tokens: We thank B. Ochsendorf and K. Tchernyshyov for useful discussions. We thank the referee for the useful comments. This publication makes use of data products from the Wide-field Infrared Survey Explorer , which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. SvV is supported by NASA through a Hubble Fellowship (HST-HF2-51350).
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