Application of Gas Dynamical Friction for Planetesimals: II. Evolution of Binary Planetesimals

TL;DR

This paper investigates how gas dynamical friction influences the evolution and merger of binary planetesimals in protoplanetary disks, revealing that GDF can cause mergers and complex orbital evolutions affecting planetesimal growth.

Contribution

It introduces a detailed N-body simulation study of binary planetesimals under gas dynamical friction, highlighting effects like mergers, orbital expansion, and chaotic evolution, which were not thoroughly explored before.

Findings

GDF induces binary mergers within disk lifetime for masses above 10^22 g.

Eccentric binaries can expand due to supersonic GDF torque reversal.

Chaotic evolution occurs in highly inclined binaries with small mass ratios.

Abstract

One of first the stages of planet formation is the growth of small planetesimals and their accumulation into large planetesimals and planetary embryos. This early stage occurs much before the dispersal of most of the gas from the protoplanetary disk. At this stage gas-planetesimal interactions play a key role in the dynamical evolution of \emph{single} intermediate-mass planetesimals ($m_{p}\sim10^{21}-10^{25}g$) \emph{through gas dynamical friction} (GDF). A significant fraction of all Solar system planetesimals (asteroids and Kuiper-belt objects) are known to be binary planetesimals (BPs). Here, we explore the effects of GDF on the evolution of \emph{binary} planetesimals embedded in a gaseous disk using an N-body code with a fiducial external force accounting for GDF. We find that GDF can induce binary mergers on timescales shorter than the disk lifetime for masses above $m_{p}\gtrsim10^{22}g$ at 1AU, independent of the binary initial separation and eccentricity. Such mergers can affect the structure of merger-formed planetesimals, and the GDF-induced binary inspiral can play a role in the evolution of the planetesimal disk. In addition, binaries on eccentric orbits around the star may evolve in the supersonic regime, where the torque reverses and the binary expands, which would enhance the cross section for planetesimal encounters with the binary. Highly inclined binaries with small mass ratios, evolve due to the combined effects of Kozai-Lidov cycles with GDF which lead to chaotic evolution. Prograde binaries go through semi-regular Kozai-Lidov evolution, while retrograde binaries frequently flip their inclination and $\sim50\%$ of them are destroyed.