On collision course: The nature of the binary star cluster NGC2006/SL 538

Abstract

Context. The Large Magellanic Cloud (LMC) is known to be the host of a rich variety of star clusters of all ages. A large number of them is seen in close projected proximity. Ages have been derived for few of them showing differences up to few million years, hinting at being binary star clusters. However, final confirmation through spectroscopy measurements and dynamical analysis is needed. Aims. In the present work we focus on one of these LMC cluster pairs (NGC 2006–SL 538) and aim to determine whether the star cluster pair is a bound entity and, therefore, a binary star cluster or a chance alignment. Methods. We used the Magellan Inamori Kyocera Echelle (MIKE) high-resolution spectrograph on the 6.5 m Magellan-II Clay telescope at Las Campanas Observatory to acquire integrated-light spectra of the two clusters, measuring their radial velocities with individual absorption features and cross-correlation of each spectrum with a stellar spectral library. Results. We measured radial velocities by two methods: first by direct line-profile measurement yields νr = 300.3 ± 5 ± 6 km s−1 for NGC 2006 and νr = 310.2 ± 4 ± 6 km s−1 for SL 538. The second one is derived by comparing observed spectra with synthetic bootstrapped spectra yielding νr = 311.0 ± 0.6 km s−1 for NGC 2006 and νr = 309.4 ± 0.5 km s−1 for SL 538. Finally when spectra are directly compared, we find a Δν = 1.08 ± 0.47 km s−1. Full-spectrum spectral energy distribution fits reveal that the stellar population ages of both clusters lie in the range 13–21 Myr with a metallicity of Z = 0.008. We find indications for differences in the chemical abundance patterns as revealed by the helium absorption lines between the two clusters. The dynamical analysis of the system shows that the two clusters are likely to merge within the next ∼150 Myr to form a star cluster with a stellar mass of ∼104 M⊙. Conclusions. The NGC 2006–SL 538 cluster pair shows radial velocities, stellar population and dynamical parameters consistent with a gravitational bound entity and, considering that the velocity dispersion of the stars in LMC is ≲20 km s−1, we reject them as a chance alignment. We conclude that this is a genuine binary cluster pair, and we propose that their differences in ages and stellar population chemistry is most likely due to variances in their chemical enrichment history within their environment. We suggest that they may have formed in a loosely bound star-formation complex which saw initial fragmentation but then had its clusters become a gravitationally bound pair by tidal capture.