The Structure of the Young Star Cluster NGC 6231. II. Structure,
  Formation, and Fate
  The young cluster NGC 6231 (stellar ages ~2-7 Myr) is observed shortly after
star-formation activity has ceased. Using the catalog of 2148 probable cluster
members obtained from Chandra, VVV, and optical surveys (Paper I), we examine
the cluster's spatial structure and dynamical state. The spatial distribution
of stars is remarkably well fit by an isothermal sphere with moderate
elongation, while other commonly used models like Plummer spheres, multivariate
normal distributions, or power-law models are poor fits. The cluster has a core
radius of $1.2\pm0.1$ pc and a central density of ~200 stars pc$^{-3}$. The
distribution of stars is mildly mass segregated. However, there is no radial
stratification of the stars by age. Although most of the stars belong to a
single cluster, a small subcluster of stars is found superimposed on the main
cluster, and there are clumpy non-isotropic distributions of stars outside ~4
core radii. When the size, mass, and age of NGC 6231 are compared to other
young star clusters and subclusters in nearby active star-forming regions, it
lies at the high-mass end of the distribution but along the same trend line.
This could result from similar formation processes, possibly hierarchical
cluster assembly. We argue that NGC 6231 has expanded from its initial size but
that it remains gravitationally bound.
