tokens: The authors acknowledge useful discussions with G. Nelemans and M. Dan. We acknowledge an anonymous referee for suggestions that helped us to improve the presentation of our results. LP acknowledges support from the PRIN-INAF 2011 project ‘Multiple populations in Globular Clusters: their role in the Galaxy assembly’. AT acknowledges support from the PRIN-MIUR 2010-2011 project ‘The Obscure Universe and the Cosmic Evolution of Barions’. LRY acknowledges support by RFBR grants 14-02-00604, 15-02-04053 and Presidium of RAS program P-41. This research has made use of NASA's Astrophysics Data System.
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