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tokens: The authors thank a referee, H. J. Melosh, for useful comments that substantially improve the quality of this work. Numerical computations were in part carried out on the GRAPE system at Center for Computational Astrophysics, National Astronomical Observatory of Japan. Y.H. is supported by an EACOA Fellowship that is supported by East Asia Core Observatories Association, which consists of the Academia Sinica Institute of Astronomy and Astrophysics, the National Astronomical Observatory of Japan, the National Astronomical Observatory of China, and the Korea Astronomy and Space Science Institute.