On the Dispersal of Young Stellar Hierarchies
  Hierarchical structure in young star fields has been demonstrated in a
variety of ways, including two point correlation functions (TPCFs) that are
power laws for spatial scales up to at least several hundred parsecs. As the
stars age, this power law decreases in slope until it becomes nearly flat at
~100 Myr, at which point the hierarchical structure has disappeared. The fact
that the TPCF remains nearly a power law during this time implies that the
dispersal mechanism is somewhat independent of scale. This rules out dispersal
by random stellar motions at either the local gas turbulent speed or a constant
speed, because in both cases the hierarchy would disappear at small scales
first, causing the TPCF to bend over. Destruction by shear has the right
property as the shear rate in a galaxy is independent of scale for kpc-size
regions, but shear converts the hierarchy into an azimuthal stream which still
has a power-law TPCF. What does explain the observation is the overlapping of
several independent hierarchies from successive generations of star formation
in the same region. If stellar age is determined from magnitude intervals on
the main sequence of a color-magnitude diagram, or if clusters ages are grouped
together logarithmically into bins, then multiple generations will overlap more
and more as the grouped populations age, and this overlap will lower the
spatial correlations between group members. Models of these processes
illustrate their relative roles in removing the appearance of young stellar
hierarchies.
