A statistical study of binary and multiple clusters in the LMC^*

¹ Sternwarte der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² Institut für Photogrammetrie, Universität Bonn, Nußallee 15, 53115 Bonn, Germany
³ MPI für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Corresponding author: A. Dieball, adieball@astro.uni-bonn.de

Received: 20 March 2002
Accepted: 28 May 2002

Abstract

Based on the Bica et al. ([CITE]) catalogue, we studied the star cluster system of the LMC and provide a new catalogue of all binary and multiple cluster candidates found. As a selection criterion we used a maximum separation of corresponding to 20 pc (assuming a distance modulus of 18.5 mag). We performed Monte Carlo simulations and produced artificial cluster distributions that we compared with the real one in order to check how many of the found cluster pairs and groups can be expected statistically due to chance superposition on the plane of the sky. We found that, depending on the cluster density, between (bar region) and (outer LMC) of the detected pairs can be explained statistically. We studied in detail the properties of the multiple cluster candidates. The binary cluster candidates seem to show a tendency to form with components of similar size. When possible, we studied the age structure of the cluster groups and found that the multiple clusters are predominantly young with only a few cluster groups older than 300 Myr. The spatial distribution of the cluster pairs and groups coincides with the distribution of clusters in general; however, old groups or groups with large internal age differences are mainly located in the densely populated bar region. Thus, they can easily be explained as chance superpositions. Our findings show that a formation scenario through tidal capture is not only unlikely due to the low probability of close encounters of star clusters, and thus the even lower probability of tidal capture, but the few groups with large internal age differences can easily be explained with projection effects. We favour a formation scenario as suggested by Fujimoto & Kumai ([CITE]) in which the components of a binary cluster formed together and thus should be coeval or have small age differences compatible with cluster formation time scales.