\section*{Acknowledgments} 
All the authors of this paper thank the KITP and the organizers of the ``Galactic Archaeology and Precision Stellar Astrophysics{\textquotedblright} program held from January to April 2015. JF acknowledges partial support from NSF under grant no. AST-1205732 and through a Lee DuBridge Fellowship at Caltech. RAG acknowledge the support of the European Community{'}s Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 312844 (SPACEINN), and from the CNES. This project was supported by NASA under TCAN grant number NNX14AB53G and the NSF under grants PHY 11-25915 and AST 11-09174. 