Investigating gravitational collapse of a pebble cloud to form transneptunian binaries

¹ Astrophysics Research Centre, Queen’s University Belfast, University Road, BT7 1NN, Belfast, UK
e-mail: j.e.robinson@qub.ac.uk
² Herzberg Astronomy and Astrophysics Research Centre, National Research Council of Canada, 5071 West Saanich Road, Victoria BC V9E 2E7, Canada

Received: 7 January 2020
Accepted: 8 August 2020

Abstract

Context. A large fraction of transneptunian objects are found in binary pairs, ~30% in the cold classical population between a_hel ~ 39 and ~48 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 bi-lobate nature of 2014 MU₆₉ (Arrokoth) was discovered, interest in gravitational collapse as a pathway for forming contact binaries has also grown.

Aims. We investigate the formation of binary systems through gravitational collapse by 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 for 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, about 1.5 bound systems were produced per cloud in the mass range studied here. In addition to large equal-sized binaries, we found that gravitational collapse produces massive bodies with small satellites and low-mass binaries with a high mass ratio. Our results disfavour the collapse of high-mass clouds, in line with reported upper mass limits of clouds formed by the streaming instability. 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.