The gravitational collapse formation theory directly creates planetesimals up to a few hundred km in size from cm-sized pebbles by bypassing the radial drift, fragmentation, and bouncing barriers, which limit pairwise growth to pebble sizes. Gravitationally unstable clouds of pebbles likely form in the protoplanetary disk via the streaming instability (Youdin & Goodman 2005; Johansen et al. 2007). It has been hypothesized that Kuiper Belt binaries are evidence of this collapse process (Nesvorný et al. 2010; Nesvorný & Vokrouhlický 2019) because the excess angular momentum of the cloud prevents coalescence into a single body. However, previous work has only explored a limited space of initial cloud conditions. Here, we examine how a collapsing cloud’s initial velocity and mass distributions affect the created planetesimal systems. In particular, we focus on the multiplicity and system dynamics of the collapsed planetesimals as well as individual planetesimal morphology and spin dynamics.
We use PKDGRAV (Stadel 2001; Richardson et al. 2000)—a parallel tree gravitational N-body integrator—to model a cloud undergoing gravitational collapse, employing the soft-sphere discrete element method (SSDEM) package in PKDGRAV (Schwartz et al. 2012) so that colliding particles may stick and rest on one another. With this model, we accurately track the rotational dynamics and shape of individual planetesimals, including contact binaries. Our particles also have realistic densities, i.e. no artificial inflation factor to enhance the collision rate. The effect of inflation factors is difficult to assess but the induced artificial growth enhancement may be too vigorous, prevent the formation of tightly orbiting systems, and bias final system architectures towards binarity rather than higher number multiplicity.
Our results show that over a wide range of initial angular velocity values, gravitational collapse can form planetesimal systems composed of: a single large primary planetesimal; aggregates bound to each primary, with an increasing number of aggregates (20-100 times less massive than the primary) with increasing angular velocity; and single particles bound to each primary ranging from 40 particles for the lowest initial angular velocity value to over 300 for the highest. Given an extended simulation time, the remaining bound aggregates may coalesce and form into a large secondary planetesimal.