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