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The Effects of Particle Contact Physics on the Gravitational Collapse Process

Presentation #506.05 in the session Things that Figuratively “Go Bump in the Kuiper Belt, Chapter One”.

Published onOct 20, 2022
The Effects of Particle Contact Physics on the Gravitational Collapse Process

Growth barriers limit the pairwise growth of pebbles in the protoplanetary disk to cm sizes, hindering planetesimal formation. Growth up to 100-km planetesimal sizes may occur beyond these barriers via the gravitational collapse of clouds of pebbles gathered by interactions with nebular gas, as in the streaming instability (Youdin & Goodman 2005; Johansen et al. 2007; Carrera et al. 2021a). The abundance of Kuiper belt binaries is evidence of this process, as excess angular momentum leftover from the SI would prevent the coalescence of a pebble cloud into a single body (Nesvorný et al. 2010, 2019, 2021; Robinson et al. 2020). Yet, the role of contact physics between colliding pebbles during gravitational collapse is not well-explored. In this work, we examined the effects of varying these parameters on the formation of planetesimal systems, focusing particularly on individual planetesimal morphology and rotation states, system dynamics, and planetesimals’ collisional histories.

We have modeled gravitational collapse using the PKDGRAV N-body integrator (Richardson et al. 2000; Stadel 2001) and its soft-sphere discrete element method, which ensures that colliding particles stick and rest upon one another rather than merging to form a single larger spherical particle (Schwartz et al. 2012). Because we do not use an inflation factor to enhance the collision rate, our particles maintain realistic densities. The use of inflation factors may also induce overly vigorous planetesimal growth, prevent the formation of tightly orbiting systems, and bias final systems towards binarity rather than higher number multiplicity. Moreover, one cannot accurately determine planetesimal shapes and spins in perfect-merging models. The SSDEM’s ability to monitor contact forces between particles is therefore required to form a complete theory of planetesimal formation.

All of our simulations successfully create binary systems, each containing a single large primary and bound secondary planetesimal as well as many smaller binary systems. Gravitational collapse is very efficient. With the inclusion of varied particle frictional forces and coefficients of restitution we see the formation of planetesimal systems with a wide array of dynamics. Our largest systems exhibit mild dynamics with mutual orbital distances ranging from 102-103 km. Smaller systems exhibit a wide range of inclinations, moderate to high eccentricities, and some maintain even tighter orbits.

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