Interacting star clusters in the Large Magellanic Cloud - Overmerging problem solved by cluster group formation

We present the tidal tail distributions of a sample of candidate binary clu sters located in the bar of the Large Magellanic Cloud (LMC). One isolated cluster, SL 268, is presented in order to study the effect of the LMC tidal field. All the candidate binary clusters show tidal tails, confirming that the pairs are formed by physically linked objects. The stellar mass in the tails covers a large range, from 1.8 x 10(3) to 3 x 10(4)M.. We derive a t otal mass estimate for SL 268 and SL 356. At large radii, the projected den sity profiles of SL 268 and SL 356 fall off as r(-gamma), with gamma = 2.27 and gamma = 3.44, respectively. Out of 4 pairs or multiple systems, 2 are older than the theoretical survival time of binary clusters (going from a f ew 10(6) years to 10(8) years). A pair shows too large age difference betwe en the components to be consistent with classical theoretical models of bin ary cluster formation (Fujimoto & Kumai 1997). We refer to this as the "ove rmerging" problem. A different scenario is proposed: the formation proceeds in large molecular complexes giving birth to groups of clusters over a few 10(7) years. In these groups the expected cluster encounter rate is larger , and tidal capture has higher probability. Cluster pairs an not born toget her through the splitting of the parent cloud, but formed later by tidal ca pture. For 3 pairs, we tentatively identify the star cluster group (SCG) me mberships. The SCG formation, through the recent cluster starburst triggere d by the LMC-SMC encounter, in contrast with the quiescent open cluster for mation in the Milky Way can be an explanation to the paucity of binary clus ters observed in our Galaxy.