Investigating Gravitational Collapse of a Pebble Cloud to form Transneptunian Binaries

Context. A large fraction of transneptunian objects are found in binary pairs, $\sim30\%$ in the cold classical population between $a_\mathrm{hel}\sim 39$ and $\sim48\, \mathrm{AU}$. Observationally, these binaries generally have components of similar size and colour. Previous work has shown that gravitational collapse of a pebble cloud is an efficient mechanism for producing such systems. Since the discovery of the bi-lobate nature of 2014 MU$_{69}$ (Arrokoth) there is also interest in gravitational collapse as a pathway to form contact binaries.

Aims. Our aim was to investigate the formation of binary systems via gravitational collapse, considering a wider range of binary masses than previous studies. We analysed in detail the properties of the bound systems that are formed and compared them to observations.

Methods. We performed $N$-body simulations of gravitational collapse of a pebble cloud using the \rebound package, with an integrator designed for rotating reference frames and robust collision detection. We conducted a deep search for gravitationally bound particles at the end of the gravitational collapse phase and tested their stability. For all systems produced, not just the most massive binaries, we investigated the population characteristics of their mass and orbital parameters.

Results. We found that gravitational collapse is an efficient producer of bound planetesimal systems. On average there were $\sim1.5$ bound systems produced per cloud, in the mass range studied here. As well as the large, equal-sized binaries, we found that gravitational collapse produces massive bodies with small satellites and low mass binaries with a high mass ratio. Gravitational collapse can create binary systems analogous to Arrokoth and collisions in a collapsing cloud should be gentle enough to preserve a bi-lobed structure.