  Some scenarios for planetesimal formation go through a phase of collapse of
gravitationally bound clouds of mm-cm-sized pebbles. Such clouds can form for
example through the streaming instability in protoplanetary disks. We model the
collapse process with a statistical model to obtain the internal structure of
planetesimals with solid radii between 10 and 1,000 km. In the collapse,
pebbles collide and, depending on relative speed, collisions have different
outcomes. A mixture of particle sizes inside a planetesimal leads to better
packing capabilities and higher densities. In this paper we apply results from
new laboratory experiments of dust aggregate collisions (presented in a
companion paper) to model collision outcomes. We find that the internal
structure of a planetesimal is strongly dependent on both its mass and the
applied fragmentation model. Low-mass planetesimals have no/few fragmenting
pebble collisions in the collapse phase and end up as porous pebble-piles. The
amount of fragmenting collisions increases with increasing cloud mass,
resulting in wider particle size distributions and higher density. The collapse
is nevertheless "cold" in the sense that collision speeds are damped by the
high collision frequency. This ensures that a significant fraction of large
pebbles survive the collapse in all but the most massive clouds. Our results
are in broad agreement with the observed increase in density of Kuiper belt
objects with increasing size as exemplified by the recent characterization of
the highly porous comet 67P/Churyumov-Gerasimenko.
