On the evolution of a star cluster and its multiple stellar systems
  following gas dispersal
  We investigate the evolution, following gas dispersal, of a star cluster
produced from a hydrodynamical calculation. We find that when the gas,
initially comprising 60% of the mass, is removed, the system settles into a
bound cluster containing 30-40% of the stellar mass surrounding by an expanding
halo of ejected stars. The bound cluster expands from an initial radius of
<0.05 pc to 1-2 pc over 4-10 Myr, depending on how quickly the gas is removed,
implying that stellar clusters may begin with far higher stellar densities than
usually assumed. With rapid gas dispersal the most massive stars are found to
be mass segregated for the first ~1 Myr of evolution, but classical mass
segregation only develops for cases with long gas removal timescales.
Eventually, many of the most massive stars are expelled from the bound cluster.
Despite the high initial stellar density and the extensive dynamical evolution
of the system, we find that the stellar multiplicity is almost constant during
the 10 Myr of evolution. This is because the primordial multiple systems are
formed in a clustered environment and, thus, by their nature are already
resistant to further evolution. The majority of multiple system evolution is
confined to the decay of high-order systems and the formation of a significant
population of very wide (10^4-10^5 AU) multiple systems in the expanding halo.
This formation mechanism for wide binaries potentially solves the problem of
how most stars apparently form in clusters and yet a substantial population of
wide binaries exist in the field. Many of these wide binaries and the binaries
produced by the decay of high-order multiple systems have unequal mass
components, potentially solving the problem that hydrodynamical simulations of
star formation are found to under-produce unequal-mass solar-type binaries.
