Astron. Astrophys. 339, 773-781 (1998)

1. Introduction

The existence of gravitationally bound pairs of star clusters is important for the understanding of formation and evolution of star clusters. Since the probability of tidal capture of one cluster by another one is very small (Bhatia et al. 1991), we can assume that the components of a true binary star cluster have a common origin. Star clusters form in giant molecular clouds (GMCs) (e.g. Elmegreen & Elmegreen 1983), but the details of cluster formation are not yet well understood. If the components of a cluster pair or multiple cluster formed simultaneously or sequentially in the same fragmenting GMC they should have similar properties like age, metallicity and stellar content.

In the Milky Way only a few binary clusters are known, though Lyngå & Wramdemark (1984) suggest the common origin of a group of six Galactic open clusters. Later Pavloskaya & Filippova (1989), and more recently Subramaniam et al. (1995), propose the existence of more possible Galactic binary clusters and cluster complexes.

The apparent lack of binary clusters in our own Galaxy may be explained in different ways. Subramaniam et al. (1995) argue that since we are looking at the Galaxy from inside, double clusters may be harder to detect than in the distant Magellanic Clouds, where binary clusters can easily be detected due to the closeness of their projected positions on the sky. The distance to the Galactic clusters must also be taken into account, but only approximately 400 of 1400 open clusters have known distances (Lyngå 1987). Subramaniam et al. (1995) found 16 Galactic binary cluster candidates on the base of the Lyngå catalogue, which corresponds to 8 % of the investigated number of clusters. From this they conclude that binary clusters in the Milky Way may not be uncommon.

The evolution of a gravitationally bound pair of star clusters depends on the interaction between the components as well as on the tidal forces of the parent galaxy. If the tidal field is strong, the binary system will not survive for long but soon will get disrupted. From some preliminary considerations Innanen et al. (1972) conclude that due to stronger tidal forces in the Milky Way a binary cluster will execute only a fraction of a single orbit around the barycentre before its components are detached, but it will survive for several orbits in the less dense, less massive Magellanic Clouds. Surdin (1991) came to the same conclusion, especially for massive clusters. The investigation of binary clusters may help to evaluate the tidal field of the parent galaxy.

Fujimoto & Kumai (1997) suggest that globular and populous star clusters form through strong collisions between massive gas clouds in high-velocity random motion. Shear and momentum of oblique cloud-cloud collisions lead to break-up into compressed sub-clouds revolving around each other, which may form binary or multiple clusters. Binary star clusters are expected to form more easily in galaxies like the Magellanic Clouds with high-velocity random gas motions, whereas in the Milky Way such large-scale high-velocity random motions are lacking.

Bhatia & Hatzidimitriou (1988), Hatzidimitriou & Bhatia (1990), and Bhatia et al. (1991), have surveyed the Magellanic Clouds in order to catalogue the binary cluster candidates. The maximum projected centre-to-centre separation of the components of a pair was chosen to be 18 pc, which corresponds to in the LMC. A binary cluster with larger separation may become detached by the external tidal forces while shorter separations may lead to mergers (Sugimoto & Makino 1989, and Bhatia 1990). In these studies 69 pairs in the LMC and 9 pairs in the SMC were identified. Two clusters may appear to be a binary cluster due to chance line-up while in fact being at different distances within the Magellanic Clouds and not gravitationally bound to each other. The number of chance-pairs of objects uniformly distributed in space can be estimated with a formula presented by Page (1975). Taking into account also a non-uniform distribution of star clusters (at least for the LMC), Bhatia & Hatzidimitriou (1988) and Hatzidimitriou & Bhatia (1990) found that statistically 31 pairs in the LMC and 3 pairs in the SMC could be explained due to mere chance line-up. As considerably more pairs have been found, this strongly suggests that at least a certain amount of them must be true binary clusters.

While it is difficult to measure true distances between apparent binary clusters an analysis of their age and stellar content can give clues to a possible common origin.

The star cluster pair NGC 2006 (also known as SL 537) and SL 538 is located in the northwestern part of the OB association LH 77 in supergiant shell LMC 4, and has a projected centre-to-centre separation of corresponding to 13.3 pc. This double cluster has already been the subject of investigations concerning its binarity. Bhatia (1992) and Bica et al. (1996) found from integrated photometry that both clusters have the same age. Kontizas et al. (1993) analyzed the stellar content and the cores of the components using low resolution objective prism spectra and integrated IUE spectra. They suggested this cluster pair constitutes a true binary cluster, which moreover may merge in some years. This raises another question, which has also been discussed in Bhatia & MacGillivray (1988): could mergers of former binary star clusters be responsible for at least some of the blue populous clusters in the LMC?

We investigated the double cluster NGC 2006 and SL 538 in an attempt to find further affirmation - or disaffirmation - of the binarity of the two clusters. We analyze the star density in the clusters and the surrounding field (Sect. 3). For the first time we derive ages for these clusters from isochrone fits to colour-magnitude diagrams (CMDs) (Sect. 4), which is a much more reliable age determination than using integrated photometry. In Sect. 5 we investigate the content of Be stars in the clusters as well as in the surrounding field. In Sect. 6 we give a summary and conclusions.

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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