The orbits of grains over 10⁵ yr, from a point indicated by an “x” in black along the z axis at a distance of 10⁴ AU from the star (which is at the origin of the coordinate system), and with different grain radii a as indicated by the different colours. The dust grains were given an initial velocity identical to the gas free-fall velocity. The turbulent mean speed was set to 0.1 km s⁻¹. We note that the scales of the x and y axes are different from that of the z axis.