  The sticking of micron sized dust particles due to surface forces in
circumstellar disks is the first stage in the production of asteroids and
planets. The key ingredients that drive this process are the relative velocity
between the dust particles in this environment and the complex physics of dust
aggregate collisions. Here we present the results of a collision model, which
is based on laboratory experiments of these aggregates. We investigate the
maximum aggregate size and mass that can be reached by coagulation in
protoplanetary disks. We model the growth of dust aggregates at 1 AU at the
midplane at three different gas densities. We find that the evolution of the
dust does not follow the previously assumed growth-fragmentation cycles.
Catastrophic fragmentation hardly occurs in the three disk models. Furthermore
we see long lived, quasi-steady states in the distribution function of the
aggregates due to bouncing. We explore how the mass and the porosity change
upon varying the turbulence parameter and by varying the critical mass ratio of
dust particles. Particles reach Stokes numbers of roughly 10^-4 during the
simulations. The particle growth is stopped by bouncing rather than
fragmentation in these models. The final Stokes number of the aggregates is
rather insensitive to the variations of the gas density and the strength of
turbulence. The maximum mass of the particles is limited to approximately 1
gram (chondrule-sized particles). Planetesimal formation can proceed via the
turbulent concentration of these aerodynamically size-sorted chondrule-sized
particles.
