  We investigated fundamental processes of collisional sticking and
fragmentation of dust aggregates by carrying out N-body simulations of
submicron-sized icy dust monomers. We examined the condition for collisional
growth of two colliding dust aggregates in a wide range of the mass ratio,
1-64. We found that the mass transfer from a larger dust aggregate to a smaller
one is a dominant process in collisions with a mass ratio of 2-30 and impact
velocity of \approx 30-170 m s^-1. As a result, the critical velocity, v_fra,
for fragmentation of the largest body is considerably reduced for such
unequal-mass collisions; v_fra of collisions with a mass ratio of 3 is about
half of that obtained from equal-mass collisions. The impact velocity is
generally higher for collisions between dust aggregates with higher mass ratios
because of the difference between the radial drift velocities in the typical
condition of protoplanetary disks. Therefore, the reduced v_fra for
unequal-mass collisions would delay growth of dust grains in the inner region
of protoplanetary disks.
