Dust size growth and settling in a protoplanetary disk

We have studied dust evolution in a quiescent or turbulent protoplanetary disk by numerically solving a coagulation equation for settling dust particles, using the minimum mass solar nebula model. As a result, if we assume an ideally quiescent disk, the dust particles settle toward the disk midplane to form a gravitationally unstable layer within 2 x 10(3) to 4; 10(4) yr at 1-30 AU, which is in good agreement with an analytic calculation by Nakagawa et al., although they did not take the particle size distribution into account explicitly. In an opposite extreme case of a globally turbulent disk, on the other hand, the dust particles fluctuate owing to turbulent motion of the gas and most particles become large enough to move inward very rapidly within 70 to 3 x 10(4) yr at 1 - 30 AU, depending on the strength of the turbulence. Our result suggests that global turbulent motion should cease for planetesimal formation to be possible in protoplanetary disks.