The binary companion mass ratio distribution: an imprint of the star
  formation process?
  We explore the effects of dynamical evolution in dense clusters on the
companion mass ratio distribution (CMRD) of binary stars. Binary systems are
destroyed by interactions with other stars in the cluster, lowering the total
binary fraction and significantly altering the initial semi-major axis
distribution. However, the shape of the CMRD is unaffected by dynamics; an
equal number of systems with high mass ratios are destroyed compared to systems
with low mass ratios. We might expect a weak dependence of the survivability of
a binary on its mass ratio because its binding energy is proportional to both
the primary and secondary mass components of the system. However, binaries are
broken up by interactions in which the perturbing star has a significantly
higher energy (by a factor of >10, depending on the particular binary
properties) than the binding energy of the binary, or through multiple
interactions in the cluster. We therefore suggest that the shape of the
observed binary CMRD is an outcome of the star formation process, and should be
measured in preference to the distributions of orbital parameters, such as the
semi-major axis distribution.
