Planetesimal Accretion in Binary Systems: The Effects of Gas Dissipation

TL;DR

This study demonstrates that gas dissipation in binary star systems can facilitate planetesimal growth by reducing impact velocities, thus improving conditions for planet formation despite strong stellar perturbations.

Contribution

The paper introduces a mechanism showing that gas dissipation leads to orbital convergence of planetesimals, promoting accretion in close binary systems like $ ext{γ}$ Cephei.

Findings

Gas dissipation causes planetesimals to converge to similar orbits.

Lower impact velocities enhance mass accretion and runaway growth.

Size distribution significantly influences collisional outcomes.

Abstract

Currently, one of major problems concerning planet formation theory in close binary systems is, the strong perturbation from the companion star can increase relative velocities ($\triangle V$) of planetesimals around the primary and thus hinder their growth. According to previous studies, while gas drag can reduce the $\triangle V$ between bodies of the same sizes by forcing orbital alignment to planetesimals, it increases the $\triangle V$ among bodies of different sizes. In this paper, focusing on the $\gamma$ Cephei binary system, we propose a mechanism that can overcome this difficulty. We show that in a dissipating gas disk (with a typical dissipating timescale of $\sim 10^5-10^6$ years), all the planetesimals eventually converge towards the same forced orbits regardless of their sizes, leading to much lower impact velocities among them. These $\triangle V$ decrease processes progressively increase net mass accretion and even trigger runaway growth for large bodies (radius $>15$ km). The effect of size distribution of planetesimals is discussed, and found to be one of the dominant factors that determine the outcome of collisional evolution. Anyway, it can be concluded that by including the gas dissipation in the early stage of disk evolution, the conditions for planetesimal accretion become much better, and the process from planetesimal to planet-embryo can be carried out in close binary systems like $\gamma$ Cephei.