tokens: results, the most important one being that due to the photoelectric charging. This is further illustrated below. Two typical runs, though limited to 100 grains for each a value and with integration times of only 0.1 Myr for clarity, are illustrated in Figs. 4 and 5 . The spread of the orbits into broom-like structures is entirely due to the turbulence; if v turb is set equal to 0 the orbits collapse to a single vertical line. It is obvious from these figures that the choice of initial conditions for the velocity is important for the orbits, although the fraction of grains falling into the star as a function of grain radius is not as dependent on these conditions. Fig. 4. The orbits of grains over 10 5 yr, from a point indicated by an “ x ” in black along the z axis at a distance of 10 4 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. Fig. 5. The orbits of grains over 10 5 yr, from a point indicated by an “ x ” in white along the z axis at a distance of 10 4 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 initial velocity of the dust grains was set to zero. 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. The main results of these numerical experiments are displayed in Fig. 6 . Here we have plotted a quantity called “the defect”, D ( a ), of the run, as a function of a for different choices of the defining parameters. D ( a ) measures the fraction of grains that leave the proto-solar cloud as a result of the radiative cleansing. It is defined as where N l is the number of dust grains lost inside 100 AU, and N 0 is the corresponding number lost in a run with identical parameters with the exception that the stellar luminosity L is set equal to zero. In this way, we try to correct for the fact that even at zero luminosity some of the gas may depart at high turbulence speeds from the stellar neighbourhood. It is seen from the figure that the variation of D with a is smoothed when v turb increases, from a very steep change when a passes the limiting value in Eq. ( 9 ) for small turbulence, to a less steep variation. We also note that an increased inflow of gas decreases the deficit at a given a , as does a change of the initial condition from v ( r 0 )=0 to setting v ( r 0 ) equal to the free fall velocity of the gas flow. Also, the coulomb forces diminish D but not drastically so. None of these effects is unexpected. Obviously, these variations of the deficit with the different parameters are not very dramatic: the conclusions concerning the cleansing effects in
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