The Unique History of the Globular Cluster Omega Centauri
  Using current observational data and simple dynamical modeling, we demonstrate that Omega Cen is not special among the Galactic globular clusters in its ability to produce and retain the heavy elements dispersed in the AGB phase of stellar evolution. The multiple stellar populations observed in Omega Cen cannot be explained if it had formed as an isolated star cluster. The formation within a progenitor galaxy of the Milky Way is more likely, although the unique properties of Omega Cen still remain a mystery. 
Oleg Y. Gnedin, HongSheng Zhao, J. E. Pringle, S. Michael Fall, Mario   Livio, and Georges Meylan