Millimeter interferometry provides evidence for the presence of mm to cm size ‘ pebbles ’ in the outer parts of disks around pre-main-sequence stars .
The observations suggest that large grains are produced relatively early in disk evolution ( < 1 Myr ) and remain at large radii for longer periods of time ( 5 to 10 Myr ) .
Simple theoretical estimates of the radial drift time of solid particles , however , imply that they would drift inward over a time scale of less than 0.1 Myr .
In this paper , we address this conflict between theory and observation , using more detailed theoretical models , including the effects of sedimentation , collective drag forces and turbulent viscosity .
We find that , although these effects slow down the radial drift of the dust particles , this reduction is not sufficient to explain the observationally determined long survival time of mm/cm-sized grains in protoplanetary disks .
However , if for some reason the gas to dust ratio in the disk is reduced by at least a factor of 20 from the canonical value of 100 ( for instance through photoevaporation of the gas ) , then the radial drift time scales become sufficiently large to be in agreement with observations .