The Spatial Structure of Young Stellar Clusters. III. Physical
  Properties and Evolutionary States
  We analyze the physical properties of stellar clusters that are detected in
massive star-forming regions in the MYStIX project--a comparative,
multiwavelength study of young stellar clusters within 3.6 kpc that contain at
least one O-type star. Tabulated properties of subclusters in these regions
include physical sizes and shapes, intrinsic numbers of stars, absorptions by
the molecular clouds, and median subcluster ages. Physical signs of dynamical
evolution are present in the relations of these properties, including
statistically significant correlations between subcluster size, central
density, and age, which are likely the result of cluster expansion after gas
removal. We argue that many of the subclusters identified in Paper I are
gravitationally bound because their radii are significantly less than what
would be expected from freely expanding clumps of stars with a typical initial
stellar velocity dispersion of ~3 km/s for star-forming regions. We explore a
model for cluster formation in which structurally simpler clusters are built up
hierarchically through the mergers of subclusters--subcluster mergers are
indicated by an inverse relation between the numbers of stars in a subcluster
and their central densities (also seen as a density vs. radius relation that is
less steep than would be expected from pure expansion). We discuss implications
of these effects for the dynamical relaxation of young stellar clusters.
