The Dynamics of an Expanding OB Association
  We present 3-dimensional kinematical observations of the massive OB association Cygnus OB2 to identify the mechanisms responsible for disrupting young star clusters. The picture revealed by these observations is of a highly-substructured, dynamically unmixed OB association that does not exhibit the position-velocity correlations predicted by the theories of infant mortality or tidal stripping. These observations would appear to support a picture of hierarchical star formation. 
Nicholas J. Wright, Herve Bouy, Jeremy J. Drake, Janet E. Drew, Mario   Guarcello, David Barrado y Navacues
astro-ph.SR