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How to create planetesimals from tiny dust particles in a proto-planetarydisk before the dust particles spiral to the central star is one of the mostchallenging problems in the theory of planetary system formation. In ourprevious paper Kato et al. (2009), we have shown that a steady angular velocityprofile that consists of both super and sub-Keplerian regions is created in thedisk through non-uniform excitation of Magneto-Rotational Instability (MRI).Such non-uniform MRI excitation is reasonably expected in a part of disks withrelatively low ionization degree. In this paper, we show throughthree-dimensional resistive MHD simulations with test particles that thisradial structure of the angular velocity indeed leads to prevention ofspiral-in of dust particles and furthermore to their accumulation at theboundary of super-Keplerian and sub-Keplerian regions. Treating dust particlesas test particles, their motions under the influence of the non-uniform MRIthrough gas drag are simulated. In the most favorable cases (meter-size dustparticles in the disk region with a relatively large fraction of MRI-stableregion), we found that the dust concentration is peaked around thesuper/sub-Keplerian flow boundary and the peak dust density is 10,000 times ashigh as the initial value. The peak density is high enough for the subsequentgravitational instability to set in, suggesting a possible route toplanetesimal formation via non-uniformly excited MRI in weakly ionized regionsof a disk.

Dust Concentration at the Boundary Between Steady Super/Sub-Keplerian Flow Created by Inhomogeneous Growth of MRI