tokens: We thank Phil Armitage, Ilse Cleeves, Ilaria Pascucci, Richard Teague, and Upasana Das for useful discussions related to this work. We also thank the anonymous referee whose comments improved the quality of this paper. J.B.S. acknowledges support from NASA grants NNX13AI58G and NNX16AB42G, and from NSF grant AST 1313021. The computations were performed on Stampede , Stampede 2 , and Maverick through XSEDE grants TG-AST120062 and TG-AST140001. We thank the Kavli Institute for Theoretical Physics, supported in part by the National Science Foundation under Grant No. NSF PHY-1125915, for hospitality during the completion of the paper. Software: Athena (Stone et al. 2008 ).
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