  Planetesimals are compact astrophysical objects roughly 1-1000 km in size,
massive enough to be held together by gravity. They can grow by accreting
material to become full-size planets. Planetesimals themselves are thought to
form by complex physical processes from small grains in protoplanetary disks.
The streaming instability (SI) model states that mm/cm-size particles (pebbles)
are aerodynamically collected into self-gravitating clouds which then directly
collapse into planetesimals. Here we analyze ATHENA simulations of the SI to
characterize the initial properties (e.g., rotation) of pebble clouds. Their
gravitational collapse is followed with the PKDGRAV N-body code, which has been
modified to realistically account for pebble collisions. We find that pebble
clouds rapidly collapse into short-lived disk structures from which
planetesimals form. The planetesimal properties depend on the cloud's scaled
angular momentum, l=L/(M R_H^2 Omega, where L and M are the angular momentum
and mass, R_H is the Hill radius, and Omega is the orbital frequency. Low-l
pebble clouds produce tight (or contact) binaries and single planetesimals.
Compact high-l clouds give birth to binary planetesimals with attributes that
closely resemble the equal-size binaries found in the Kuiper belt.
Significantly, the SI-triggered gravitational collapse can explain the angular
momentum distribution of known equal-size binaries -- a result pending
verification from studies with improved resolution. About 10% of collapse
simulations produce hierarchical systems with two or more large moons. These
systems should be found in the Kuiper belt when observations reach the
threshold sensitivity.
