Binaries traveling through a gaseous medium: Dynamical drag forces and internal torques

TL;DR

This paper uses linear theory to analyze how a binary system moving through gas experiences dynamical drag and internal torques, affecting its migration and orbital evolution.

Contribution

It provides a detailed calculation of the gravitational wake and drag forces on a binary in gaseous media, highlighting differences from point-mass models and the dependence on Mach numbers.

Findings

Dynamical friction on the binary's center of mass is weaker than for a point mass.

Internal torque depends on the Mach numbers of binary components and the center of mass.

Internal torque generally decreases with higher binary velocity relative to gas.

Abstract

Using time-dependent linear theory, we investigate the morphology of the gravitational wake induced by a binary, whose center of mass moves at velocity Vcm against a uniform background of gas. For simplicity, we assume that the binary's components are on circular orbits about their common center of mass. The consequences of dynamical friction is twofold. First, gas dynamical friction may drag the binary's center of mass and cause the binary to migrate. Second, drag forces also induce a braking torque, which causes the orbits of the binary components to shrink. We compute the drag forces acting on one component of the binary due to the gravitational interaction with its own wake. We show that the dynamical friction force responsible to decelerate the binary's center of mass is smaller than it is in the point-mass case because of the loss of gravitational focusing. We show that the braking internal torque depends on the Mach numbers of each binary component about their center of mass, and also on the Mach number of the center of mass of the binary. In general, the internal torque decreases with increasing the velocity of the binary relative to the ambient gas cloud. However, this is not always the case. We also mention the relevance of our results on the period distribution of binaries.