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T. Takeda, S. Ida (Tokyo Institite of Tech.)
The Moon would have formed through accretion from an impact-debris disk. N-body simulations (Ida et al. 1997, {\em Nature}, 389, 353; Kokubo et al. 2000, {\em Icarus}, in press) showed that time scale of lunar accretion is regulated by diffusion of material out from the Roche limit due to angular momentum transfer. Ida et al.(N ~ 1,000) and Kokubo et al. (2000) (N=10,000) suggested that the time scale is a month to a year in the case of a particulate disk neglecting vaporization. Here we formulate angular momentum transfer in an impact-debris disk and performed global N-body simulation up to 100,000 particles to investigate the details of angular momentum transfer.
The protolunar disk is generally optically thick, and spiral structure develops within Roche limit. With N=100,000, the spiral structure is beautifully resolved. In such a disk, (1) gravitational torque, (2) translation of particles, and (3) physical collisions play an important role in angular momentum transfer. Except in innermost region near Earth's surface, (1) and (2) dominate the angular momentum transfer. They are both regulated by the spiral pattern, which depends on surface density, but not on particle size, as long as, optical depth \tau is \agt 0.2. \tau \agt 0.2 is equivalent to N \agt a few thousand when 2-3 lunar mass is within Roche limit. This indicates that the result of N-body simulations that lunar accretion time scale is a month to a year would be robust. We will also comment on the effects of vaporization of disk particles caused by collisional heating.
The author(s) of this abstract have provided an email address for comments about the abstract: ttakeda@geo.titech.ac.jp