We presented an isophotal atlas for 75 star cluster pairs and multiplets in the Small Magellanic Cloud, comprising 176 objects.
It was possible to derive ages from Colour-Magnitude Diagrams using the OGLE-II photometric survey for 91 objects. In addition we included in the analysis ages for 21 embedded objects in HII regions. The age distribution has a maximum in the youngest bin with a profile related to the cluster formation/destruction rates, in particular cluster multiple systems can have a fast destruction rate caused by their internal dynamical evolution. There is a second peak around 220 Myr which is probably related to the SMC/LMC last encounter.
We find that 55% of the pairs and multiplets in the sample are in general coeval, indicating that captures are a rare phenomenon. Most of the cluster multiplets occur in OB stellar associations and/or HII region complexes which indicates a common origin and suggests that multiplets coalesce into pairs or single clusters in a short time scale ( 100 Myr).
The majority of the cluster members have comparable sizes, with a diameter ratio ranging mostly between 1 - 2. The projected separation distribution between the members of a pair has a pronounced peak at 13 pc. These observational results are important constraints to theoretical models of star cluster pair encounters and could be related with the formation process and subsequent dynamical evolution of cluster systems.
The angular distribution of cluster pairs and multiplets shows that most of the objects are located in the SMC main body. The overall SMC cluster sample presents a similar distribution and there is evidence that it is related to a nearly edge-on disk in the SMC. Considering this, it is expected an increase of projection effects as compared to estimates for the LMC disk where physical pairs would be about 50% (Bhatia & Hatzidimitriou 1988).
The atlas shows that around 25% of the isophote maps present relevant structures like bridges, common envelopes or detached distorted isophotes. N-body simulations have indicated that these structures arise from interactions between the members of the cluster systems. Indeed cluster pairs as NGC 241/NGC 242 and B39/BS30 show in their isophotal maps bridges linking their members and have comparable ages.
We conclude that multiplicity may have an important role in the early dynamical evolution of star clusters in general, and signatures of that may survive in the long term structure of large single clusters (de Oliveira et al. 2000).
We thank the referee Dr. B. Westerlund for interesting remarks. We acknowledge support from the Brazilian institutions CNPq, CAPES and FINEP.
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