Blood Ties: The Real Nature of the LMC Binary Globular Clusters NGC 2136 and NGC 2137

We have used a sample of high-resolution spectra obtained with the multi-fiber facility FLAMES at the Very Large Telescope of the European Southern Observatory to derive the kinematical and chemical properties of the two young Large Magellanic Cloud (LMC) globular clusters NGC2136 and NGC2137. These two clusters represent a typical example of LMC cluster pair suspected to be bound in a binary system: indeed, the cluster centers of gravity have an angular separation of less than 1.4 arcmin in the sky. The spectral analysis of seven giants in NGC2136 and four in NGC2137 reveals that the two clusters share very similar systemic radial velocities, namely, V r = 271.5±0. 4kms -1 (σ = 1.0kms -1) and V r = 270.6±0.5kms -1 (σ = 0.9kms -1) for NGC2136 and NGC2137, respectively, and they have also indistinguishable abundance patterns. The iron content is [Fe/H] = -0.40±0.01dex (σ = 0.03dex) for NGC2136 and -0.39±0.01dex (σ = 0.01dex) for NGC2137, while the [α/Fe] ratios are roughly solar in both clusters. These findings suggest that the two clusters are gravitationally bound and that they formed from the fragmentation of the same molecular cloud that was chemically homogeneous. This is the first firm confirmation of the binary nature of an LMC cluster pair. The most likely fate of this system is to merge into a single structure in a timescale comparable with its orbital period.