How to Form (Twin) Globular Clusters?
  Though it is generally assumed that massive molecular clouds are the progenitors of globular clusters, their detailed formation mechanism is still unclear. Standard scenarios based on the collapse of a smooth matter distribution suffer from strong requirements with respect to cluster formation time scale, binding energy and star formation efficiency. An alternative model assuming cluster formation due to the recollapse of a supernova-induced, fragmented shell can relax these difficulties.   In this paper the final collapse stages of the different scenarios are compared by N-body simulations for shells and spheres. It is shown that fragmentation is much more pronounced for shells. Taking a galactic tidal field into account shells preferably form twin (or multiple) systems, whereas spheres end up as single clusters. The twins are characterized by identical metallicities, and stellar mass functions; some of them show counter-rotating cores. Their orbital evolution can result in both, a final merger or well separated twins sharing a common galactic orbit. 
Christian Theis (Institut f. Theoretische Physik und Astrophysik,   University of Kiel, Kiel, Germany)