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Astron. Astrophys. 344, 450-458 (1999)


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Interacting star clusters in the Large Magellanic Cloud *

Overmerging problem solved by cluster group formation

Stéphane Leon 1,2, Gilles Bergond 2,3 and Antonella Vallenari 4

1 DEMIRM, Observatoire de Paris, 61, Avenue de l'Observatoire, F-75014 Paris, France
2 CAI-MAMA, Observatoire de Paris, 61, Avenue de l'Observatoire, F-75014 Paris, France
3 DASGAL, Observatoire de Paris-Meudon, 5, Place J. Janssen, F-92195 Meudon, France
4 Astronomical Observatory of Padova, Vicolo dell'Osservatorio 5, I-35122 Padova, Italy

Received 15 October 1998 / Accepted 15 December 1998

Abstract

We present the tidal tail distributions of a sample of candidate binary clusters 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 [FORMULA] to [FORMULA]. We derive a total mass estimate for SL 268 and SL 356. At large radii, the projected density profiles of SL 268 and SL 356 fall off as [FORMULA], with [FORMULA] and [FORMULA], respectively. Out of 4 pairs or multiple systems, 2 are older than the theoretical survival time of binary clusters (going from a few [FORMULA] years to [FORMULA] years). A pair shows too large age difference between the components to be consistent with classical theoretical models of binary cluster formation (Fujimoto & Kumai 1997). We refer to this as the "overmerging" problem. A different scenario is proposed: the formation proceeds in large molecular complexes giving birth to groups of clusters over a few [FORMULA] years. In these groups the expected cluster encounter rate is larger, and tidal capture has higher probability. Cluster pairs are not born together through the splitting of the parent cloud, but formed later by tidal capture. For 3 pairs, we tentatively identify the star cluster group (SCG) memberships. The SCG formation, through the recent cluster starburst triggered by the LMC-SMC encounter, in contrast with the quiescent open cluster formation in the Milky Way can be an explanation to the paucity of binary clusters observed in our Galaxy.

Key words: stars: formation – stars: Hertzsprung – Russel (HR) and C-M diagrams – galaxies: Magellanic Clouds – galaxies: star clusters – galaxies: stellar content

* Based on observations collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: S. Leon (sleon@mesunb.obspm.fr)

© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999

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