tokens: We acknowledge financial support from PRIN MIUR 2010-2011, project ‘The Chemical and dynamical Evolution of the Milky Way and Local Group Galaxies’, prot. 2010LY5N2T. NR acknowledges support from the Swedish Research Council, VR (project number 621-2014-5640), Funds from Kungl. Fysiografiska Sällskapet i Lund. (Stiftelsen Walter Gyllenbergs fond and Märta och Erik Holmbergs donation). NR acknowledges the supports from Knut och Alice Wallenbergs stiftelse. We thank I. J. Danziger for reading the manuscript. Finally, we thank an anonymous referee for his/her important suggestions which improved the paper.
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