  The formation of planetesimals in the early Solar System is hardly
understood, and in particular the growth of dust aggregates above millimeter
sizes has recently turned out to be a difficult task in our understanding [Zsom
et al. 2010, A&A, 513, A57]. Laboratory experiments have shown that dust
aggregates of these sizes stick to one another only at unreasonably low
velocities. However, in the protoplanetary disk, millimeter-sized particles are
known to have been ubiquitous. One can find relics of them in the form of solid
chondrules as the main constituent of chondrites. Most of these chondrules were
found to feature a fine-grained rim, which is hypothesized to have formed from
accreting dust grains in the solar nebula. To study the influence of these
dust-coated chondrules on the formation of chondrites and possibly
planetesimals, we conducted collision experiments between millimeter-sized,
dust-coated chondrule analogs at velocities of a few cm/s. For 2 and 3 mm
diameter chondrule analogs covered by dusty rims of a volume filling factor of
0.18 and 0.35-0.58, we found sticking velocities of a few cm/s. This velocity
is higher than the sticking velocity of dust aggregates of the same size. We
therefore conclude that chondrules may be an important step towards a deeper
understanding of the collisional growth of larger bodies. Moreover, we analyzed
the collision behavior in an ensemble of dust aggregates and non-coated
chondrule analogs. While neither the dust aggregates nor the solid chondrule
analogs show sticking in collisions among their species, we found an enhanced
sicking efficiency in collisions between the two constituents, which leads us
to the conjecture that chondrules might act as "catalyzers" for the growth of
larger bodies in the young Solar System.
