On the Disruption of Star Clusters in a Hierarchical Interstellar Medium
  The distribution of the number of clusters as a function of mass M and age T
suggests that clusters get eroded or dispersed in a regular way over time, such
that the cluster number decreases inversely as an approximate power law with T
within each fixed interval of M. This power law is inconsistent with standard
dispersal mechanisms such as cluster evaporation and cloud collisions. In the
conventional interpretation, it requires the unlikely situation where diverse
mechanisms stitch together over time in a way that is independent of
environment or M. Here we consider another model in which the large scale
distribution of gas in each star-forming region plays an important role. We
note that star clusters form with positional and temporal correlations in giant
cloud complexes, and suggest that these complexes dominate the tidal force and
collisional influence on a cluster during its first several hundred million
years. Because the cloud complex density decreases regularly with position from
the cluster birth site, the harassment and collision rates between the cluster
and the cloud pieces decrease regularly with age as the cluster drifts. This
decrease is typically a power law of the form required to explain the mass-age
distribution. We reproduce this distribution for a variety of cases, including
rapid disruption, slow erosion, combinations of these two, cluster-cloud
collisions, cluster disruption by hierarchical disassembly, and partial cluster
disruption. We also consider apparent cluster mass loss by fading below the
surface brightness limit of a survey. In all cases, the observed log M - \log T
diagram can be reproduced under reasonable assumptions.
