tokens: Dust studies at Leiden Observatory are supported through the Spinoza Premie of the Dutch science agency, NWO. S.K. would like to thank C.P. Dullemond, J. Dra ̧żkowska and T. Birnstiel for useful discussions, and S. Okuzumi for comments and helping with the comparison to his work. The authors thank B. Gundlach and J. Blum for sharing a version of their manuscript. C.W.O. acknowledges support for this work by NASA through Hubble Fellowship grant No. HST-HF-51294.01-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555.
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