tokens: We thank the referee for comments that help us to improve the manuscript. We acknowledge the 2011 PRIN-INAF grant for financial support. We acknowledge the financial support of the UnivEarthS Labex program at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). G.G. thanks the Observatoire de Paris (Meudon) for support and hospitality during the completion of this work. We thank S. Campana and G. Tagliaferri for contributing to useful discussions.
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