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Astron. Astrophys. 358, 897-909 (2000)

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Studies of binary star cluster candidates in the bar of the LMC. II *

A. Dieball 1 and E.K. Grebel **  2

1 Sternwarte der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
2 University of Washington, Department of Astronomy, Box 351580, Seattle, WA 98195-1580, USA

Received 3 August 1999 / Accepted 30 March 2000


Binary clusters account for more than 10% of the cluster population in the Magellanic Clouds. Statistically fewer than 50% of the found pairs are expected to be chance superpositions. We estimated the cluster encounter rate and suggest that tidal capture is an unlikely formation scenario for the formation of binary clusters. Thus, most true binary clusters can be expected to have formed together.

Here we present a study of three binary cluster candidates which are located in the bar of the LMC. NGC 1971 & NGC 1972 are situated in the association LH 59 in the eastern part of the bar. A third star cluster, NGC 1969, is close enough to this pair that all three objects may constitute a triple system. We present the first age determination that is based on CMDs for these star clusters. Our findings suggest that all three clusters are young (40-70 Myr) and may have been formed in the same GMC. It cannot clearly decided whether the clusters are physically interacting or not.

NGC 1894 & SL 341 are located at the south-western rim of the LMC bar. This pair is studied in detail for the first time: The isopleths of both clusters reveal an elliptical shape. Whether this might be interpreted as a sign of interaction or is a peculiarity which is shared with a large amount of LMC star clusters which show higher ellipticities than their counterparts in the Milky Way remains unclear. From our age determination we find that both clusters are coeval with an age of [FORMULA] Myr. This makes a formation from the same GMC a likely scenario.

SL 385 & SL 387 are a close pair in the western part of the LMC bar. We derived ages of [FORMULA] Myr for SL 385 and [FORMULA] for SL 387. The large age difference makes it unlikely that these two clusters formed in the same GMC.

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

* Based on observations taken at the European Southern Observatory, La Silla, Chile in program 54.D-0730 and during time allocated by the MPIA, Heidelberg.
** Hubble Fellow

Send offprint requests to: A. Dieball (adieball@astro.uni-bonn.de)

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000