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 globular clusters NGC 2136 and NGC 2137. 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 NGC 2136 and four in NGC 2137 reveals that the two clusters share
very similar systemic radial velocities, namely Vrad=271.5\pm0.4 km/s
(sigma=1.0 km/s) and Vrad=270.6\pm0.5 km/s (sigma=0.9 km/s) for NGC 2136 and
NGC 2137, respectively, and they have also indistinguishable abundance
patterns. The iron content is [Fe/H]=-0.40\pm0.01 dex (sigma=0.03 dex) for NGC
2136 and -0.39\pm0.01 dex (sigma=0.01 dex) for NGC 2137, while the [alpha/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 a LMC cluster pair. The most likely
fate of this system is to merge into a single structure in a time-scale
comparable with its orbital period.
