tokens: This work is supported by the NSF Astronomy and Astrophysics Division (grant No.1311958 to G.V.). We thank Dr. J. Brucato of the Astrophysical Observatory of Arcetri (Italy) for providing the sample used in these experiments and Zhirou Zhang, Jianming Shi, and Tyler Hopkins for technical help. R.T.G. acknowledges the support of the NASA Astrophysics Theory Program (grant No. NNX11AC38G).
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