tokens: Acknowledgments We are grateful to the referee, Raffaele Gratton, whose comments greatly improved the paper. Support for this work (proposal number GO-10775) was provided by NASA through a grant from the Space Telescope Science Institute that is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. This material is based upon work supported by the National Aeronautics and Space Administration under Grant NNX11AF34G issued through the Office of Space Science and through the University of Central Floridas NASA Florida Space Grant Consortium. DSs work was supported by NSF grants DMS 1208791. DvD was partially supported by a Wolfson Research Merit Award (WM110023) provided by the British Royal Society and by Marie-Curie Career Integration (FP7-PEOPLE-2012-CIG-321865) and Marie-Skodowska-Curie RISE (H2020-MSCA-RISE-2015-691164) Grants both provided by the European Commission.
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