would be rated as binaries. At 1-2 Myrs, the supposed age of the ONC,
the ``transient'' massive systems become increasingly stable, lasting on
average several 10^6 yrs. Despite the ONC being so young, the observed binary
frequency for massive stars -- unlike that of solar-mass stars -- is not
identical to the primordial binary frequency but is increased by at least
10-15% through dynamical interaction processes. This value might be increased
to at least 20-25% by taking disc effects into account. The primordial binary
frequency could well be the same for massive and solar mass stars because the
observed difference can be explained by capture processes alone.
