N-Body Simulations of Growth from 1 km Planetesimals at 0.4 AU

TL;DR

This study uses N-body simulations to explore planetary growth from 1 km planetesimals at 0.4 AU, revealing rapid growth, high rotation rates, and limitations of the patch approximation in modeling planetary accretion.

Contribution

First detailed N-body simulation of 1 km planetesimal growth at 0.4 AU, highlighting rapid accretion, rotation dynamics, and the limitations of perfect accretion assumptions.

Findings

Some particles grow over 100 times in mass

Most merger remnants rotate faster than breakup speed

Runaway growth indicated but no large detached body formed

Abstract

We present N-body simulations of planetary accretion beginning with 1 km radius planetesimals in orbit about a 1 solar mass star at 0.4 AU. The initial disk of planetesimals contains too many bodies for any current N-body code to integrate; therefore, we model a sample patch of the disk. Although this greatly reduces the number of bodies, we still track in excess of 10^5 particles. We consider three initial velocity distributions and monitor the growth of the planetesimals. The masses of some particles increase by more than a factor of 100. Additionally, the escape speed of the largest particle grows considerably faster than the velocity dispersion of the particles, suggesting impending runaway growth, although no particle grows large enough to detach itself from the power law size-frequency distribution. These results are in general agreement with previous statistical and analytical results. We compute rotation rates by assuming conservation of angular momentum around the center of mass at impact and that merged planetesimals relax to spherical shapes. At the end of our simulations, the majority of bodies that have undergone at least one merger are rotating faster than the breakup frequency. This implies that the assumption of completely inelastic collisions (perfect accretion), which is made in most simulations of planetary growth at sizes 1 km and above, is inappropriate. Our simulations reveal that, subsequent to the number of particles in the patch having been decreased by mergers to half its initial value, the presence of larger bodies in neighboring regions of the disk may limit the validity of simulations employing the patch approximation.