tokens: Acknowledgements We are grateful to Mark Booth for commenting on the manuscript and Hiroshi Kobayashi for providing us with the details of his planetesimal growth simulations. A speedy and insightful referee report by an anonymous reviewer is very much appreciated. AVK, TL, and JB thank the Deutsche Forschungsgemeinschaft (DFG) for financial support through grants Kr 2164/13-1, Lo 1715/2-1, and Bl 298/24-1, respectively. AJ thanks the Knut and Alice Wallenberg Foundation (grants 2012.0150, 2014.0017, 2014.0048), the Swedish Research Council (grant 2014-5775) and the European Research Council (ERC Consolidator Grant 724687-PLANETESYS) for their financial support.
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